| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
0 Department of Zoology, Box 90325, Duke University, Durham, North Carolina 27708-0325 USA
Received for publication September 21, 1998. Accepted for publication March 30, 1999.
ABSTRACT
In this paper, I investigate whether the presence or absence of anthocyanin is neutral with respect to reproduction in Datura stramonium. The observations concern the portion of the life cycle spanning pollination to germination. Pollinators do not appear to distinguish between floral morphs, as revealed by nonbiased distribution of fluorescent powder used as a pollen analogue. Pollen-tube growth is also equal for the two morphs. Seed germination is affected by the presence of anthocyanin, but apparently only by the genotype of the mother at the anthocyanin locus, and not by the genotype of the embryo itself. In addition, there was an interaction between maternal morph and the maternal source population, with seeds from high-elevation anthocyanin-producing mothers germinating most rapidly and seeds from low-elevation anthocyanin-producing mothers germinating most slowly in a common garden at low elevation. However, because germination of anthocyanin-producing and anthocyaninless progeny proceed equally quickly, the anthocyanin marker provides unbiased estimates of outcrossing rates. The results overall support the use of anthocyanin as a neutral marker, with the alternate phenotypes unlikely to be differentially impacted by the processes of pollination, pollen-tube growth, and germination. Fluctuating selection on conditions for dormancy release may be partially responsible for the maintenance of the anthocyanin polymorphism in Datura stramonium.
Key Words: anthocyanin • Datura stramonium • fluorescent powder • germination • neutral marker • outcrossing rates • pollen tubes • pollination
Neutral genetic markers are essential for inferring population-level processes such as gene flow, paternity, and outcrossing rates (Avise, 1994
). Allozymes have long been the standard tool of choice due to their often high levels of variability, codominance, and low cost. More recently, molecular markers such as sequence polymorphisms, randomly amplified polymorphic DNAs (RAPDs) and microsatellites have become invaluable in some situations where the level of variability present in allozymes is insufficient for the question at hand. It is generally expected that these classes of markers are neutral; that is, variation at marker loci is uncorrelated with any phenotypic variation that affects fitness. If this assumption is violated, then conclusions about population genetic processes will be biased.
Morphological variation is infrequently used as a tool to follow population-level genetic processes for a number of reasons. Relatively few species possess clear alternative morphs in a single population. For those species that do, the marker only gives information about a single locus, which is inadequate for estimates of parameters such as paternity or population structure. However, morphological markers can be extremely effective as estimators of outcrossing rates, especially when used in artificial populations where a homozygous recessive morph is surrounded by homozygous dominant individuals (e.g., Rick, Holle, and Thorp, 1978
). In this situation, a morphological marker can actually surpass nonvisible markers in precision, for the simple reason that many more progeny can be assayed.
The presence of anthocyanin has been used as a marker to estimate outcrossing rates of individual flowers and plants in Datura stramonium (Motten and Antonovics, 1992
). Plants producing anthocyanin have lavender flowers and purplish-green stems ("purple" plants), while plants lacking anthocyanin have white flowers and pale green stems ("white" plants). The anthocyanin marker is dominantly expressed by a single locus (Blakeslee and Avery, 1917
) and can be scored at the cotyledon stage. Outcrossing rates are estimated by examining progeny arrays from anthocyaninless maternal plants and dividing the frequency of offspring that produce anthocyanin by the frequency of the marker allele in the population. Hundreds of progeny can be scored from each flower with very little effort. Large sample sizes are especially valuable for detecting rare outcrossing events in predominantly selfing species. In D. stramonium, population-level estimates of outcrossing rates range from 0.0 (Stone, unpublished data) to 8.5% (Motten and Antonovics, 1992
).
However, one must wonder about the neutrality of markers that have gross impacts on the phenotype. It is impossible to determine, a priori, whether or not the presence of the marker influences the process that one is measuring. It is perhaps most worrisome that anthocyanin changes pigmentation in the flower, because pollinators frequently express corolla-color preferences (e.g., Waser and Price, 1981
; Brown and Clegg, 1984
; Stanton, 1987
). If pollinators flew preferentially between white flowers, for example, outcross pollen from anthocyaninless plants would be attributed to self-pollination, and outcrossing rates would be underestimated. I chose to use fluorescent powders to track pollinator movements as an indication of the presence or absence of assortative pollination.
Following pollination, fertilization, seed maturation, and germination must proceed before outcrossing rates can be estimated. If the allele for anthocyanin production has pleiotropic effects on pollen-tube growth rates, outcrossing rates could again be biased. If pollen tubes from anthocyanin-producing plants grew more quickly than pollen tubes from anthocyaninless plants, outcrossing rates would be overestimated, as the assumption that both types of outcross pollen have equal siring success would be false. I made pairwise comparisons of pollen-tube growth rates from plants with and without the anthocyanin allele. Finally, I monitored germination rates for seedlings of both morphs. If seedlings containing anthocyanin systematically germinated at different rates from those lacking anthocyanin, estimates of outcrossing rates would be biased. Even if eventual germination percentages are the same, a difference in timing could influence estimates made within a certain time following sowing of seed.
Outcrossing rate has traditionally been estimated at the population level (e.g., Ritland and Jain, 1981
). However, individuals within populations may differ in outcrossing rate, due either to ecological or stochastic influences, or to genetically based differences in floral traits (e.g., Smyth and Hamrick, 1984
; Ritland and El-Kassaby, 1985
; Warwick and Thompson, 1989
; Morgan and Barret, 1990
; Cruzan et al., 1994
; Karron et al., 1997
; Vrieling et al., 1997
). While mean male and female outcrossing rates are, by definition, equivalent at the population level, they may differ at the level of individuals (Horovitz and Harding, 1972
). Maternal outcrossing refers to seeds produced by outcross pollen, whereas paternal outcrossing refers to the successful siring of seeds on other plants. Maternal outcrossing rate is of particular interest in D. stramonium, because it is influenced by anther-stigma separation, a heritable trait (Motten and Stone, 2000
). Heritable differences in maternal outcrossing rates may lead to the development of lineages differing in inbreeding history, with potential implications for the evolution of mating systems (Uyenoyama, Holsinger, and Waller, 1993
).
In this paper, I investigate the neutrality of anthocyanin production for reproduction in D. stramonium. I make no attempt to evaluate the fitness consequences of the trait over the entire life of the plant, but restrict my attention to the portion of the life cycle relevant for estimates of outcrossing rates: pollinator visitation, pollen-tube growth, and germination. If "white" and "purple" variants perform indistinguishably during these stages, then the anthocyanin marker should yield unbiased estimates of outcrossing rates. I focus primarily on maternal outcrossing rates of individuals, but also include tests for bias in estimates of paternal or population-level outcrossing rates where appropriate.
MATERIALS AND METHODS
Study species and sites
Datura stramonium (jimsonweed; Solanaceae) is a rank annual weed distributed worldwide (Holm et al., 1997
). It bears large trumpet-shaped flowers that open at dusk, are visited by hawk moths and bees, and close the following morning. Corollas and styles are ~10 cm in length, and the spiny capsules normally contain hundreds of seeds. In North Carolina, D. stramonium infests soybean fields and pastures on dairy farms, with most populations consisting uniformly of plants containing anthocyanin. For the pollination aspects of this work, I created two artificial populations in 1998, using plants propagated from a population in Durham, North Carolina containing both morphs. The artificial populations were established at Duke University's Botany Plot and Zoology Field Station, sites separated by ~1 km. In each population, plants were laid out in a 5 x 12 grid at 1-m spacing. In the first, third, and fifth rows, the two morphs were alternated. The second and fourth rows consisted of anthocyanin-producing plants so that anthocyaninless plants on the interior were surrounded by anthocyanin-producing plants at all eight nearest-neighbor positions. There were a total of 18 "white" and 42 "purple" plants in each population. This total size and density is representative of that found in natural populations, although natural populations sometimes contain hundreds of plants that are not so regularly spaced, with resulting variation in the size of individual plants. The germination data come from seeds collected in the fall of 1997 from two naturally occurring polymorphic populations, the previously mentioned one in Durham (~100-m elevation), and the other near Mountain Lake, Virginia (~750-m elevation).
Pollinator visitation
Fluorescent powder served as an indicator of pollinator movement. On six mornings during the first 10 d of July 1998, I selected adjacent "purple" and "white" plants of similar sizes having buds that would open the following night. Using a toothpick, I applied powder to the anthers of one bud on each of the two plants. At one artificial population, orange and yellow dyes were alternated nightly between the two floral morphs, and at the other population, blue and green dyes were alternated. The following morning, I collected stigmas from all flowers that had been open the previous night. Stigmas were scored for presence or absence of each of the four colors by viewing them under a dissecting microscope in a dark room with a hand-held UV light.
Log-likelihood ratios were used to test both for pollinator preference and for biased dye movement. In order to increase the power of the tests, I pooled data across dates and across sites, as well as analyzing data for the two sites separately. This pooling could obscure distinct patterns between the two sites if pollinator assemblages were different, for example, if honey bees dominated one site and hawk moths the other. However, populations of honey bees have declined dramatically in recent years and were not observed in either population. Pollinator preference was examined using a 2 x 2 contingency table with morph as one factor and presence or absence of dye as the other factor, with the null hypothesis being that the probability of receipt of dye was independent of morph. Assortative dye movement was examined using another 2 x 2 contingency table with the null hypothesis that the donor morph was independent of the recipient morph.
The fact that all stigmas were collected from plants with known locations also allowed me to examine distances traveled by dye particles. The two populations were treated separately for these analyses. I first used t tests to see whether plants that received dye were the same distance from donors as plants that did not receive dye. I then pooled plants with open flowers into distance classes at 1-m intervals from the donor plants. A series of Kolmogorov-Smirnov tests determined whether the distribution of distance classes for open flowers of a given morph differed from the distribution of distance classes for flowers of that morph receiving dye. A significant difference in the distributions of available flowers and flowers receiving dye would indicate non-random dye movement. The Kolmogorov-Smirnov test is a useful complement to the contingency-table analyses, because both morphs are likely to have a majority of anthocyanin-producing plants as nearest neighbors, and therefore the null expectation is not necessarily that the two morphs will be visited in proportion to their frequency across the entire population.
Pollen-tube growth
Because pollen-tube growth could be affected by microenvironmental factors such as temperature or relative humidity, comparisons of outcross pollen-tube growth from homozygous "purple" and "white" donors were paired on single maternal plants. I performed a total of 60 pollinations on 30 plants, half of each morph. Pollinations were performed between 0900 and 1000 on pairs of flower buds that would normally open the following night. A single anther from either a "white" or a "purple" donor was removed with forceps and immediately swept across the stigma of an emasculated recipient flower, completely coating it with pollen. Styles were collected 4 h later, stained with aniline blue dye (Martin, 1959
), and viewed under a fluorescent microscope. Dozens of pollen tubes in each style were visible due to fluorescing callose tissue; their farthest extent was recorded. I analyzed the data from both maternal morphs in a single paired-comparisons t test.
The paired-comparisons test described above reveals potential bias in estimates of maternal outcrossing rates of individuals based on the assumption that "white" and "purple" outcross sires have representation in the progeny proportional to their frequency in the population. However, unequal pollen-tube growth rates could also lead to a bias in population-level estimates. For example, if tubes from anthocyanin-producing plants grow more quickly in "purple" than in "white" mothers, an extrapolation from anthocyaninless mothers would underestimate population-level outcrossing rates. A two-way Model I ANOVA was used to examine whether pollen-tube growth rate was independent of the identity of donor and recipient morphs.
Germination
I collected mature fruits in the fall of 1997 from open-pollinated plants in polymorphic populations in Durham, North Carolina and Mountain Lake, Virginia. I collected a single fruit from each of about half of all anthocyanin-producing plants bearing fruit in each population, yielding 63 fruits from Durham and 28 fruits from Mountain Lake. I collected all ripe fruits from all anthocyaninless plants in both populations, yielding 21 fruits from ten plants in the Durham population and 20 fruits from nine plants in the Mountain Lake population. Seeds were removed from capsules and stored at room temperature. Seeds were planted 1 cm deep in flats in the greenhouse in March 1998. I planted 100 seeds from each "white" fruit and 20 seeds from each "purple" fruit. Germination rates of both morphs were recorded 2, 4, and 6 wk after sowing.
Because seed tissues are composed of genotypes from diverse origins, I wanted to separate effects of the embryonic genotype on germination from effects of the maternal genotype. Differences in germination rates according to embryonic morph could threaten the neutrality of the marker. To see whether premature censuses could skew mating system estimates, I looked at progeny of anthocyaninless mothers, using a paired t test to compare the proportions at 2 wk and again at 6 wk of seedlings that were anthocyaninless. This analysis was only possible for seed parents producing progeny arrays with both types of offspring, and was therefore restricted to nine fruits from four anthocyaninless maternal plants.
Differences in germination rates due to maternal color morph would not influence the neutrality of the marker, but would still be of interest. I used a two-way analysis of variance to compare overall germination rates at 4 wk, with maternal morph being a fixed factor and site being a random factor.
RESULTS
Pollinator visitation
The majority of the 142 stigmas examined showed evidence of insect movement between flowers (Table 1). Dye was found on white and purple flowers in proportion to their abundance in the populations (Table 1; Botany Plot G = 0.75, Zoology Field Station G = 1.78, pooled data G = 0.21; for each analysis, df = 1, P > 0.1). It was also dispersed nonassortatively between morphs, with dye from both white and purple flowers being dispersed randomly to stigmas of flowers of both morphs (Table 1; Botany Plot G = 0.12, Zoology Field Station G = 2.28, pooled data G = 0.34; for each analysis, df = 1, P > 0.1). There was a small amount of movement detectable between the two populations; seven stigmas, or ~5% of those examined, showed dye colors originating from the population at 1 km distance.
|
|
, pp. 165–167). The curve shows that a difference between growth rates as small as 2 mm could have been detected with almost complete certainty, given the sample size and standard error observed (Fig. 2). Pollen tubes grew the same rate regardless of which morph was the maternal plant, and there was no interaction between maternal and paternal growth rates (Table 3), suggesting that patterns of pollen-tube growth do not bias population-level estimates derived from data on anthocyaninless mothers.
|
|
|
|
|
|
|
Phenotypic markers have a long and distinguished history in genetics and breeding, dating back to Mendel's legendary experiments. For genetic mapping and demonstrating concepts of inheritance, their utility is uncontroversial, if limited. For population-level processes, in contrast, they are infrequently used, mainly because visible polymorphism with a simple genetic basis is rare in most natural populations (but see Marshall and Abbott, 1982
; Snow and Mazer, 1988
). In addition, there is legitimate concern that a dramatic phenotypic difference may influence or be influenced by the processes that one is trying to measure, leading to skewed estimates. However, visible markers, if indeed neutral, are highly desirable, especially for attempting to measure rare events. In Datura stramonium, where outcrossing rates are very low, the use of a visible marker allows unusually precise estimates of outcrossing rates for individual plants and even individual flowers (Motten and Antonovics, 1992
; Motten and Stone, 2000
).
The results from the fluorescent powder confirm earlier reports based on pollinator observations that flower color does not appear to influence pollen movement (Motten and Antonovics, 1992
). Purple and white flowers are visited in proportion to their abundances, and dye was transported at random between flowers of the two morphs (Table 1). This is perhaps not surprising. Although flower color has been shown to influence pollinator behavior in other studies (e.g., Levin and Kerster, 1970
; Miller, 1981
; Waser and Price, 1981
; Brown and Clegg, 1984
; Stanton, 1987
), the color difference between morphs of Datura flowers is subtle. The anthocyanin in the stems of young plants lends them a dark purple color, but the flowers are a pale lavender, not much different from white flowers of the anthocyaninless morph. At night, when the flowers open and are visited by hawk moths, the two morphs are indistinguishable to the human eye. Neither are the morphs distinguishable in UV light; both appear dark under UV illumination.
When focusing on pollinator attraction, it is logical to consider anthocyanin mainly from the perspective of its effects on corolla color. However, anthocyanin is the end product of a complex biochemical pathway (see Koes, Quattrocchio, and Mol, 1994
, and Shirley, 1996
, for recent reviews), and its production or lack thereof may be associated with pleiotropic effects (Rausher and Fry, 1993
). Regulatory genes control the transcription of structural pigmentation genes; a mutation in any of these can lead to an anthocyaninless form. Phenotypes differing in anthocyanin production have been found to differ in numerous other traits, including resistance to UV-B radiation, pollen performance, germination rates, and plant–microbe interactions (Shirley, 1996
). Pleiotropic effects of anthocyanin loss will presumably vary depending on whether the causal mutations occur in regulatory genes affecting transcription or structural pigmentation genes (Tiffin, Miller, and Rausher, 1998
). It is not yet known where along the pathway anthocyaninless morphs of Datura differ from pigmented morphs, although the fact that anthocyanin production is eliminated in all plant parts suggests modification of one of the structural genes.
In this study, pollen-tube growth was uncorrelated with anthocyanin production (Fig. 1). The use of a power test demonstrates that the sample size was indeed adequate to find a distinction in growth rates between morphs if one did, in fact, exist. The literature is too sparse at this point to determine whether or not these findings are typical for most species. A mutant of maize and a transgenic petunia that are deficient in activity of chalcone synthase, an enzyme near the beginning of the anthocyanin pathway, produce pollen incapable of forming normal pollen tubes (Coe, McCormick, and Modena, 1981
; Taylor and Jorgensen, 1992
). However, in Arabidopsis, Antirrhinum, and parsley, lack of chalcone synthase activity does not affect male fertility (Burbulis, Iacobucci, and Shirley, 1996
; Shirley, 1996
; Ylstra, Muskens, and Van Tunen, 1996
).
The search for the effect of a mutation on seed germination is complicated by the fact that the seed is a composite of genotypes from different sources: the seed coat consists of maternal tissue, the embryo possesses the genotype of the new individual, and the endosperm is a triploid entity with 2N coming from maternal and 1N from paternal sources. Any of these tissues can influence the cessation of dormancy. In Datura ferox, a model system for seed germination, endosperm softening is necessary for germination (de Miguel and Sánchez, 1992
; Mella, Maldonado, and Sánchez, 1995
); this softening is promoted by the far-red absorbing form of phytochrome and an unknown co-factor originating in the embryo (Sánchez and de Miguel, 1997
). In addition to these physiological factors, it appears that mechanical barriers to germination are imposed by the seed coat in Datura stramonium, which during dormant periods is only partially permeable (Reisman, Kigel, and Rubin, 1989
; Benvenuti and Macchia, 1995
).
In this study, both the genotype of the mother at the anthocyanin locus and her site of origin influenced germination rates, suggesting that the seed coat played an important role in regulating dormancy. For the Durham population, a higher proportion of seeds from anthocyaninless mothers germinated than from anthocyanin-producing mothers (Table 4), and this difference is eliminated if the seed coat is mechanically removed (unpublished observations). This is consistent with the results for three anthocyaninless mutants of tomato, which germinate more quickly than anthocyanin-containing lines (Atanassova, Shtereva, and Molle, 1997
). In F1 hybrids between anthocyaninless and anthocyanin-producing lines of tomato, germination rates of the progeny are similar to those of their mothers, suggesting that the impact of anthocyanin on germination is either maternally inherited, due to maternal environmental effects, or due to characteristics of the seed coat, which is of maternal origin. Effects on germination of the three anthocyaninless genes in tomatoes are similar in spite of the fact that they appear to be involved in disparate stages of the anthocyanin pathway.
It is not obvious why seeds from Mountain Lake germinated in Durham should present a pattern reverse to that found in tomatoes and in Datura from Durham, with progeny of anthocyanin-producing mothers having dormancy broken more easily than progeny of anthocyaninless mothers. We do know that plants exposed to high UV-B wavelengths produce elevated levels of anthocyanins (Li et al., 1993
), and that various products of the anthocyanin pathway impede germination (Koes, Quattrocchio, and Mol, 1994
). It may be that plants grown at high elevations possess genetic modifications to aid germination given that high levels of germination-inhibiting anthocyanin are produced in response to UV-B stress. Easy germination of seeds from anthocyaninless mothers is not universal; in an Ontario study, seeds from a totally anthocyaninless population of Datura had dramatically lower germination rates than seeds from three populations monomorphic for anthocyanin-producing plants (Weaver, Dirks, and Warwick, 1985
).
Germination was not affected by the phenotype of the embryo with respect to anthocyanin production (Fig. 2). "Purple" and "white" seedlings from anthocyaninless mothers germinated at equivalent rates, so the timing of the census is unlikely to bias estimates of outcrossing rates. Along with the findings that pollination and pollen-tube growth are not affected by the absence of anthocyanin in Datura, I conclude that anthocyanin variants act neutrally with respect to outcrossing rates. In this paper, I have emphasized the use of the anthocyanin marker in Datura for measuring female outcrossing rates. It also has possibilities for use in male outcrossing rates sensu Horovitz and Harding (1972)
. That is, anthocyanin-producing plants varying in traits of interest such as plant size or anther–stigma separation could be surrounded by anthocyaninless plants in replicate artificial arrays. Screening of the progeny of anthocyaninless individuals would indicate how successful the pigmented plants had been in siring outcross seeds. The anthocyanin marker could also be used in estimates of gene flow, for example, by creating artificial anthocyaninless populations at varying distances from anthocyanin-producing ones.
The finding that anthocyanin does not affect outcrossing rates in Datura begins to address the larger question of whether or not anthocyanin production is selectively neutral. Given that Datura experiences almost complete selfing, eroding genetic variation to the extent that no allozyme polymorphisms for the species have been detected (Conklin, 1976
), it seems likely that the anthocyanin polymorphism is maintained by selection. Putative selective agents remain to be identified, but may include biotic interactions as well as abiotic ones, including the effect of anthocyanin production on seed dormancy and germination rates.
FOOTNOTES
1 The author thanks Elena Kuhn for help in the field and laboratory, M. K. Uyenoyama for logistical support, Don Stone for sharing his fluorescent microscope, A. Motten and J. Antonovics for help locating the population at Mountain Lake, Shirley Price and Mrs. W. MacFarlands for permission to work on their land, and C. Coberly, L. McDade, P. Tiffin, and an anonymous reviewer for comments on the manuscript. This work was supported by NSF DEB-9707684. 
LITERATURE CITED
Atanassova, B., L. Shtereva, and E. Molle. 1997 Effect of three anthocyaninless genes on germination in tomato (Lycopersicon esculentum Mill.) I. Seed germination under optimal conditions. Euphytica 95: 89–98. [CrossRef][ISI]
Avise, J. C. 1994 Molecular markers, natural history, and evolution. Chapman and Hall, New York, New York, USA.
Benvenuti, S., and M. Macchia. 1995 Effect of hypoxia on buried weed seed germination. Weed Research 35: 343–351. [CrossRef][ISI]
Blakeslee, A. F., and B. T. Avery. 1917 Adzuki beans and Jimson weeds. Journal of Heredity 8: 125–131.
Brown, B. A., and M. T. Clegg. 1984 Influence of flower color polymorphism on genetic transmission in a natural population of the common morning glory, Ipomea purpurea (Convolvulaceae). Evolution 38: 796–803. [CrossRef][ISI]
Brunet, J., and C. G. Eckert. 1998 Effects of floral morphology and display on outcrossing in Blue Columbine, Aquilegia caerulea (Ranunculaceae). Functional Ecology 12: 596–606. [CrossRef][ISI]
Burbulis, I. E., M. Iacobucci, and B. W. Shirley. 1996 A null mutation in the first enzyme of flavonoid biosynthesis does not affect male fertility in Arabidopsis. Plant Cell 8: 1013–1025. [Abstract]
Coe, E. H., S. M. McCormick, and S. A. Modena. 1981 White pollen in maize. Journal of Heredity 72: 318–320.
Conklin, M. E. 1976 Genetic and biochemical aspects of development in Datura. Monographs in developmental biology 12. S. Karger, New York, New York, USA.
Cruzan, M. B., J. L. Hamrick, M. L. Arnold, and B. D. Bennett. 1994 Mating system variation in hybridizing irises - effects of phenology and floral densities on family outcrossing rates. Heredity 72: 95–105. [ISI]
de Miguel, L., and R. A. Sánchez. 1992 Phytochrome-induced germination, endosperm softening and embryo growth potential in Datura ferox seeds: sensitivity to low water potential and time to escape to FR reversal. Journal of Experimental Botany 43: 969–974.
Holm, L., J. Doll, E. Holm, J. Pancho, and J. Herberger. 1997 World weeds: natural histories and distribution. Wiley, New York, New York, USA.
Horovitz, A., and J. Harding. 1972 The concept of male outcrossing in hermaphrodite higher plants. Heredity 29: 223–236. [ISI]
Karron, J. D., R. T. Jackson, N. N. Thumser, and A. J. Hessenauer. 1997 Outcrossing rates of individual Mimulus ringens genets are correlated with anther-stigma separation. Heredity 79: 365–370. [CrossRef][ISI]
Koes, R. E., F. Quattrocchio, and J. N. M. Mol. 1994 The flavonoid biosynthetic pathway in plants: function and evolution. BioEssays 16: 123–132.
Levin, D. A., and H. W. Kerster. 1970 Phenotypic dimorphism and populational fitness in Phlox. Evolution 24: 128–134. [CrossRef]
Li, J., T. M. Lee, R. Raba, R. G. Amundson, and R. L. Last. 1993 Arabidopsis flavonoid mutants are hypersensitive to UV-B irradiation. Plant Cell 5: 171–179. [Abstract]
Marshall, D. F., and R. J. Abbott. 1982 Polymorphism for outcrossing frequency in Senecio vulgaris L. I. Evidence. Heredity 48: 227–235. [ISI]
Martin, F. M. 1959 Staining and observing pollen tubes in the style by means of fluorescence. Stain Technology 34: 125–128. [ISI][Medline]
Mella, R. A., S. Maldonado, and R. A. Sánchez. 1995 Phytochrome-induced structural changes and protein degradation prior to radicle protrusion in Datura ferox seeds. Canadian Journal of Botany 73: 1371–1378.
Miller, R. B. 1981 Hawkmoths and geographic patterns of floral variation in Aquilegia caerulea. Evolution 35: 763–774.
Morgan, M. T., and S. C. H. Barrett. 1990 Outcrossing rates and correlated mating within a population of Eichhornia paniculata (Pontederiaceae). Heredity 64: 271–280. [ISI]
Motten, A. F., and J. Antonovics. 1992 Determinants of outcrossing rate in a predominantly self-fertilizing weed, Datura stramonium (Solanaceae). American Journal of Botany 79: 419–427. [CrossRef][ISI]
———, and J. L. Stone. 2000 Heritability of stigma position and the effect of stigma-anther separation on outcrossing in a predominantly self-fertlizing weed, Datura stramonium (Solanaceae). American Journal of Botany 87: 339–347.
Rausher, M. D., and J. D. Fry. 1993 Effects of a locus affecting floral pigmentation in Ipomoea purpurea on female fitness components. Genetics 134: 1237–1247. [Abstract]
Reisman, B. O., J. Kigel, and B. Rubin. 1989 Short soaking in water inhibits germination of Datura ferox L. and Datura stramonium L. seeds. Weed Research 29: 357–364. [CrossRef][ISI]
Rick, C. M., M. Holle, and R. W. Thorp. 1978 Rates of cross-pollination in Lycopersicon pimpinellifolium: impact of genetic variation in floral characters. Plant Systematics and Evolution 129: 31–44. [CrossRef][ISI]
Ritland, K., and Y. A. El-Kassaby. 1985 The nature of inbreeding in a seed orchard of Douglas fir as shown by an efficient multilocus model. Theoretical and Applied Genetics 71: 375–384. [ISI]
———, and S. Jain. 1981 A model for the estimation of outcrossing rate and gene frequencies using n independent loci. Heredity 47: 35–52. [ISI]
Sánchez, R. A., and L. de Miguel. 1997 Phytochrome promotion of mannan-degrading enzyme activities in the micropylar endosperm of Datura ferox seeds requires the presence of the embryo and gibberellin synthesis. Seed Science Research 7: 27–33.
Shirley, B. W. 1996 Flavonoid biosynthesis: 'new' functions for an 'old' pathway. Trends in Plant Science 1: 377–382.
Smyth, C. A., and J. L. Hamrick. 1984 Variation in estimates of outcrossing in musk thistle populations. Journal of Heredity 75: 303–307.
Snow, A. A., and S. J. Mazer. 1988 Gametophytic selection in Raphanus raphanistrum: a test for heritable variation in pollen competitive ability. Evolution 42: 1065–1075. [CrossRef][ISI]
Sokal, R. R., and F. J. Rohlf. 1981 Biometry, 2nd edition. W. H. Freeman, New York, New York, USA.
Stanton, M. L. 1987 Reproductive biology of petal color variants in wild populations of Raphanus sativus. I. Pollinator response to color morphs. American Journal of Botany 74: 178–187. [CrossRef][ISI]
Taylor, L. P., and R. Jorgense. 1992 Conditional male fertility in chalcone synthase-deficient petunia. Journal of Heredity 83: 11–17.
Tiffin, P., R. E. Miller, and M. D. Rausher. 1998 Control of expression patterns of anthocyanin structural genes by two loci in the common morning glory. Genes and Genetic Systems 73: 105–110. [CrossRef]
Uyenoyama, M. K., K. E. Holsinger, and D. M. Waller. 1993 Ecological and genetic factors directing the evolution of self-fertilization. In D. J. Futuyma and J. Antonovics [eds.], Oxford Surveys in Evolutionary Biology, 327–381. Oxford University Press, Oxford, UK.
Vrieling, K., P. Saumitou Laprad, E. Meelis, and J. T. Epplen. 1997 Multilocus DNA fingerprints in the plant Cynoglossum officinale L. and their use in the estimation of selfing. Molecular Ecology 6: 587–593. [CrossRef]
Warwick, S. I., and B. K. Thompson. 1989 The mating system in sympatric populations of Carduus nutans, C. acanthoides and their hybrid swarms. Heredity 63: 329–337. [ISI]
Waser, N. M., and M. V. Price. 1981 Pollinator choice and stabilizing selection for flower color in Delphinium nelsonii. Evolution 35: 376–390.
Weaver, S. E., V. A. Dirks, and S. J. Warwick. 1985 Variation and climatic adaptation in northern populations of Datura stramonium. Canadian Journal of Botany 63: 1303–1308.
Ylstra, B., M. Muskens, and A. J. Van Tunen. 1996 Flavonols are not essential for fertilization in Arabidopsis thaliana. Plant Molecular Biology 32: 1155–1158.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
