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Biology Department, Muhlenberg College, Allentown, Pennsylvania 18104-5586
Received for publication February 23, 1998. Accepted for publication July 28, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: competition • fitness • fruit • fruit abortion • maternal effect • Mirabilis jalapa; • Nyctaginaceae • pollen
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Sutherland and Delph, 1984
). The abortion of initiated fruits should theoretically be selective so that limited resources are allocated to fruits containing embryos of higher genetic quality (Janzen, 1977
; Charnov, 1979
; Stephenson, 1981
; Lee, 1984
; Sutherland, 1987
). The degree to which pollen tubes compete for access to ovules is a possible basis of selective fruit abortion that could result in investment in fruits with higher quality embryos, and higher average offspring fitness (Janzen, 1977
; Lee, 1984
). Under high levels of pollen competition, fertilization is more likely to occur by faster growing pollen tubes (Mulcahy and Mulcahy, 1975
; Mulcahy, 1979
; McKenna, 1986
; Schlicting et al., 1987
; Thomson, 1989
). A growing body of evidence suggests that faster growing pollen tubes sire offspring of higher genetic quality and greater vigor (Tanksley, Zamir, and Rick, 1981
; McKenna and Mulcahy, 1983
; Stephenson, Winsor, and Davis, 1986
; Davis, Stephenson, and Winsor, 1987
; Schlicting et al., 1987
; Winsor, Davis, and Stephenson, 1987
; Willing, Bashe, and Mascarhenas, 1988
; Ottaviano and Mulcahy, 1989
; Ottaviano et al., 1991
; Richardson and Stephenson, 1992
; Quesada, Winsor, and Stephenson, 1993
) and that plants selectively mature fruits from flowers in which there has been more pollen competition (Niesenbaum and Casper, 1994
; Niesenbaum, 1997
).
Studies of the effects of the level of pollen competition on selective fruit abortion and progeny vigor have been limited by the confounding effect of partial seed set within a fruit on fruit set. Because lower pollen load treatments can result in lower per fruit seed set and the number of seeds within a developing fruit is often a determinant of fruit abortion, separating pollen competition effects from seed set effects can be difficult. This problem has been dealt with, at least in part, by manipulating flowers so that only the fastest growing pollen tubes can enter the style or by being able to identify ovules fertilized by the fastest growing pollen tubes, which occur in a particular region of the ovary (Stephenson, Devlin, and Horton, 1988
; Quesada et al., 1991
; Quesada, Winsor, and Stephenson, 1993
). However, by working on plants with single-ovulate ovaries this confounding problem can be completely eliminated (Lee, 1984
; Niesenbaum and Casper, 1994
; Niesenbaum, 1997
).
It has also been difficult to differentiate between genetic and nongenetic causes of selective fruit abortion based on pollen tube number. High pollen loads may simply stimulate the maternal plant to allocate more resources to the ovary and developing embryo in flowers receiving that treatment (Charlesworth, 1988
; Walsh and Charlesworth, 1992
). Although this could be an adaptive mechanism by which plants mature seeds sired by faster growing pollen tubes, this should be differentiated from selective maturation where the embryos sired by faster growing pollen tubes are more vigorous, serve as stronger resource sinks, and outcompete developing embryos fertilized under conditions of reduced pollen competition because of genetic differences.
In this paper, I report results of experimental manipulations of pollen load to test whether abortion and progeny vigor are related to the level of pollen competition in Mirabilis jalapa. The complication of partial seed set within fruits and its relationship with pollen load and fruit maturation are eliminated because this species has a single-ovulate ovary making seed set the equivalent of fruit set. Both pollen load size and the diversity of pollen donors were manipulated to help differentiate between genetic and nongenetic mechanisms of selective abortion based on pollen tube number. I examined the effects of pollen load size and donor diversity on pollen germination and growth, the maturation or abortion of initiated fruit, seed size, and seedling performance.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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This species provides useful traits for studying pollen competition in relation to fruit maturation. The long style (3–4 cm) provides ample opportunity for pollen competition and the expression of differential pollen tube growth rates, and pollen tube growth rates are readily quantified using standard florescent microscopic techniques. The pollen grains are large enough to be viewed with a 10x hand lens so that exact numbers of pollen grains, including single grains, can be applied to the stigma. The plant is self-compatible (Cruden, 1973
, Niesenbaum and Schueller, 1997
), and the absence of an incompatibility system eliminates the possibility of self-incompatibility reactions obscuring levels of competition among compatible pollen. The single-ovulate ovary eliminates potential complication of partial seed set within some fruits, i.e., fruit set is equal to seed set in this species.
Individual seeds obtained from four different commercial sources were planted in separate 15-cm pots with Metro-Mix 510 growing medium. Pots were placed in the Muhlenberg College greenhouse, which effectively excludes insects that might serve as pollinators. Fifteen plants were used for the pollen performance experiments and 20 additional plants were used for the seed set and seed size experiments. All plants were systematically rearranged to average out local greenhouse environmental effects and all were watered twice daily. Experiments were performed from May to July on the majority of flowers on the experimental plants.
Pollination treatments
In both experiments, individual flowers received one of five pollination treatments varying in load size and diversity. The treatments were: (1) large load—multiple donors; (2) large load—single donor; (3) small load—multiple donors; (4) small load—single donor; and (5) a singled outcross pollen grain (Fig. 1). Large loads ranged from 20 to 30 pollen grains per stigma, and small loads always consisted of five pollen grains. Multiple donor loads always consisted of pollen from five different donors. Treatments were randomly assigned to flowers, and each treatment was replicated at least three times per plant. Experimental pollen loads were comparable to the range of pollen loads reported in natural populations of this species (Cruden, 1973
; del Rio and Burquez, 1986
).
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; Kearns and Inouye, 1993
). To determine whether our methods of pollination damaged the pollen, viability tests were conducted with pollen applied to a microscope slide with the dissecting probe in the same manner that grains were applied to stigmas. These tests revealed that on most nights >90% of the examined pollen grains fluoresced brightly and were deemed viable. On five separate nights, <90% of the pollen grains were viable, and data from these nights were not used in the subsequent analyses. Application of pollen grains to a microscope slide using the pollination technique employed in the experiments did not affect viability.
Pollen performance
Preliminary experiments indicated that the first pollen tubes reach the base of the style within 60 min. To examine pollen germination, pollen tube penetration into the style, and pollen tube growth rates during this initial period, styles were removed with fine forceps from the ovary 45 min after pollination and fixed in 70% ethanol for 24 h. After rinsing in distilled water, the styles were softened and cleared in 8 mol/L NaOH for 48–72 h. The softened styles were placed in distilled water for at least 1 h before staining with a 0.1% solution of aniline blue in 0.1 mol/L K3PO4 for 4 h. Each style was then examined at 100x under UV light. The number of germinated pollen grains with pollen tubes penetrating the stigmatic surface and the number of pollen tubes that penetrated the stigma and entered the style were counted. The length of every pollen tube, from the stigmatic surface to the tip of the pollen tube, was measured using an ocular reticle, and pollen tube growth rates were calculated based on the 45-min growth period. Some ungerminated pollen grains or partially germinated pollen grains that did not have tubes penetrating the stigmatic surface may have been washed off in this process, but careful treatment of the styles and examination of staining dishes indicated that loss of pollen was minimal. A total of 250 treated flowers and 407 individual pollen tubes were observed in this part of the study.
Seed set, seed size, and seedling performace
To assess treatment effects on seed set and seed size, pollination treatments were performed as above, but styles were not removed. Flowers were permanantly marked and monitored until fruit maturation or abortion occurred. The percentage of flowers that matured fruits with each treatment was calculated. Flowers that dropped off prior to the enlargement of the ovary were assumed not to have been fertilized or to have initiated embryos, and were not included in the calculations of fruit set.
Upon maturation, seeds were harvested, dried at room temperature for 2 wk, and then weighed to the nearest 0.1 mg. Seeds were then planted in the same way and under similar conditions as described for the original experiment. Counting the day of planting as day 0, the following were obtained: the number of days until emergence; height and number of leaves at 90 d; and height and number of leaves at 120 d.
Analyses
Prior to analysis all proportional data were arcsine transformed to meet the assumptions of normality and homogeneity of variances (Sokal and Rohlf, 1981
). The proportion of pollen grains that germinated per style and the proportion that had pollen tubes penetrate the stigma and enter the style were compared among treatments using a one-way analysis of variance (ANOVA). A posteriori comparisons of means among treatments were made for each significant ANOVA using the Tukey-Kramer HSD test (SAS, 1987
). The lengths of individual pollen tubes were square root transformed and compared among treatments using a one-way ANOVA. Because most pollen tubes of germinated pollen grains do not penetrate the stigma and enter the style, a better estimate of competitive environment within the style is the actual number of pollen tubes rather than the initial number of pollen grains. A mixed-model analysis of variance of pollen tube growth rate (in millimetres after 45 min) was conducted with the number of pollen tubes per style, maternal plant, and their interaction as the classification variables. Pollen tube number was tested over the interaction, and individual plant was tested over the error term.
A relationship between pollen tube number and tube growth rates might be due to the increased probability that a highly vigorous pollen grain is deposited on a stigma with large pollen loads. This would be the case especially when larger pollen loads result in higher numbers of pollen tubes per style. I was able to examine this because all treatments resulted in some styles that had only a single pollen tube. If selection at the stigma was occurring and driving the relationship between tube number and growth rate, a single pollen tube in a style would grow faster with treatments with larger stigmatic pollen loads (greater selection at the stigma) than a single pollen tube in a style with small stigmatic pollen loads. This was examined with a one-way ANOVA of the effects of treatment on transformed tube lengths for all styles across all pollination treatments that had only one tube. Also, by examining the relationship between tube number and growth rate on a subset of data that had only high levels of selection at the stigma, it could be determined whether sorting at the stigma reduced or eliminated any effect observed for the entire data set. This was achieved with a one-way ANOVA of the effects of tube number on transformed growth rate for treatment 1 only (strong selection at the stigma).
Fruit set was analyzed as the presence or absence of fruit at each flower using the maximum liklihood analyis of variance in the CATMOD procedure of SAS (SAS, 1987
). The model included maternal plant, treatment, and their interaction. All measures of seedling performance were analyzed with a two-way ANOVA including maternal plant and treatment as the main effects. No test for the interaction of these effects could be conducted because many maternal plants had no seedlings at one or more treatments.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The pollen tubes of most pollen grains applied to the stigmas with each treatment did not penetrate the stigma and enter the style. Thus, to analyze the effects of pollen competitive environment within the style on pollen tube growth rate, pollen tube number per style rather than treatment was used in the subsequent analysis. The number of pollen tubes per style ranged from one to six. Pollen tube growth rates within a particular style were affected by the number of pollen tubes in that style. Pollen tube growth rate significantly increased with the number of tubes per style, but appeared to plateau with higher pollen tube numbers (F1,281 = 77.12, P < 0.001; Fig. 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Pollen performance
The pollen competitive environment influenced pollen performance at three stages of pollen growth. Pollen germination, the percentage of pollen grains that penetrated the stigma and grew into the style, and pollen tube growth rates all increased with load size. Independent of treatment, pollen tube growth rates were higher when there were more pollen tubes in a style.
Increased pollen germination and penetration with larger pollen loads suggest that pollen germination is positively density dependent. Density-dependent pollen germination has been observed in vitro (Brewbaker and Majumder, 1961
) and on the stigmas of a variety of species (Hornby and Charles, 1966
; Jennings and Topham, 1971
; Levin, 1975
; Cruzan, 1986
; Winsor and Stephenson, 1995
). The increase in germination rates with pollen density could be due to positive chemical interactions among pollen grains such as a reduction in stigmatic pH caused by a critical number of pollen grains. The reduction in pH results in increased germination by causing the inhibition or destruction of pH-sensitive pollen germination inhibitors (Ganeshaiah and Shaanker, 1988
).
Pollen tube growth rates increased with the number of pollen tubes per style. This could have been due to facilitation of pollen tube growth from positive interactions among growing pollen tubes (Sari-Gorla and Rovida, 1980
) or from interaction of pollen tubes with the style (Mulcahy and Mulcahy, 1985
). An alternative explanation is that with larger pollen load sizes, pollen tubes might have grown faster because of greater selection for more vigorous pollen on the stigma or a greater probability of there being more vigorous pollen on a stigma with higher pollen loads (Mulcahy, Mulcahy, and Searcy, 1992
). Because pollen tube growth rate varied significantly with pollen tube number within treatment 1 (Table 2) and a single pollen tube grew at the same rate regardless of the initial pollination treatment, I conclude that the increased growth rates were due to interactions within the style rather than to selection at the stigma.
There is evidence that pollen tube growth rate is at least in part genetically determined. A correlation between pollen tube growth rate and offspring quality (Willing, Bashe, and Mascarenhas, 1988
; Ottaviano and Mulcahy, 1989
), that growth rates may be under strong selection (Ottaviano, Sari-Gorla, and Villa, 1988
; Walsh and Charlesworth, 1992
; Quesada et al., 1996), and that pollen tube growth requires the expression of 20000–23000 individual genes (Stephenson et al., 1992
) imply pollen performance is an inherited trait. However, recent studies also show a large environmental influence on pollen tube growth rates. Environmental factors such as temperature, nutrients, and pH may also strongly influence pollen tube growth rate (Stephenson et al., 1992
; ; Delph, Johannsson, and Stephenson, 1997). This study and others (Cruzan, 1986
; Winsor and Stephenson, 1995
; Niesenbaum and Schueller, 1997
) show that another environmental effect, pollen competitive environment, does affect pollen performance.
Selective abortion based on pollen tube numbers per style
When resources are insufficient to mature all fruits, plants should abort some fruits and selectively mature those of high quality (Janzen, 1977
; Charnov, 1979
; Stephenson, 1981
; Bawa and Webb, 1984
; Lee, 1984
). It follows that those fruits produced after more intense pollen competition should be more likely to mature because under these conditions, fertilization by genetically superior pollen is more likely (Janzen, 1977
; Lee, 1984
; Niesenbaum and Casper, 1994
; Niesenbaum, 1997
). The relationship between pollen tube numbers and fruit maturation found here is consistent with this prediction.
Selective fruit abortion in relation to pollen tube number could be based on the number of ovules fertilized or the genetic composition of seeds (Sutherland, 1987
), and in past studies it has been difficult to differentiate between these two factors (Stephenson, 1981
; Bertin, 1990
). As in this study, this problem can be eliminated by working on plants with single-ovulate ovaries (Lee, 1984
; Niesenbaum and Casper, 1994
; Niesenbaum, 1997
) or can be dealt with by identifying or ensuring that certain ovules are fertilized by tubes of particular growth rates (Stephenson, Devlin, and Horton, 1988
; Quesada et al., 1991
; Quesada, Winsor, and Stephenson, 1993
).
Selective abortion based on pollen tube numbers could also be attributed to non-genetic causes such as high pollen loads stimulating the maternal plant to allocate more resources to those fruits (Charlesworth, 1988
; Walsh and Charlesworth, 1992
). It is also possible that faster pollen tube growth rates result in an increase in the speed of fertilization. This may allow the embryo to achieve rapid, early development such that abscission isn't begun by the maternal plant. This too could reduce the probability of fruit abortion when there is more pollen competition. In previous studies examining the relationship between pollen tube number and abortion it has been difficult to differentiate between genetic and nongenetic causes (Niesenbaum and Casper, 1994
; Niesenbaum, 1997
). The direct effect of both pollen load size and donor diversity on abortion seen here suggests that there is, at least in part, a genetic component to this phenomenon. Future studies where paternity by specific pollen donors can be assessed and where abortion can be attributed to pollen genotypes will allow us to further differentiate between any genetic and nongenetic basis to the kind of selective abortion observed here.
Fitness measures
Determining the fitness consequences of selective abortion based on pollen tube numbers is essential in testing the hypothesis that plants can increase fitness by selectively maturing fruits from flowers that have had greater levels of competition (Lee, 1984
; Schlicting et al., 1990
; Snow, 1990
). One obvious effect in this study was that none of the plants from seeds resulting from pollinations with a single pollen grain survived. This suggests a linkage between the level of pollen competition and fitness, but there were no other discernible fitness differences in offspring from different pollination treatments. Other effects on progeny vigor may have been more readily detected under a wider range of competitive conditions.
Maternal effects seem to be driving the variation in seed size and germination and seedling growth observed here. It may be that individual maternal plants respond to resource levels by adjusting seed number through abortion and invest equally among the developing seeds and fruits that survive the resource limitation. It is also possible that fitness differences due to the pollination treatments applied here cannot be detected until later in the plant lifecycle or may not be reflected until offspring produce flowers and seeds (Dudash, 1990
). Also, observed differences in gametophyte performance might be masked in the diploid state, thereby obscuring expected fitness differences. This was observed by Havens and Delph (1996)
who found that a transformation that affected gametophyte vigor did not affect sporophyte vigor.
If all seeds of reduced fitness because of lower levels of competition are aborted, then fitness differences among matured seeds from flowers with different levels of competition may be difficult to detect. Future work to test the fitness consequences of selective abortion based on pollen tube numbers must in some sense "rescue" aborting seeds and fruits that come from flowers with lower levels pollen competition. This might be achieved through the manipulation of developing embryos (Casper, 1988
) or resource levels (Marshall, 1988
; Zimmerman and Pyke, 1988
; Marshall and Folsum, 1992
; Marshall and Fuller 1994
; Niesenbaum, 1997
).
This study shows that pollen competitive environment affects both pollen performance and patterns of abortion. The effect on pollen performance is likely to be an environmental effect, but because donor diversity significantly influenced abortion, there could be a significant genetic component to this process. This is consistent with the hypothesis that increased pollen competition is resulting in an increased likelihood that superior pollen genotypes achieve fertilization and that selective abortion is based on this process. However, stronger evidence for a genetic basis of pollen tube growth rates and a relationship between the level of competition and offspring success will permit a more conclusive test of this hypothesis.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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C. M. Herrera Censusing natural microgametophyte populations: variable spatial mosaics and extreme fine-graininess in winter-flowering Helleborus foetidus (Ranunculaceae) Am. J. Botany, October 1, 2002; 89(10): 1570 - 1578. [Abstract] [Full Text] [PDF]
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M. Paschke, C. Abs, and B. Schmid Effects of population size and pollen diversity on reproductive success and offspring size in the narrow endemic Cochlearia bavarica (Brassicaceae) Am. J. Botany, August 1, 2002; 89(8): 1250 - 1259. [Abstract] [Full Text] [PDF]
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M. Quesada, E. J. Fuchs, and J. A. Lobo Pollen load size, reproductive success, and progeny kinship of naturally pollinated flowers of the tropical dry forest tree Pachira quinata (Bombacaceae) Am. J. Botany, November 1, 2001; 88(11): 2113 - 2118. [Abstract] [Full Text] [PDF]
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C. Melser and P. G. L. Klinkhamer Selective seed abortion increases offspring survival in Cynoglossum officinale (Boraginaceae) Am. J. Botany, June 1, 2001; 88(6): 1033 - 1040. [Abstract] [Full Text] [PDF]
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D. L. Marshall and P. K. Diggle Mechanisms of differential pollen donor performance in wild radish, Raphanus sativus (Brassicaceae) Am. J. Botany, February 1, 2001; 88(2): 242 - 257. [Abstract] [Full Text]
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D. L. Marshall, J. J. Avritt, M. Shaner, and R. L. Saunders Effects of pollen load size and composition on pollen donor performance in wild radish, Raphanus sativus (Brassicaceae) Am. J. Botany, November 1, 2000; 87(11): 1619 - 1627. [Abstract] [Full Text]
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