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2 Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA; and 3 Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, Colorado 80309-0334 USA
Received for publication November 23, 1999. Accepted for publication May 2, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Brassicaceae • mate choice • pollen competition • Raphanus sativus • seed paternity • sexual selection • wild radish
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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, but see Bierzychudek, 1981
; Young and Young, 1992
). This excess pollen creates an opportunity for selection among pollen and pollen tube phenotypes to produce nonrandom paternity of seeds (e.g., Mulcahy, 1974, 1979
; Charnov, 1979
; Willson, 1979, 1982, 1990, 1994
; Queller, 1983, 1987
; Stephenson and Bertin, 1983
; Willson and Burley, 1983
; Snow, 1994
). Such sorting may occur by competition among growing pollen tubes for access to ovules or by choice of maternal plants among potential mates. These mechanisms can cause differences in mating success among pollen donors and, if the differences are genetically based, may result in sexual selection (e.g., Marshall and Folsom, 1991
). Whether sexual selection actually occurs in plants, however, remains controversial (e.g., Charlesworth, Schemske, and Sork, 1987
; Lyons et al., 1989
; Grant, 1995
).
The existence of some kinds of nonrandom seed paternity is not in question. Plants do not simply mate at random. Mechanisms that regulate mating with close relatives, i.e., self-incompatibility, are very well known (e.g., Richards, 1986
; Holsinger, 1992
; Waller, 1993
; de Nettencourt, 1997
), and much finer scale nonrandom mating also occurs. For example, if pollen from several compatible donors is used to pollinate a stigma, the donors typically do not sire equal numbers of seeds (Bemis, 1959
; Barnes and Cleveland, 1963b
; Pfahler, 1967, 1974a, b, 1975
; Levin, 1975
; Pfahler and Linskens, 1972
; Ottaviano, Sari-Gorla, and Mulcahy, 1975
; Ottaviano, Sari-Gorla, and Arenair, 1983
; Sari-Gorla, Ottaviano, and Faini, 1975
; Currah, 1981
; Sarr, Fraleigh, and Sandmeir, 1983
; Marshall and Ellstrand, 1986, 1988
; Bertin, 1990
; Cruzan, 1990
; Young and Stanton, 1990
; Marshall, 1991, 1998
; Snow and Spira, 1991, 1996
; Rigney et al., 1993
; Björkman, Samimy, and Pearson, 1995
; Mitchell and Marshall, 1998
). The question then is not whether nonrandom mating occurs, but rather how it occurs, whether these mechanisms fit the accepted models of sexual selection, and what plant characters may be affected by sexual selection
A major problem in evaluating the possibility of sexual selection in plants is that it is very difficult to determine which of the potential mechanisms of sexual selection operate: competition among pollen donors, mate choice, or both. There is good evidence that rates of pollen tube growth vary among individuals both in vitro and in vivo (Barnes and Cleveland, 1963a
; Sayers and Murphy, 1966
; Mulcahy, 1971, 1974
; Ottaviano, Sari-Gorla, and Mulcahy, 1975, 1980
; Sari-Gorla, Ottaviano, and Faini, 1975
; Wolff, 1975
; Mulcahy, Curtis, and Snow, 1983
; Ottaviano, Sari-Gorla, and Arenair, 1983
; Schemske and Fenster, 1983
; Snow, 1986
; Aizen, Searcy, and Mulcahy, 1990
, Cruzan, 1990
; Snow and Spira, 1991
; Waser and Price, 1991
; Walsh and Charlesworth, 1992
; Winsor and Stephenson, 1995
; Niesenbaum, 1999
). Thus, raw material necessary for differential success of pollen donors exists. Moreover, application of mixed pollen loads results in differential siring success among pollen donors (see references above). These data, however, could result from competition among pollen donors, choice by maternal plants, or other, postfertilization mechanisms.
The second potential mechanism of selection among pollen donors, mate choice, has proved difficult to test. Since pollen from different donors must be present for mate choice to occur, experiments designed to measure mate choice are confounded with the possibility of competition among pollen donors. Other types of evidence, however, suggest that mate choice may occur in plants. First, some studies show that the ability of a pollen donor to sire seeds varies with the maternal plant that is used (Bertin, 1982, 1985
; Stephenson and Bertin, 1983
; Waser et al., 1987
). Second, in Raphanus sativus, number of mates per fruit affects resource allocation to fruits. This cannot be due solely to the identity of individual pollen donors (Marshall and Ellstrand, 1986
; Marshall, 1988
). Third, in R. sativus and Campsis radicans changes in the condition of maternal plants alter the ability of pollen donors to sire seeds (Bertin, 1985
; Marshall and Ellstrand, 1988
; Marshall and Fuller, 1994
). All three kinds of data suggest that maternal tissues may play a role in determining seed paternity.
While both pollen competition and mate choice may occur, it is not clear that either can completely explain previous results since several other processes may occur. During fertilization and seed filling, complementation between maternal and paternal genotypes may be important (Waser et al., 1987
). Once seeds are fertilized, maternal plants may choose among offspring rather than among mates (Queller, 1987
). In addition, developing seeds may compete among themselves for resources, causing differential seed abortion and seed filling (Kress, 1981
; Queller, 1983
). Finally, if the interests of the maternal plant and individual offspring do not coincide, parent-offspring conflict may affect the pattern of seed maturation (Westoby and Rice, 1982
; Law and Cannings, 1984
; Mazer, 1987a
).
Much of the problem in assessing whether any or all of these mechanisms operate lies in the kinds of evidence available to test among them. Many of the potential mechanisms that sort among mates and seeds occur inside the gynoecium long before final seed number and size are determined. Thus, direct observation and analysis of these events have been difficult (but see Rigney, 1995
). However, if we are to understand which of the potential mechanisms that can affect seed paternity occur, we must have more detailed information about the sequence of events that occur in the gynoecium following single and mixed pollinations. Previous studies have focused on one character, speed of pollen tube growth. Yet, siring success may also be affected by interactions among growing pollen tubes, interactions of pollen tubes with styles, ovaries, or ovules, and interactions among developing seeds. Studies of the mechanisms of differential fertilization and seed filling in the context of known variation in ability to sire seeds are critical to understanding whether and how sexual selection occurs in plants.
Previous studies with wild radish have demonstrated that, after mixed pollination, seed paternity is nonrandom with respect to differences in both number of seeds and position of seeds within fruits (Marshall and Ellstrand, 1986, 1988
; Marshall, 1991, 1998
; Marshall and Fuller, 1994
). The performance of pollen donors is affected by both the identity and the condition of the maternal plants (Marshall and Ellstrand, 1988
). These patterns could thus be the result of any of the proposed mechanisms of nonrandom seed paternity. Here, we examine the sequence of developmental events during mating in wild radish in an attempt to explain these observations. After describing the time course of events from pollination to seed filling, we ask: (1) are there differences in pollen germination, pollen tube growth, ovule fertilization, and seed development among pollen donors that correlate with differences in success of pollen donors in siring seeds, (2) are there differences in those characters after mixed and single pollination that correlate with selective filling of multiply sired fruits, and (3) does variation in maternal condition affect performance of pollen donors, as measured by these early characters, suggesting a role of the maternal plant in mating success?
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Stanton, 1987a, b
; Stanton and Preston, 1988
; D. Marshall, personal observation). Flowers are numerous (hundreds per plant), allowing many kinds and replications of crosses. Seed masses, 
10 mg, are large enough that individual seeds can be weighed precisely. Seeds germinate rapidly, making studies of offspring success feasible. Wild radish has high levels of isozyme polymorphism, making paternity analysis of seeds possible (Ellstrand, 1984
; Ellstrand and Marshall, 1985a, b, 1986
).
Experimental design
Sixteen maternal plants, four each from four full-sibships, and four pollen donors were selected. These plants were chosen primarily to minimize any obvious or cryptic effects of the incompatibility (S locus) genotype and, secondarily, on the basis of suitable genotypes at two electrophoretic loci (PGI and LAP). One generation of plants was grown prior to the experiment to allow us to obtain these plants. First, two plants from each of 100 field-collected lineages were grown in the greenhouse; 50 families were used from each of two wild radish populations near Riverside, California, USA. We started the crosses by selecting several plants haphazardly. Five reciprocal crosses were done between every pair of plants by rubbing an anther from one plant across the stigma of the other. If three or more crosses produced fruits, the cross was deemed compatible, if one out of five crosses set fruit, the cross was retested and if no fruits were set, the cross was scored as incompatible. Since wild radish has sporophytic self-incompatibility (Samson, 1957
; Hinata and Nishio, 1980
; Pundir, Abbas, and Al-Attar, 1983
), cross-compatible plants must have completely different S alleles. That is, S1S2 x S3S4 is a compatible cross, but S1S2 x S1S3 is not. In incompatible crosses, the pollen tubes do not penetrate the stigma (Richards, 1986
). Once a set of a few reciprocally intercompatibile plants was established, additional plants were tested only against that set until a group of 16 fully intercompatible plants was identified (Karron, Marshall, and Oliveras, 1990
). Finding this set of plants required nearly 10 000 test crosses. Once this set of plants was selected, the plants were screened for their isozyme phenotypes at PGI and LAP [methods as in Ellstrand (1984)
] and additional crosses were done to generate seed. For the experiment, four pairwise combinations (eight of the 16 plants) were selected to be maternal families and four to be pollen donor lineages. This was done based on combinations that could produce distinguishable homozygous genotypes at a locus of PGI and a locus of LAP.
Experimental crosses
In order to test for effects of maternal condition on mating, the 16 maternal plants were divided into two treatments. Two maternal plants from each family were control plants. The other two plants from each family were given reduced water. Control and stressed plants were selected at random within families. Stressed or low-water plants were given half of the control amount of water daily. This resulted in nearly daily wilting. The treatment was initiated before pollinations began.
The four pollen donors (named A–D) were then used to make four kinds of single pollination and six kinds of mixed pollination (A, B, C, D, A & B, A & C, A & D, B & C, B & D, C & D) on each maternal plant. Replicate crosses were made so that ovaries and developing fruits could be collected at a variety of time intervals. The products of both single and mixed pollination were collected after the following intervals: 30 min to score pollen germination, 6 h to score pollen tube length, and 12, 16, 20, and 24 h to score pollen tube location within ovaries and ovule fertilization, and at maturity to score seed size, seed number, and seed paternity (from mixed pollinations). All types of crosses collected at 30 min to 24 h were repeated four times on each plant. In addition, each single pollination was repeated 10 times to generate mature fruits and each mixed pollination was repeated 15 times to generate fruits for analysis of seed paternity and allocation to seeds. These crosses totaled 136 single pollinations and 240 mixed pollinations per plant or 6016 crosses for the experiment. Crosses were also made for collection of tissue at 3 d and 10 d after pollination in order to measure size of developing ovules and other characteristics of developing embryos. Because we anticipated that processing these tissues would be particularly time consuming, we reduced the sample size. These crosses were performed on only eight of the maternal plants (one from each treatment from each family, selected at random). All single and mixed pollinations were repeated four times on each of the eight plants for each collection interval, a total of 640 crosses.
Pollination methods
Single pollinations were performed by collecting pollen in a small petri dish and applying the pollen with tissue-wrapped forceps. Stigmas were thoroughly coated with pollen. Pollen mixes for mixed pollinations were made by collecting pollen from equal numbers of anthers from each donor in a small petri dish and then mixing thoroughly with tissue-covered forceps. Pollen was applied as in single pollinations. The method of pollen application used places several hundred pollen grains on the stigma, producing dozens of pollen tubes in the style (D. Marshall, personal observation). Both the number of pollen grains and pollen tubes are larger than the number of ovules. In this experiment, mean ovule number was 6.79.
Tissue preparation and examination
Pollen viability was scored by collecting pollen onto slides and staining with Alexander's stain (Alexander, 1980
). Pollen was collected from three flowers from each pollen donor. A minimum of 200 pollen grains was counted per slide.
Stigmas collected at 0.5 h were immediately placed on microscope slides in a drop of stain solution and examined under a compound microscope at 100x total magnification. The solution contained 1 mg/mL of sodium azide (to inhibit further pollen germination) and 1 mg/mL aniline blue (to stain germinating pollen tubes) in 0.33 mol/L K3 PO4. Pollen grains were scored as germinated if a pollen tube could be seen emerging from the pollen coat. At least 200 pollen grains were counted per stigma.
Gynoecia collected at intervals of 6 h to 10 d were fixed in 3:1 70% ethanol:acetic acid for 24 h and then stored in 70% ethanol until scoring. Stored tissue was prepared for examination by rehydration and clearing in 0.8 mol/L NaOH at 60°C for 30–50 min at 60°C, and staining overnight in 1 mg/mL aniline blue with 0.1 mg/mL ethidium bromide in 0.33 mol/L K3 PO4. Gynoecia collected at 3 and 10 d were dissected to remove ovules prior to processing.
In order to score pollen tube growth, style length, and ovule fertilization, tissue was examined at 50–100x total magnification under a fluorescence microscope. For gynoecia collected at 6 h, tissues were identified at 100x total magnification, and lengths of styles and pollen tubes were measured at 50x after calibration with a stage micrometer. The number of full screen lengths occupied by the structure to be measured was counted (calibrated at 1.56 mm/screen), and the length of the structure in the last screen was measured using a video image analysis system (Jandel Scientific, 1989
). Style length was measured as a straight line from the crest of the stigma to the top of the first ovule. Pollen tubes were followed down the style into the ovary, and the lengths of the five longest pollen tubes were measured in a straight line from the lowest point that the tubes could be seen in the ovary or style up to the crest of the stigma.
Patterns of fertilization and pollen tube growth were examined in 12 h to 24 h tissue. The number of ovules per ovary was counted. Fertilization of ovules and location of pollen tubes was scored by examination of the ovules at 50–100x total magnification. Ovules were considered reached by pollen tubes if the pollen tube had grown through the transmitting tissue at least as far as that ovule. Ovules were counted as fertilized if a pollen tube had, in addition to growing through the transmitting tissue, exited the transmitting tissue and entered the micropyle. Location of ovules, from the basal to stylar end of the ovary, was also recorded.
Size of ovules at 3 d and embryos at 10 d after pollination was measured by dissecting ovules from ovaries and measuring their size at 25–50x. For the 3-d treatment, we measured three ovules per ovary, one each from the basal, central, and stylar regions of the fruit. For fruits with even numbers of ovules, the middle ovule closest to the basal end of the ovary was measured. Area of 3-d ovules was measured at 25x. Ovules were placed on a microscope slide and covered lightly with a cover slip. A video camera was used to capture the image of the ovule. Then an image analysis program (MorphoSys; Meacham and Duncan, 1990
) was used to trace the outline of the ovule and calculate area. Thus our measure of ovule size is essentially a two-dimensional projection of the surface area. The 10-d tissue was handled a bit differently. Ovules were cleared in 0.8 mol/mL NaOH for 30 min at 60°C. They were then flattened slightly under a cover slip so that the embryo could be seen within the ovule. Length of the embryo was measured at 50x under Nomarski optics using the same computer program described above.
All seeds were counted, scored for position from the basal to the stylar end of the fruit, and weighed to the nearest 0.01 mg. Total seed mass per fruit was calculated as mean individual seed mass x number of good seeds per fruit. Seeds from mixed pollinations were then genotyped for PGI and/or LAP using starch gel electrophoresis. Both loci were resolved on the same gel (Ellstrand, 1984
).
Analysis
Comparisons of reproductive parameters among maternal plants, pollen donors, maternal families, and stress treatments were made using a series of ANOVAs. Because the numbers of pollen donors and maternal families were modest, due to the difficulty of assessing events within ovaries, some decisions had to be made about the analysis. We viewed this experiment as somewhat exploratory and wished our analyses to be conservative enough to be believable, but not so conservative that we missed trends that would lead to further experiments concentrating on larger sample sizes for fewer variables.
Our first concern was whether to treat maternal plants or families and pollen donors as random or fixed effects. These plants were clearly not selected at random as they were chosen based on careful screening for S-alleles and electrophoretic patterns. Therefore, we chose to treat all independent variables as fixed. This limits our conclusions to this set of plants.
Second, because our work was exploratory, we measured a large number of variables, some of which were likely to be correlated. To examine relationships among these variables and to look at multivariate effects, we could not use the separate data sets for each variable. Individual measurements were made on different pieces of tissue and are associated only at the level of the plant and the type of pollinations. Therefore, we created a composite data set containing the mean values of each mechanistic variable for each plant by pollination type category. This data set had a total sample size of 160 (16 maternal plants x 10 kinds of pollinations). The combined data set was used to examine correlations among all of the dependent variables and to perform MANOVAs to examine overall differences among pollen donors, maternal families, and stress treatments.
After considering overall differences among categories, the dependent variables were then analyzed individually with ANOVAs of the individual data sets. For these analyses, the fruit or in some cases the fruit region was the unit of analysis.
Our third concern was with specification of the models. It was possible to use either maternal plant or maternal family and stress treatment as independent variables in the models. While maternal family and stress treatment typically explain a substantial amount of variation, individual maternal plants explain even more. This is because there is considerable variation among plants within families, and the effect of stress treatment often varied among families. Models specifying all three (by including a maternal plant within family effect) appeared to be overspecified and were difficult to interpret. However, models that do not specify maternal plant explain less of the variation in most variables, making tests of pollen donor effects less powerful.
We solved this problem by specifying different models for tests of pollen donor effects and test of effects of maternal family and stress treatment. For direct tests of pollen donor effects, we could use only the data from the four single pollinations. With this reduced sample size we were concerned about accounting for as much variation as possible so that we could have a powerful test of the effects of pollen donor identity. Therefore, we specified maternal plant and pollen donor as main effects in the model (Table 3). These models are straightforward to interpret and explain the most variance. However, they do not provide for tests of stress effects on pollen donor performance as those differences are lumped within the maternal plant effects. Therefore, we also used models that specified maternal family, stress treatment, pollen donor, and their interactions (Table 6).
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In contrast to the more complicated models described above, the proportion of viable pollen was compared among pollen donors in a one-way ANOVA where pollen donor was the independent variable and proportion viable pollen was the dependent variable.
Some variables were analyzed as proportions. These were arcsine square-root transformed prior to analysis. None of the other variables required transformation.
Two variables proved even more difficult to measure than we anticipated and final sample sizes were reduced further. We measured pollen tube length directly from just two replicates of the 6-h crosses; however, the proportion of ovules reached by pollen tubes at 12, 16, and 20 h provides an additional, albeit indirect, measure of pollen tube growth rate. In addition, only the single pollen donor crosses were examined for embryo length at 10 d.
Because our experiment was exploratory, we tested several dependent variables. We were primarily interested in discovering which of these variables might show differences in the time course of mating among pollination treatments and maternal plants. However, for completeness, we indicate in the tables, which variables have tablewide significance after a Bonferroni correction (Rice, 1989
).
Finally, we wished to compare ability of pollen donors to sire seeds with other variables such as pollen tube growth and ovule fertilization. For the purposes of this paper, numbers of seeds sired by pollen donors were compared in chi-square tests so that they could be tested against two expectations. First, they were compared against an expectation of equal paternity and then they were compared against an expectation that was corrected for number of pollen grains per flower produced by each plant. Additional and somewhat more conservative analyses of seed paternity are available in Marshall (1998)
.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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In order to understand how the events during pollen tube growth, fertilization, and seed filling might be related, we examined correlations among the variables described above (Table 1). Interestingly, the very earliest measures, pollen germination and pollen tube growth at 6 h, were correlated with few other variables. Pollen germination was weakly positively correlated with the number of ovules pollen tubes had reached at 16 h and negatively correlated with ovule surface area and final individual seed mass. Pollen tube length was strongly correlated with only the proportion of the style reached by pollen tubes at 6 h. There were many positive correlations among proportion of ovules reached at 12 h to 20 h and proportion of ovules fertilized at 12 h to 24 h. However, the correlations among the measures of ovules fertilized at different time intervals (mean r = 0.71) tend to be higher than the correlations between measures of position of pollen tubes in the ovary and measures of ovules fertilized (mean r = 0.35), suggesting that these two categories of variables may measure different aspects of mating. Measures of pollen tube position within the ovary and of ovule fertilization were negatively correlated with individual seed mass, but typically positively correlated with number of seeds per fruit and total seed mass. Early fertilization was also correlated with embryo size at 10 d; however, embryo size at 10 d had no bearing on features of mature seeds and fruits.
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) and analyses reported elsewhere indicated that these differences were highly consistent across the maternal plants (Marshall, 1998
). Pollen donors also differed in the locations within fruits where they sired the most seeds. Pollen donors A and D sired a disproportionate number of their seeds in the stylar region of the fruit, while pollen donors C and B sired more central and stylar seeds (Table 4). The frequencies of seeds sired by the four pollen donors are not independent of fruit region (
2 = 31.67, P < 0.001, df = 6). In examining the events during pollen tube growth, fertilization, and seed filling, we were concerned with finding differences in these characters that corresponded to the differences in seed paternity.
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We found differences among pollen donors in some, but not all, of the characteristics examined (Table 3). Pollen donors differed significantly in percentage nonaborted pollen and all pairwise comparisons among pollen donors were also significant. We found no significant differences among donors in mean pollen tube length, maximum pollen tube length, or proportion of style that had been reached 6 h after pollination. Pollen tube length was difficult to measure directly and, hence, sample sizes were small. As differences in the highest and lowest values were 10–20% a larger sample size might have revealed a significant difference.
We also assessed pollen tube growth by scoring the proportion of ovules that had been reached by pollen tubes at 12, 16, and 20 h after pollination. These pollen tubes had grown far enough through the transmitting tissue to reach the position of an ovule, but had not exited the transmitting tissue and fertilized the ovules. This scores relative rather than absolute pollen tube growth; because the ovules are arranged in a linear series, donors with faster growing pollen tubes should reach a greater proportion of ovules at each time interval than donors with slower pollen tube growth rates. The proportion of ovules reached by pollen donors differed significantly at 12 and 16 h after pollination, but not at 20 h after pollination. At 12 h after pollination, pollen tubes of donor B had reached the most ovules and those of donor A, the fewest. At 16 h after pollination, no pairwise comparisons among donors were significant even though there was an overall effect of pollen donor; the rank order of pollen donor performance was the same as at 12 h (Table 3).
Differences in the proportion of ovules fertilized by the four pollen donors approached significance at 16 h, but were not significantly different at any other time interval. This is true even though the relative differences among pollen donors in proportion of ovules fertilized were large at 12 h (Table 3).
Since the proportions of ovules fertilized at 12–24 h were strongly correlated (Table 1), we also examined the overall pattern of ovule fertilization among pollen donors using an ANOVA where the independent variables were maternal plant, pollen donor, hours after fertilization and their interactions. However, neither the overall effect of pollen donor (P < 0.42) nor the pollen donor by hours past pollination interaction (P < 0.15) were significant. Since this analysis provided no new information, the ANOVA table is not shown.
Although the overall analysis revealed that pattern of fertilization within three regions of the ovary differed significantly (Fig. 1; Table 2), when we examined this pattern among pollen donors we found no significant effects. That is, there were no significant effects of pollen donor by ovary region or pollen donor by ovary region by hours past pollination in a four-way ANOVA where maternal plant, individual pollen donor, ovary region, and hours past pollination were the independent variables. (P >> 0.05; the ANOVA tables are not shown).
Three days after pollination, the developing seed surface area did not differ among pollen donors. However, embryo length did differ among pollen donors 10 d after pollination (Table 3).
In mature fruits resulting from single pollinations, there were significant effects of pollen donor on individual seed mass (Table 3). However, the pollen donors that produced larger seeds were those that had smaller embryos at 10 d. This negative relationship was not significant (Table 1); however, the sample size was quite small. Total seed mass per fruit did not differ among donors, probably because there was a negative relationship between seed mass and number of seeds per fruit.
In addition to our analyses of single-donor crosses, it was possible to assess differences in seed mass among donors by looking at the seeds they sired after mixed pollinations. In this case individual seed mass was the dependent variable and the pollen donor that sired the seed and the maternal plant were the independent variables. Both the identity of the pollen donor and the maternal plant by pollen donor interaction had significant effects on individual seed mass in the fruits that resulted from mixed pollination.
There were strong maternal plant effects on most of the variables measured (Table 3). The exceptions were the pollen tube characteristics measured at 6 h, where sample size was modest and percentage nonaborted pollen for which no maternal plant effect was possible. In this analysis maternal plant effects included differences among plants, maternal families, and stress treatments.
Although maternal plant effects were uniformly strong and donor effects were significant for several variables, maternal plant by pollen donor interactions were uncommon. The only significant effect was on final mass of seeds produced in mixed pollinations.
We were also interested in whether maternal condition affected the expression of pollen donor characters. Therefore, we performed additional analyses where the independent variables were maternal family, stress treatment, and pollen donor. However, the only significant stress treatment by pollen donor interaction effect was on embryo length after 10 d (P < 0.024). Since there is so little new information, these lengthy tables are not included. The main effect of stress is discussed below.
Effects of number of pollen donors per cross
Crosses for this experiment used pollen from one donor or a mixture of pollen from two donors. A MANOVA that used maternal plant and pollination type as the independent variables revealed that there were overall differences among the pollination types in the characters measured (Pillai's trace, F13,123 = 2.49, P < 0.0047). A number of individual characters differed among pollination types as well (Table 5)
As in all previous studies with wild radish, total seed mass per fruit differed significantly among the pollination types and multiply sired fruits were larger (Table 5). In this case, the larger fruits were a result of multiply sired fruits having significantly more seeds and tending to have larger seeds. Some of the variables measured during pollination and fruit development also differed among cross types. However, it is important to keep in mind that the test is somewhat different than the tests of differences in seed number per fruit and total seed mass per fruit. For mature fruits, we compared multiply and singly sired fruits, but for all the early variables we can only compare one-donor and two-donor pollen loads. Two-donor pollen loads can produce either multiply or singly sired fruits. Only multiply sired fruits have larger total seed mass (Ellstrand and Marshall, personal observations).
A higher proportion of pollen had germinated 30 min after single pollination than 30 min after mixed pollination (see also, Marshall et al., 1996
). There were no significant differences detected in pollen tube length at 6 h after pollination; however, the trend was for single pollinations to have longer pollen tubes. The trend had reversed by 12 to 20 h after pollination when pollen tubes from mixed pollinations had reached a greater number and proportion of ovules (Table 5). This difference was significant by 20 h after pollination.
The difference among pollination types in proportion of ovules fertilized approached significance at 12 and 24 h, with more ovules being fertilized after mixed than single pollination at 12 h and the reverse at 24 h. Three days after pollination, seed surface area was slightly, but not significantly larger after mixed pollination.
The effect of maternal plant was significant for almost all characters (Table 5). The only exception was length of the longest pollen tube 6 h after pollination. At 24 h there was also a maternal plant by pollination type interaction, indicating that the difference between single and mixed pollination varied among maternal plants.
The analyses above used models with maternal plant and pollination type as the independent variables because these explain the most variance in the characters. However, we also tested for stress by pollination type effects in an additional set of models (Table 6). None of these interactions were significant.
Effects of maternal family
Maternal families differed overall in the time course of events during pollination and fertilization. A MANOVA that used maternal family, stress treatment, and pollination type as independent variables revealed a significant maternal family effect (Pillai's trace, F39,378 = 20.2, P < 0.0001).
We found significant effects on several individual variables (Tables 6 and 7). Proportion pollen germinated at 30 min differed among the families, but the measures of pollen tube length at 6 h did not. There were differences in the proportion of ovules reached at 12 h through 20 h after pollination. Proportion of ovules fertilized was highest for family 3 and lowest for family 4 at 16 h through 24 h. The differences among families in ovule fertilization were significant at all collection times (Tables 6 and 7). Three days after pollination mean ovule size differed by one-third between the families with the largest and smallest values (Table 7), and these differences were statistically significant (Table 6). In addition, at maturity there were significant differences in number of seeds per fruit, individual seed mass, and total seed mass per fruit among maternal families (Table 6).
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). However, the rank order of pollen donor performance was identical for each maternal family. In addition to the overall effects of maternal family, there were maternal family by stress treatment effects on most variables (Table 6). This indicates that the effects of stress, described below, differed somewhat among the maternal families.
Effects of maternal stress treatment
The time course of events differed overall among the control and low-water plants. A MANOVA that used maternal family, stress treatment and pollination type as independent variables revealed a significant stress treatment effect (Pillai's trace, F 13,124 = 16.0, P < 0.0001). The extent of the differences among treatments is apparent in an examination of the correlations among the characters under the two conditions (Table 8). In addition to the changes in significant correlations among the treatments, over one-third of the correlations changed sign between the control and low-water treatments.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Comparisons among donors
The pollen donors used in this analysis clearly differed in their abilities to sire seeds in mixed pollinations. The rank order of pollen donor performance in siring seeds after mixed pollinations was C, D, B, A. Since the pollen donors appeared to differ in pollen production per flower, and this might affect number of seeds sired, we corrected expected seed paternity to take this difference into account (Table 10). Following this adjustment, C was still the most successful pollen donor, siring well over the expected number of seeds, but D was performing poorly if its high pollen production is considered. Using this correction, the rank order of pollen donor performance is C, B, A, D. In addition to this difference in number of seeds sired, pollen donors differed in the proportion of their seeds that were sired in different regions of the fruit. Donor B sired more of its seeds in the central and basal regions of the fruit while donor A sired more of its seeds in the stylar region (Table 4).
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Paradoxically, these intriguing differences among pollen donors in single-donor crosses do not appear to explain the differential success of pollen donors in mixed pollinations. Only one of the reproductive characters analyzed, proportion of ovules fertilized at 24 h, gives the same rank order of donor performance as in mixed donor pollinations (C, B, A, D). In fact, the 20 different measures of pollen donor performance give nine different rank orders among the pollen donors and each donor ranks first for some variable (Table 3). In addition, although we detected differences in location of seeds sired within ovaries among the pollen donors, we did not detect differences among those donors in the tendency to sire seeds in different fruit regions. All four donors followed the pattern of earliest fertilizations in the central region of the ovary, as has been observed in R. raphanistrum (Hill and Lord, 1986
; Mazer, Snow, and Stanton, 1986
).
We predicted that measures of pollen donor performance during the postpollination process would explain variation in ability to sire seeds after mixed pollination because of the focus in the literature on differences among donors in the speed of pollen tube growth (reviewed in Walsh and Charlesworth, 1992
). Additionally, in an earlier experiment, pollen from two donors of R. sativus grew much farther through the stylar tissue in 4 h than pollen from a third donor (Marshall and Ellstrand, 1986
). In mixed pollinations, this third donor sired few seeds overall and many of these seeds were located in the stylar portion of the ovary. However, we detected no significant variation in pollen tube growth rate in the current experiment (Table 3), despite considerable differences in seed siring ability among the pollen donors in mixed pollinations.
One explanation for the lack of correspondence between pollen tube growth rate in single pollinations and siring ability in mixed pollinations in this experiment is that there are interactions among pollen donors. The lower pollen germination we observed in mixed compared to single pollinations suggests that interference may occur among pollen donors at this very early stage of the reproductive process (Marshall et al., 1996
). Perhaps interference occurs at later stages as well. Thus, measuring characters for single pollen donors might not provide an accurate assessment of pollen donor performance in mixed-pollen loads. For wild radish, multiple paternity of fruits is very common in field populations (Ellstrand and Marshall, 1986
) and this multiple paternity is likely the result of deposition of mixed pollen loads by pollinators (Marshall and Ellstrand, 1985
). Therefore, mixed pollination must be the rule rather than the exception in the field, and interference competition could well be an important influence on nonrandom seed paternity in wild radish.
Another explanation for the lack of correspondence between early pollen tube growth and seed siring ability is the possibility that other pollen donor characters may affect seed siring ability. Analysis of fertilization dynamics suggests that factors other than pollen tube growth rates contributed to variation in siring success in these crosses. At 12 h postpollination, comparison among donors showed significant variation for the proportion of ovules reached and, by inference, differences in later pollen tube growth rate (Table 3; Fig. 1). However, these differences did not translate into similar differences in number of ovules fertilized or number of seeds sired after mixed pollination. Donor B reached the highest proportion of ovules after 12 h, yet it fertilized the smallest proportion of ovules at 12 through 20 h postpollination. After 24 h, donor B ranked only second in the number of ovules fertilized. Comparison of the proportion of ovules reached with the proportion fertilized (Fig. 1) shows a considerable delay between the arrival of the fastest pollen tubes in the ovary and fertilization of ovules. By 16 h postpollination nearly 100% of ovules have been reached by pollen tubes, yet at this time only 25–32% of ovules have been fertilized. Even at 24 h following pollination (8 h after all ovules are reached), just over 50% of ovules have been fertilized, yet we expect virtually all ovules were eventually fertilized (Nakamura and Stanton, 1987
). This protracted delay between the arrival of the fastest pollen tubes in the ovary and fertilization of ovules provides ample opportunity for interactions of microgametophytes with one another, with the maternal ovary tissues, and with the ovules, all of which may affect the likelihood of fertilization. Interestingly, the rank order of proportion of ovules fertilized at 24 h is consistent with the rank order of siring ability.
A third explanation for the lack of correspondence between prezygotic measures of donor performance in single pollinations and siring success in mixed pollinations is interactions of the pollen tubes with maternal tissues. Although we did not detect maternal plant by pollen donor interactions in the initial ANOVAs, the water stress treatment provides evidence for the involvement of maternal tissues (Table 8) (discussed below under Maternal plant effects on mating success). Additional evidence for the role of maternal tissues is the tendency for the variation among pollen donors in embryo length at 10 d to be negatively correlated with final individual seed mass. Earlier work with R. sativus (Nakamura and Stanton, 1989
) showed that, when embryos were grouped by pollen donor, rank order of embryo size in culture was different from that of final seed mass of intact seeds, perhaps due to strong maternal effects on seed mass.
It is also possible that our inability to associate mechanistic characters such as pollen tube growth rate and proportion of ovules reached with ability to sire seeds is due to the small number of pollen donors used, although Hill and Lord (1986)
found differences in fertilization success among only two pollen donors. Given the difficulty in measuring these characters and the number of characters we wished to explore, a larger sample size was not feasible. Adding only one donor increases the number of kinds of crosses from ten (four single-donor and six two-donor) to 15 (five single-donor and ten two-donor), increasing by 50% the number of crosses to perform and tissue samples to be analyzed. Having explored numerous variables in the postpollination process, our future studies will concentrate on a few variables that can be measured for a larger number of pollen donors. At this point we are particularly interested in proportion of ovules reached at 12 h because this is a good measure of relative pollen tube growth rate and because there is still considerable variation in proportion reached. Proportion of ovules fertilized at 24 h remains of interest because this was the only variable for which the rank order of pollen donor performance matched that of proportion of seeds sired. However, without this initial exploration we would not have known which characters were most informative.
It is somewhat difficult to compare these results to the literature, as there are few studies that couple the measurement of mechanistic variables such as pollen tube growth with the outcome of mating. Previous work with R. sativus and R. raphanistrum gives some evidence for variation in similar characters. We found an indication of variation in pollen tube growth among donors of R. sativus (Marshall and Ellstrand, 1986
), while no variation in pollen tube growth was found among donors of R. raphanistrum (Mazer, 1987b
). Number of ovules fertilized by pollen donors varied in one study with R. raphanistrum (Hill and Lord, 1986
), but not in another (Mazer, Snow, and Stanton, 1986
; Mazer, 1987b
). However, none of the studies on R. raphanistrum included measures of seed paternity.
Looking to other genera, both variation in pollen tube growth and variation in ability to sire seeds have been shown in Fagopyrum esculentum (Björkman, Samimy, and Pearson, 1995
). However, the two variables were not measured for the same plants. A more complete example is work on Hibiscus moscheutus (Snow and Spira, 1991, 1996
). For this species, differences in pollen tube growth, measured on one donor at a time correlated with differences in seed paternity after pollination of flowers with pollen from two donors. In contrast, in Oenothera organensis, fertilization ability and seed siring success were not correlated, probably due to differential seed abortion (Havens and Delph, 1996
).
Single vs. multiple donors
We have found in all previous experiments with wild radish that multiply sired fruits are selectively filled. That is, fruits in which the seeds are sired by more than one pollen donor are more massive than singly sired fruits (Marshall and Ellstrand, 1986
; Marshall, 1988, 1991
; Marshall and Fuller, 1994
). We found the same pattern of selective fruit filling in this experiment and examined postpollination processes in single and multiple-donor pollinations for evidence of when this pattern might begin to emerge. Interestingly, the patterns of early reproductive characters such as pollen germination and some measures of ovule fertilization were the opposite of what one might expect. Pollen germination and ovule fertilization seem to proceed more slowly after mixed pollination, perhaps due to negative interference among pollen of different donors. In contrast, the proportion of ovules reached by pollen tubes, which indicates the performance of the very fastest pollen tubes, tended to be higher for two donor pollinations. Perhaps the presence of both interference competition among pollen grains and exploitation competition (races for ovules) provides additional selective events in multiple-donor pollinations.
Although the trend was not significant, seed surface area was slightly larger 3 d after pollination in mixed than single pollinations. This foreshadows the trend in later seed and fruit characters, suggesting that whatever is different about fruits developing from single and mixed pollinations, it begins rather early.
Maternal plant effects on mating success
We attempted to separate the maternal contributions to mating (e.g., mate choice), from pollen competition by comparing reproductive development in control and low-water plants. Our assumption is that if maternal tissues discriminate among donors, then the condition of the maternal tissue will affect donor performance. The results are intriguing. For nine of 11 variables measured in the first 24 h after pollination, events proceeded more quickly in low-water than control plants. This is not because low-water plants were actually the least stressed since the low-water plants wilted daily (D. Marshall, personal observation). In contrast, measures taken at 10 d after pollination and later showed stronger performance in seed production in control than in low-water plants.
This difference between control and stressed plants in the time course of mating appears to have had some effect on the outcome of mixed pollinations. Seed paternity differed in control and low-water plants. While the rank order of pollen donor performance remained the same, the difference in performance of the pollen donors was less on low-water than on control plants as indicated by the lower variance in number of seeds sired (Table 9).
These two sets of observations lead us to the following interpretation. It may be that, as Herrero and Hormaza (1996)
have suggested for other species, maternal plants exert mate choice by creating barriers or gates that pollen donors may or may not breach. If the barriers are less effective in plants under stress, then the time course of fertilization may be faster in stressed plants. With less effective barriers, the capacity for maternal plants to influence donor success may be compromised. Decreased maternal selectivity is consistent with our observation that the differences in pollen donor success were less on low-water than on control plants. Similarly, two earlier experiments with wild radish showed that plant condition affected mating patterns (Marshall and Ellstrand, 1988
; Marshall and Fuller, 1994
) and that mating may have been less selective under stress.
Conclusions
Considering all of the data together, we can make some general observations about the mechanisms of nonrandom mating in R. sativus. First, pollen donors differ in their intrinsic ability to sire seeds. The differences in seed paternity seen here cannot be attributed to some subtle effect of the incompatibility system as all of the pollen donors and all of the maternal families had different S-alleles (Marshall, 1998
). The developmental mechanisms that produce nonrandom mating, however, remain unresolved. Differences in reproductive characters of donors measured in single pollinations did not predict ability to sire seeds in mixed pollinations. Thus, our second point is that simple exploitation competition is likely not the sole mechanism of nonrandom mating in wild radish. Both interference competition among pollen/pollen tubes and interactions with the maternal tissues of the stigma, style, ovary, and ovules, may be critical processes that contribute to variation in fertilization success of donors. If this is the case, then continuing to study these mechanistic characters for single pollen donors is inappropriate for wild radish because mixed pollen loads are common in the natural environment. Thus, the presence of other polle/pollen tubes is part of the "natural environment" of male gametophytes. Perhaps we should expect that pollen would behave differently in the "unnatural environment" of a single-donor pollination compared to mixed pollinations. Third, maternal plant by pollen donor interactions were not major factors in this experiment. They were infrequently found and seemed not to play a role in the early phases of fertilization. Finally, the condition of the maternal tissue played an important role in mating. Slower early events in more competent maternal tissue resulted in improved seed production and greater variation in seed paternity. Thus, it seems likely that both competition among pollen donors and choice by the maternal tissue affect the outcome of mating in wild radish.
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FOOTNOTES |
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4 Author for reprint requests (marshall{at}unm.edu
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Alexander, M. P. 1980 A versatile stain for pollen, fungi, yeast, and bacteria. Stain Technology 55: 13–18[Web of Science][Medline]
Barnes, D. K., and R. W., Cleveland. 1963a Pollen tube growth of diploid alfalfa in vitro. Crop Science 3: 291–295
———, and ———. 1963b Genetic evidence for nonrandom fertilization in alfalfa as influenced by differential pollen tube growth. Crop Science 3: 295–297
Bemis, W. P. 1959 Selective fertilization in lima beans. Genetics 44: 555–562
Bertin, R. I. 1982 Paternity and fruit production in trumpet creeper (Campsis radicans). American Naturalist 119: 694–709[CrossRef][Web of Science]
———. 1985 Nonrandom fruit production in Campsis radicans: between-year consistency and effects of prior pollination. American Naturalist 126: 750–759[CrossRef][Web of Science]
———. 1990 Paternal success following mixed pollinations of Campsis radicans. American Midland Naturalist 124: 153–163[CrossRef][Web of Science]
Bierzychudek, P. 1981 Pollinator limitation of plant reproductive effort. American Naturalist 117: 838–840[CrossRef][Web of Science]
Björkman, T., C. Samimy, and K. J. Pearson. 1995 Variation in pollen performance among plants of Fagopyrum esculentum. Euphytica 82: 235–240[CrossRef][Web of Science]
Charlesworth, D., D. W. Schemske, and V. L.. Sork, 1987 The evolution of plant reproductive characters: sexual vs. natural selection. In S. C. Stearns [ed.], Evolution of sex, 317–335. Birkhauser, Basel, Switzerland
Charnov, E. L. 1979 Simultaneous hermaphroditism and sexual selection. Proceedings of the National Academy of Sciences, USA 76: 2480–2484
Cruzan, M. B. 1990 Variation in pollen size, fertilization ability, and postfertilization siring ability in Erythronium grandiflorum. Evolution 44: 843–856[CrossRef][Web of Science]
Currah, L. 1981 Pollen competition in onion (Allium cepa L.) Euphytica 30: 687–696[CrossRef][Web of Science]
de Nettencourt, D. 1997 Incompatibility in angiosperms. Sexual Plant Reproduction 10: 185–199[CrossRef][Web of Science]
Ellstrand, N. C. 1984 Multiple paternity within the fruits of the wild radish, Raphanus sativus. American Naturalist 123: 819–828[CrossRef][Web of Science]
———, and D. L. Marshall. 1985a Interpopulation gene flow by pollen in wild radish, Raphanus sativus. American Naturalist 126: 606–616[CrossRef][Web of Science]
———, and ———. 1985b The impact of domestication on distribution of allozyme variation within and among cultivars of radish Raphanus sativus L. Theoretical and Applied Genetics 69: 393–398[CrossRef][Web of Science]
———, and ———. 1986 Patterns of multiple paternity in populations of Raphanus sativus. Evolution 40: 837–842[CrossRef][Web of Science]
Grant, V. 1995 Sexual selection in plants: pros and cons. Proceedings of the National Academy of Sciences, USA 92: 1247–1250
Havens, K., and L. F. Delph. 1996 Differential seed maturation uncouples fertilization and siring success in Oenothera organensis. Heredity 76: 623–632[Web of Science]
Herrero, M., and J. I. Hormaza. 1996 Pistil strategies controlling pollen tube growth. Sexual Plant Reproduction 9: 343–347[Web of Science]
Hill, J. P., and E. M. Lord. 1986 Dynamics of pollen tube growth in the wild radish, Raphanus raphanistrum (Brassicaceae). I. Order of fertilization. Evolution 40: 1328–1333[CrossRef][Web of Science]
Hinata, K., and T. Nishio. 1980 Self-incompatibility in crucifers. In S. Tsunoda, K. Hinata, and C. Gomez-Campo [eds.], Brassica crops and wild allies: biology and breeding, 223–234. Japanese Scientific Societies, Tokyo, Japan
Holsinger, K. E. 1992 Ecological models of plant mating systems and the evolutionary stability of mixed mating systems. In R. Wyatt [ed.], Ecology and evolution of plant reproduction, 169–191. Chapman and Hall, New York, New York, USA
Jandel Scientific. 1989 User's manual for IBM AT and Compatibles version 1.30: Jandel Video Analysis Software. Jandel Scientific, Corte Madera, California, USA
Karron, J. D., D. L. Marshall, and D. M. Oliveras. 1990 Numbers of sporophytic self-incompatibility alleles in populations of wild radish. Theoretical and Applied Genetics 79: 457–460[Web of Science]
Kay, Q. O. N. 1976 Preferential pollination of yellow-flowered morphs of Raphanus raphanistrum by Pieris and Eristalis spp. Nature 261: 230–232[CrossRef]
Kress, W. J. 1981 Sibling competition and evolution of pollen unit, ovule number, and pollen vector in angiosperms. Systematic Botany 6: 101–112
Law, R., and C. Cannings. 1984 Genetic analysis of conflicts arising during the development of seeds in Angiospermophyta. Proceedings, Royal Society of London, B, Biological Science 221: 53–70
Levin, D. A. 1975 Gametophytic selection in Phlox. In D. L. Mulcahy [ed.], Gamete competition in plants and animals, 207–217. North Holland, Amsterdam, The Netherlands
Lyons, E. E., N. M. Waser, M. V. Price, J. Antonovics, and A. F. Motten. 1989 Sources of variation in plant reproductive success and implications for concepts of sexual selection. American Naturalist 134: 409–433[CrossRef][Web of Science]
Marshall, D. L. 1988 Post pollination effects on seed paternity: mechanisms other than microgametophyte competition operate in wild radish. Evolution 42: 1256–1266[CrossRef][Web of Science]
———. 1991 Nonrandom mating in wild radish: variation in pollen donor success and effects of multiple paternity among one- to six-donor pollinations. American Journal of Botany 78: 1404–1418[CrossRef][Web of Science]
———. 1998 Pollen donor performance can be consistent across maternal plants in wild radish (Raphanus sativus, Brassicaceae): a necessary condition for the action of sexual selection. American Journal of Botany 85: 1389–1397
———, and N. C. Ellstrand. 1985 Proximal causes of multiple paternity in wild radish, Raphanus sativus. American Naturalist 126: 596–605[CrossRef][Web of Science]
———, and ———. 1986 Sexual selection in Raphanus sativus: experimental data on nonrandom fertilization, maternal choice, and consequences of multiple paternity. American Naturalist 127: 446–461[CrossRef][Web of Science]
———, and ———. 1988 Effective mate choice in wild radish: evidence for selective seed abortion and its mechanism. American Naturalist 131: 736–759
———, and M. W. Folsom. 1991 Mate choice in plants: an anatomical to population perspective. Annual Review of Ecology and Systematics 22: 37–63
———, ———, C. Hatfield, and T. Bennett. 1996 Does interference competition occur among pollen grains of wild radish? Evolution 50: 1842–1848[CrossRef][Web of Science]
———, and O. S. Fuller. 1994 Does nonrandom mating among wild radish plants occur in the field as well as in the greenhouse. American Journal of Botany 81: 439–445[CrossRef][Web of Science]
Mazer, S. J. 1987a Maternal investment and male reproductive success in angiosperms: parent-offspring conflict or sexual selection. Biological Journal of the Linnean Society 30: 115–133[CrossRef]
———. 1987b Parental effects on seed development and seed yield in Raphanus raphanistrum: implications for natural and sexual selection. Evolution 41: 355–371[CrossRef][Web of Science]
———, A. A. Snow, and M. L. Stanton. 1986 Fertilization dynamics and parental effects upon fruit development in Raphanus raphanistrum: consequences for seed size variation. American Journal of Botany 73: 500–511[CrossRef][Web of Science]
Meacham, C. A., and T. Duncan. 1990 MorphoSys, version 1.26. University of California, Berkeley, California, USA
Mitchell, R. J., and D. L. Marshall. 1998 Nonrandom mating and sexual selection in a desert mustard: an experimental approach. American Journal of Botany 85: 48–55[Abstract]
Mulcahy, D. L. 1971 A correlation between gametophytic and sporophytic characteristics in Zea mays L. Science 171: 1155–1156
———. 1974 Correlation between speed of pollen tube growth and seedling height in Zea mays L. Nature 249: 491–493[CrossRef][Web of Science]
———. 1979 The rise of the angiosperms: a genecological factor. Science 206: 20–23
———, P. S. Curtis, and A. A. Snow. 1983 Pollen competition in a natural population. In C. E. Jones and R. J. Little [eds.], Handbook of experimental pollination biology, 330–337. Van Nostrand Reinhold, New York, New York, USA
Nakamura, R. R., and M. L. Stanton. 1987 Cryptic seed abortion and the estimation of ovule fertilization. Canadian Journal of Botany 65: 2463–2465
———, and ———. 1989 Embryo growth and seed size in Raphanus sativus: maternal and paternal effect in vivo and in vitro. Evolution 43: 1435–1443[CrossRef][Web of Science]
Niesenbaum, R. A. 1999 The effects of pollen load size and donor diversity on pollen performance, selective abortion, and progeny vigor in Mirabilis jalapa (Nyctaginaceae). American Journal of Botany 86: 261–268
Ottaviano, E. M., M. Sari-gorla, and I. Arenair. 1983 Male gametophytic competition in maize: selection and implications with regard to breeding systems. In D. L. Mulcahy and E. M. Ottaviano [eds.], Pollen: biology and implications for plant breeding, 367–374. Elsevier, New York, New York, USA
———, ———, and D. L. Mulcahy. 1975 Genetic and intergametophytic influences on pollen tube growth. In D. L. Mulcahy [ed.], Gamete competition in plants and animals, 125–134. North Holland, Amsterdam, The Netherlands
———, ———, and ———. 1980 Pollen tube growth rates in Zea mays: implications of the genetic improvement of crops. Science 210: 437–438
Pundir, N. S., R. F. Abbas, and A. A. A. Al-Attar. 1983 Pollen germination and pollen tube growth in Raphanus sativus L. following self- and cross-pollinations. Phyton 43: 127–130[Web of Science]
Pfahler, P. L. 1967 Fertilization ability of maize pollen grains. II. Pollen genotype, female sporophyte, and pollen storage interactions. Genetics 57: 513–521
———. 1974a Fertilization ability of maize (Zea mays L.) pollen grains. IV. Influence of storage and the alleles at the shrunken, sugary, and waxy loci. In H. F. Linskens [ed.], Fertilization in higher plants, 15–25. North-Holland, Amsterdam, The Netherlands
———. 1974b Pollen genotype studies in maize (Zea mays L.). In H. F. Linskens [ed.], Fertilization in higher plants, 3–14. North-Holland, Amsterdam, The Netherlands
———. 1975 Factors affecting male transmission in maize (Zea mays L.). In D. L. Mulcahy [ed.], Gamete competition in plants and animals, 115–124. North Holland, Amsterdam, The Netherlands
———, and H. F. Linskens. 1972 In vitro germination and pollen tube growth of maize (Zea mays L.) pollen. VI. Combined effects of storage and alleles at the waxy (wx), sugary (su1), and shrunken (sh2) loci. Theoretical and Applied Genetics 42: 136–140[CrossRef][Web of Science]
Queller, D. C. 1983 Sexual selection in a hermaphroditic plant. Nature 305: 706–707[CrossRef]
———. 1987 Sexual selection in flowering plants. In J. W. Bradbury and M. B. Andersson [eds.], Sexual selection: testing the alternatives, 165–181. Wiley, New York, New York, USA
Rice, W. R. 1989 Analyzing tables of statistical tests. Evolution 43: 223–225[CrossRef][Web of Science]
Richards, A. J. 1986 Plant breeding systems. George Allen and Unwin, London, UK
Rigney, L. P. 1995 Postfertilization causes of differential success of pollen donors in Erythronium grandiflorum (Liliaceae): nonrandom ovule abortion. American Journal of Botany 82: 578–584[CrossRef][Web of Science]
———, J. D. Thomson, M. B. Cruzan, and J. Brunet. 1993 Differential success of pollen donors in a self-compatible lily. Evolution 47: 915–924[CrossRef][Web of Science]
Samson, D. R. 1957 The genetics of self-incompatibility in the radish. Journal of Heredity 48: 26–29
Sari-Gorla, M., E. Ottaviano, and D. Faini. 1975 Genetic variability of gametophytic growth rate in maize. Theoretical and Applied Genetics 46: 289–294[Web of Science]
Sarr, A., B. Fraleigh, and M. Sandmeier. 1983 Aspects of reproductive biology in pearl millet Pennisetum typhyoides (Burm.) Stapf & Habb. In D. L. Mulcahy and E. Ottaviano [eds.], Pollen: biology and implications for plant breeding, 381–388. Elsevier, New York, New York, USA
Sayers, E. R., and R. P. Murphy. 1966 Seed set in alfalfa as related to pollen tube growth, fertilization frequency and post-fertilization ovule abortion. Crop Science 6: 365–368
Schemske, D. W., and C. Fenster. 1983 Pollen-grain interaction in a neotropical Costus: effects of clump size and competitors. In D. L. Mulcahy and E. Ottaviano [eds.], Pollen: biology and implications for plant breeding, 405–410. Elsevier, New York, New York, USA
Snow, A. A. 1986 Pollination dynamics in Epilobium canum (Onagraceae): consequences for gametophytic selection. American Journal of Botany 73: 139–151[CrossRef][Web of Science]
———. 1994 Postpollination selection and male fitness in plants. American Naturalist 144: S69–S83[CrossRef][Web of Science]
———, and T. P. Spira. 1991 Differential pollen-tube growth rates and nonrandom fertilization in Hibiscus moscheutos (Malvaceae). American Journal of Botany 78: 1419–1426[CrossRef][Web of Science]
———, and ———. 1996 Pollen-tube competition and male fitness in Hibiscus moscheutos. Evolution 50: 1866–1870[CrossRef][Web of Science]
Stanton, M. L. 1987a The reproductive biology of petal color variants in wild populations of Raphanus sativus L. I. Pollinator response to color morphs. American Journal of Botany 74: 178–187[CrossRef][Web of Science]
———. 1987b The reproductive biology of petal color variants in wild populations of Raphanus sativus L. II. Factors limiting seed production. American Journal of Botany 74: 188–196[CrossRef][Web of Science]
———, and R. E. Preston. 1988 Ecological correlates of petal size variation in wild radish, Raphanus sativus (Brassicaceae). American Journal of Botany 75: 528–539[CrossRef][Web of Science]
Stephenson, A. G., and R. I. Bertin. 1983 Male competition, female choice, and sexual selection in plants. In L. Real [ed.], Pollination biology, 109–149. Academic Press, Orlando, Florida, USA
Waller, D. M. 1993 The statics and dynamics of mating system evolution. In N. Thornhill [ed.], The natural history of inbreeding and outbreeding, 97–117. The University of Chicago Press, Chicago, Illinois, USA
Walsh, N. E., and D. Charlesworth. 1992 Evolutionary interpretations of differences in pollen tube growth rates. Quarterly Review of Biology 67: 19–37[CrossRef]
Waser, N. M., and M. V. Price. 1991 Outcrossing distance effects in Delphinium nelsonii: pollen loads, pollen tubes, and seed set. Ecology 72: 171–179[CrossRef][Web of Science]
———, ———, A. M. Montalvo, and R. N. Gray. 1987 Female mate choice in a perennial herbaceous wildflower, Delphinium nelsonii. Evolutionary Trends in Plants 1: 29–33
Westoby, M., and B. Rice. 1982 Evolution and the fitness of plant tissues. Evolution 36: 713–724[CrossRef][Web of Science]
Willson, M. F. 1979 Sexual selection in plants. American Naturalist 113: 777–790[CrossRef][Web of Science]
———. 1982 Sexual selection and dicliny in angiosperms. American Naturalist 119: 579–583[CrossRef][Web of Science]
———. 1990 Sexual selection in plants and animals. Trends in Ecology and Evolution 5: 210–214
———. 1994 Sexual selection in plants: perspective and overview. American Naturalist 144: S13–S39[CrossRef][Web of Science]
———, and N. Burley. 1983 Mate choice in plants. Princeton University Press, Princeton, New Jersey, USA
Winsor, J. A., and A. G. Stephenson. 1995 Demographics of pollen-tube growth in Cucurbita pepo. Canadian Journal of Botany 73: 583–589
Wolff, G. 1975 Investigations on pollen tube growth of Pisum sativum and some of its mutants. In D. L. Mulcahy [ed.], Gamete competition in plants and animals, 161–175. North Holland, Amsterdam, The Netherlands
Young, H. J., and M. L. Stanton. 1990 Influence of environmental quality on pollen competitive ability in wild radish. Science 248: 1631–1633
———, and T. P. Young. 1992 Alternative outcomes of natural and experimental high pollen loads. Ecology 73: 639–647[CrossRef][Web of Science]
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L. G. Ruane and K. Donohue Environmental effects on pollen-pistil compatibility between Phlox cuspidata and P. drummondii (Polemoniaceae): implications for hybridization dynamics Am. J. Botany, February 1, 2007; 94(2): 219 - 227. [Abstract] [Full Text] [PDF]
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D. L. Marshall and D. M. Oliveras Does differential seed siring success change over time or with pollination history in wild radish, Raphanus sativus (Brassicaceae)? Am. J. Botany, December 1, 2001; 88(12): 2232 - 2242. [Abstract] [Full Text] [PDF]
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