Floral morphometrics and the evolution of sexual dimorphism in Lycium (Solanaceae)

Plants of Lycium californicum, L. exsertum, and L. fremontii produce flowers that are either male-sterile (female) or hermaphroditic, and populations are morphologically gynodioecious. As is commonly found in gynodioecious species, flowers on female plants are smaller than those on hermaphrodites for a number of floral traits. Floral size dimorphism has often been hypothesized to be the result of either a reduction in female flower size that allows reallocation to greater fruit and seed production, or an increase in hermaphroditic flower size due to the increased importance of pollinator attraction and pollen export for hermaphroditic flowers. We provide a test of these two alternatives by measuring 11 floral characters in eight species of Lycium and using a phylogeny to reconstruct the floral size shifts associated with the evolution of gender dimorphism. Our analyses suggest that female flowers are reduced in size relative to the ancestral condition, whereas flowers on hermaphrodites have changed only slightly in size. Female and hermaphroditic flowers have also diverged both from one another and from ancestral cosexual species in several shape characteristics. We expected sexual dimorphism to be similar among the three dimorphic taxa, as gender dimorphism evolved only a single time in the ancestor of the American dimorphic lineage. While the floral sexual dimorphism is broadly similar among the three dimorphic species, there are some species-specific differences. For example, L. exsertum has the greatest floral size dimorphism, whereas L. fremontii had the greatest size-independent dimorphism in pistil characters. To determine the degree to which phylogenetic uncertainty affected reconstruction of ancestral character states, we performed a sensitivity analysis by reconstructing ancestral character states on alternative topologies. We argue that investigations such as this one, that examine floral evolution from an explicitly phylogenetic perspective, provide new insights into the study of the evolution of floral sexual dimorphism.     

Gender plasticity and sexual system stability in Wurmbea

Background and aims

Sexually dimorphic populations are often located in drier habitats than cosexual populations. Gender plasticity (GP), whereby hermaphrodites alter female and male functions depending on resources, and sex-differential plasticity (SDP) between hermaphrodites and unisexuals are predicted to affect sexual system stability. Here, GP and SDP are evaluated in cosexual and gynodioecious Wurmbea biglandulosa and sub-dioecious and dioecious W. dioica.

Methods

GP was evaluated under two resource conditions, compared among sexual systems and assessed as to whether (1) males produced perfect flowers and (2) hermaphrodites altered investment in perfect (female function) and total (male function) flowers. SDP was assessed within sexual systems as differences between sex functions of hermaphrodites vs. unisexuals. Males and hermaphrodites were compared to assess whether size thresholds for female function differed among sexual systems. Plasticity costs were evaluated using correlations between female function and male traits in hermaphrodites, and in W. dioica by comparing hermaphrodite and male regressions between plant size and pollen production.

Key Results

In dioecious W. dioica no males exhibited GP, whereas 100 % did in gynodioecious and cosexual W. biglandulosa. In sub-dioecious W. dioica, resources affected GP (high, 66 %; low, 42 %). Hermaphrodites in all sexual systems reduced perfect but not total flowers under low resources. Unisexuals were unaffected, demonstrating SDP for female function only. Thresholds for female function were greater in sub-dioecious W. dioica than in W. biglandulosa. Plasticity costs were detected only in sub-dioecious W. dioica.

Conclusions

SDP for female function could assist female establishment in cosexual populations and maintain females in gynodioecious and sub-dioecious populations. Although the absence of male SDP should stabilize sub-dioecy, plasticity costs would render sub-dioecy unstable, favouring canalized males over hermaphrodites. This study highlights the importance of interactions between environmental conditions and hermaphrodite sex expression for the stability of dimorphic sexual systems.     

PHENOTYPIC GENDER IN SEX CHANGING DWARF GINSENG,PANAX TRIFOLIUM (ARALIACEAE)

Dwarf ginseng (Panax trifolium L., Araliaceae) is a diphasic (“sex changing”) species in which one phase has staminate flowers and the other has hermaphroditic flowers. In order to determine the relative allocations of the hermaphroditic gender phase to male and female functions,variation in population gender phase ratios, pollen production and viability, and ovule and seed production were documented. Gender phase ratios are highly male-biased. Dwarf ginseng is self-compatible, and both gender phases have viable pollen capable of effecting fertilization. Males produce more flowers and more viable pollen per anther than hermaphrodites. The phenotypic gender of hermaphrodites is extremely female-biased; it is likely that hermaphrodites function essentially as females. Sexual selection may have a role in the evolution and maintenance of differences between the gender phases in allocation to male function.     

Constraints on the evolution of males and sexual dimorphism: field estimates of genetic architecture of reproductive traits in three populations of gynodioecious Fragaria virginiana

Abstract To understand how genetic constraints may limit the evolution of males and sexual dimorphism in a gynodioecious species, I conducted a quantitative genetic experiment in a gynodioecious wild strawberry, Fragaria virginiana . I estimated and compared genetic parameters (narrow-sense heritabilities, between-trait and between-sex genetic correlations, as well as phenotypic and genetic variance-covariance matrices) in the two sex morphs from three populations grown in a common field garden. I measured pollen and ovule production per flower, petal size, fruit set, and flower number. My major findings are as follows. (1) The presence of a phenotypic trade-off between pollen production and fruit set in hermaphrodites reflects a negative genetic correlation in the narrow sense that is statistically significant when pooled across populations. (2) The main constraints on the evolution of males are low genetic variation for pollen per flower and strong positive correlations associated with ovule number (e.g., between pollen and ovules in hermaphrodites, and between ovules in hermaphrodites and females). (3) Traits with the lowest levels of sexual dimorphism (ovule number and flower number) have the highest between-sex genetic correlations suggesting that overlap in the expression of genes in the sex morphs constrains their independent evolution. (4) There are significant differences in G matrices between sex morphs but not among populations. However, evidence that male-female trait correlations in hermaphrodites were lower in populations with higher frequencies of females may indicate subtle changes in genetic architecture.     

Environmental stress and the evolution of dioecy: Wurmbea dioica (Colchicaceae) in Western Australia

Stressful ecological conditions have been implicated in the evolution of separate sexes in plants. Gender dimorphic species are often found in drier habitats than their sexually monomorphic relatives, and gynodioecious populations appear closer to a dioecious state as resources, particularly water, become limiting. This pattern could result if dry conditions decrease the relative seed fitness of cosexual plants, allowing female plants to become established in monomorphic populations. We studied geographical variation in gender expression and biomass allocation among 12 monomorphic and dimorphic populations of Wurmbea dioica along a latitudinal precipitation gradient in southwestern Australia to provide insight into mechanisms by which aridity might favor transitions between sexual systems. Plants in monomorphic and dimorphic populations exhibited contrasting gender expression and patterns of biomass allocation in areas with different levels of precipitation. Among dimorphic populations, lower precipitation was associated with a higher frequency of female plants, and reduced allocation to female function by hermaphrodites during flowering. In contrast, stress conditions had no effect on female allocation at flowering in monomorphic populations. Across latitudes, unisexuals and cosexuals exhibited consistent differences in above ground traits, with cosexuals having larger leaves, taller stems and larger flowers. Although all plants were smaller under drier conditions, cosexuals decreased above ground allocation to vegetative and reproductive structures with decreasing latitude. In contrast, unisexuals increased allocation to reproduction in drier areas at the expense of below ground size. Aridity was associated with reduced flower size among all gender classes, but not with changes in flower number. These data do not support the hypothesis that resource limitation of female allocation in cosexual populations favors the establishment of gender dimorphism in W. dioica. Alternative hypotheses, involving higher selfing rates and enhanced survival of unisexuals relative to cosexuals under resource-limited conditions, are discussed as possible explanations for the origin of dioecy in W. dioica. This revised version was published online in August 2006 with corrections to the Cover Date.     

Paternal leakage sustains the cytoplasmic polymorphism underlying gynodioecy but remains invasible by nuclear restorers

Cytoplasmic male sterility (CMS) in plants often results in gynodioecious populations, composed of hermaphrodites and male-sterile females. All models of gynodioecy assume maternal inheritance of the cytoplasmic alleles and postulate a variety of negatively frequency-dependent mechanisms to maintain the cytoplasmic polymorphisms observed in many natural populations. However, in some plant species, mitochondria are transmitted at least occasionally by pollen, a process called paternal leakage. We show that even a small amount of paternal leakage is sufficient to sustain a permanent, stable cytoplasmic polymorphism. Because only hermaphrodites provide pollen in gynodioecious species, the effects of paternal leakage are biased and occur more often from the non-CMS male-fertile haplotype to the CMS male-sterile haplotype. We also show that a nuclear restorer disrupts the polymorphic cytoplasmic equilibrium, leading to fixation of both the CMS allele and the restorer. Although a dominant nuclear restorer fixes, it fixes much more slowly than in the standard CMS models. Although a stable cytonuclear polymorphism is possible with "matching alleles" nuclear restoration, oscillations to low frequencies present a risk of loss by drift. Paternal leakage enhances the stability of joint cytonuclear polymorphism by reducing the chance that a CMS allele is lost by drift.     

Sex‐specific fitness variation in gynodioecious Beta vulgaris ssp. maritima: do empirical observations fit theoretical predictions?

In gynodioecious species, in which hermaphroditic and female plants co-occur, the maintenance of sexual polymorphism relies on the genetic determination of sex and on the relative fitness of the different phenotypes. Flower production, components of male fitness (pollen quantity and pollen quality) and female fitness (fruit and seed set) were measured in gynodioecious Beta vulgaris spp. maritima, in which sex is determined by interactions between cytoplasmic male sterility (CMS) genes and nuclear restorers of male fertility. The results suggested that (i) female had a marginal advantage over hermaphrodites in terms of flower production only, (ii) restored CMS hermaphrodites (carrying both CMS genes and nuclear restorers) suffered a slight decrease in fruit production compared to non-CMS hermaphrodites and (iii) restored CMS hermaphrodites were poor pollen producers compared to non-CMS hermaphrodites, probably as a consequence of complex determination of restoration. These observations potentially have important consequences for the conditions of maintenance of sexual polymorphism in B. vulgaris and are discussed in the light of existing theory on evolutionary dynamics of gynodioecy.     

MICROSPOROGENESIS IN MALE-STERILE AND HERMAPHRODITIC PLANTS OF NINE GYNODIOECIOUS TAXA OF HAWAIIAN BIDENS (ASTERACEAE)

Microsporogenesis was investigated in hermaphroditic and male-sterile plants in nine gynodioecious taxa of Hawaiian Bidens. Normal microsporogenesis in hermaphrodites and the onset of abortion in male steriles were similar in all taxa and in a hybrid between two gynodioecious species. The early abnormal vacuolation of tapetal cells is the first visible evidence leading to premeiotic abortion of microsporogenesis in male steriles. The sporogenous cells disintegrate rapidly after the vacuolation of the tapetum, resulting in a shrunken, indehiscent anther which is composed of only the epidermal layer with some remnant cells of the endothecium and the connective at anthesis. In hermaphrodites, the tapetal cells remain dense and undergo karyokinesis to become binucleate during meiosis I. The tapetum becomes plasmodial after microspores are released from tetrads and gradually disappears during pollen formation. The genetic factor(s) which cause the abortion act with remarkable precision and consistency in all taxa investigated. This suggests that gynodioecy in all Hawaiian Bidens is homologous and the establishment of male sterility in Hawaiian Bidens occurred only once. The spread of the genetic male-sterile factor(s) may be the result of adaptive radiation of the original gynodioecious species or natural interspecific hybridization.     

Plasticity in sex allocation in the plant Mercurialis annua is greater for hermaphrodites sampled from dimorphic than from monomorphic populations

Plants are notoriously variable in gender, ranging in sex allocation from purely male through hermaphrodite to purely female. This variation can have both a genetic and an adaptive plastic component. In gynodioecious species, where females co‐occur with hermaphrodites, hermaphrodites tend to shift their allocation towards greater maleness when growing under low‐resource conditions, either as a result of hermaphrodites shifting away from an expensive female function, or because of enhanced siring advantages in the presence of females. Similarly, in the androdioecious plant Mercurialis annua, where hermaphrodites co‐exist with males, hermaphrodites also tend to enhance their relative male allocation under low‐resource conditions. Here, we ask whether this response differs between hermaphrodites that have been evolving in the presence of males, in a situation analogous to that supposed for gynodioecious populations, vs. those that have been evolving in their absence. We grew hermaphrodites of M. annua from populations in which males were either present or absent under different levels of nutrient availability and compared their reaction norms. We found that, overall, hermaphrodites from populations with males tended to be more female than those from populations lacking males. Importantly, hermaphrodites' investment in pollen and seed production was more plastic when they came from populations with males than without them, reducing their pollen production at low resource availability and increasing their seed production at high resource availability. These results are consistent with the hypothesis that plasticity in sex allocation is enhanced in hermaphrodites that have likely been exposed to variation in mating opportunities due to fluctuations in the frequency of co‐occurring males.     

The consequences of gynodioecy in natural populations of Thymus vulgaris L.

Summary Gene flow between the two sexual forms is asymmetrical in gynodioecious species: genes are transferred from male-fertile individuals (mF, hermaphrodites) to male-sterile individuals (mS, females) by pollen but from mS to MF individuals by diploid seeds. This situation is expected to cause differences in the genetic make up of mF and mS individuals. The expected differences were found in an experiment on a natural population of the gynodioecious species, Thymus vulgaris L. (thyme). The effectiveness of gynodioecy as a means of regulating heterozygosity and adaptability to various conditions of the environment is discussed.     

Gender variation and the evolution of dioecy in Wurmbea dioica (Liliaceae)

A major obstacle for empirical tests of hypotheses concerning the evolution of dioecy in flowering plants is the limited number of species that possess both cosexual and dioecious populations. Wurmbea dioica (Liliaceae) is a diminutive, fly-pollinated geophyte native to temperate Australia. Marked geographical variation of floral traits is evident, particularly with respect to sex expression. A survey of phenotypic gender in 45 populations from Western Australia (WA), South Australia (SA), Victoria (Vic) and the Australian Capital Territory (ACT) revealed two contrasting patterns. Populations in SA, Vic, and ACT were uniformly dimorphic for gender, containing female and male plants, whereas populations in WA were either monomorphic or dimorphic. In most dimorphic populations varying numbers of male plants produced hermaphrodite flowers (male inconstancy). There was a significant negative relationship between female frequency and the proportion of inconstant male plants. Depending on region and population, male plants produced more flowers of larger size than females. In WA monomorphic populations often occurred on rich, moist soils at high density, whereas dimorphic populations were more commonly found at lower density on shallow soils in drier areas. In an area of sympatry, plants with contrasting sexual systems flowered at different times and were ecologically differentiated. The patterns of gender variation in W. dioica indicate that dioecy has evolved via the gynodioecious pathway. The spread of females in monomorphic populations may be favoured where ecological conditions result in increased selfing and inbreeding depression in hermaphrodites.     

POPULATION SEX STRUCTURE AND REPRODUCTIVE FITNESS IN GYNODIOECIOUS SIDALCEA MALVIFLORA MALVIFLORA (MALVACEAE)

The spatial distribution of females and hermaphrodites within gynodioecious populations is expected to exert considerable selective pressure on gender fitness through pollen limitation of seed set. If pollen flow is predominantly local, seed set in individual plants may be sensitive to the proximity of pollen donors; pollen limitation of seed set may occur if hermaphrodites are locally rare. Under such circumstances, female fitness will be negatively frequency dependent and hermaphrodite fitness will be positively frequency dependent. Given local seed dispersal, a nonrandom clumped distribution of the genders is expected in gynodioecious populations due to the heritability of gender in gynodioecious species. If gender fitness is frequency dependent, such structure should favor hermaphrodites and select against females. To test this hypothesis, I quantified the distribution of the genders in terms of nearest neighbors and neighborhood sex ratio in two populations of gynodioecious Sidalcea malviflora malviflora. I then measured the effect of neighborhood sex ratio on open-pollinated seed set and pollen limitation in both manipulated and unmanipulated neighborhoods. Results indicate that the genders have a patchy distribution and that both genders are pollen limited and show an increase in seed set with an increase in neighborhood hermaphrodite frequency. The observed population sex structure favors hermaphrodites and disadvantages females. These results highlight the importance that population-level traits can have in determining individual fitness and the evolution of sex ratios in gynodioecious species.     

Maintenance of gynodioecy in Wurmbea biglandulosa (Colchicaceae): gender differences in seed production and progeny success

 In gynodioecious species, females contribute genes to future generations only through ovules, and to persist in populations they must have a compensatory advantage compared with hermaphrodites that reproduce via ovules and pollen. This compensation can result from greater fecundity and/or superior success of progeny from females. We examined differences in seed production and progeny success between females and hermaphrodites in the geophyte Wurmbea biglandulosa to explain the maintenance of females. Females produced more ovuliferous flowers and had more ovules per flower than did hermaphrodites but this did not necessarily result in greater fecundity, in part because seed production of females was pollen-limited. Over four years in one population, open-pollinated females produced 1.32 more seeds than open-pollinated hermaphrodites (range 1.09–1.63). In two other populations examined for one year only females produced 1.07 and 0.79 as many seeds as hermaphrodites. Seed production of open-pollinated females and hermaphrodites was only 55% and 73% that of cross-pollinated plants, respectively, indicating that both genders were pollen-limited but females more so than hermaphrodites. Open-pollinated seeds from females were 1.18–1.27 times more likely to germinate than seeds from hermaphrodites. No gender differences existed in seedling growth or survival. Hermaphrodites were self-compatible, but selfed seed set was only 80% that of crossed seed set. Crossed seed set of females and hermaphrodites did not differ. Assuming nuclear control of male sterility, relative female fitness is insufficient to maintain females at their current frequencies of 17%, and substantial female fitness advantages at later life-cycle stages are required. Received May 4, 2001 Accepted February 25, 2002     

Consequences of inbreeding for offspring fitness and gender in Silene vulgaris,a gynodioecious plant

Abstract In gynodioecious plants, hermaphrodite and female plants co‐occur in the same population. In these systems gender typically depends on whether a maternally inherited cytoplasmic male sterility factor (CMS) is counteracted by nuclear restorer alleles. These restorer alleles are often genetically dominant. Although plants of the female morph are obligatorily outcrossing, hermaphrodites may self. This selfing increases homozygosity and may thus have two effects: (1) it may decrease fitness (i.e. result in inbreeding depression) and (ii) it may increase homozygosity of the nuclear restorer alleles and therefore increase the production of females. This, in turn, enhances outcrossing in the following generation. In order to test the latter hypothesis, experimental crosses were conducted using individuals derived from four natural populations of Silene vulgaris, a gynodioecious plant. Treatments included self‐fertilization of hermaphrodites, outcrossing of hermaphrodites and females using pollen derived from the same source population as the pollen recipients, and outcrossing hermaphrodites and females using pollen derived from different source populations. Offspring were scored for seed germination, survivorship to flowering and gender. The products of self‐fertilization had reduced survivorship at both life stages when compared with the offspring of outcrossed hermaphrodites or females. In one population the fitness of offspring produced by within‐population outcrossing of females was significantly less than the fitness of offspring produced by crossing females with hermaphrodites from other populations. Self‐fertilization of hermaphrodites produced a smaller proportion of hermaphroditic offspring than did outcrossing hermaphrodites. Outcrossing females within populations produced a smaller proportion of hermaphrodite offspring than did crossing females with hermaphrodites from other populations. These results are consistent with a cytonuclear system of sex determination with dominant nuclear restorers, and are discussed with regard to how the mating system and the genetics of sex determination interact to influence the evolution of inbreeding depression.     

Parental effects and gender specialization in a tropical heterostylous shrub

Abstract.— Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense , thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene.     

On the gynodioecious polymorphism in Saxifraga granulata L. (Saxifragaceae)

Sexual and vegetative fitness components in hermaphrodite and female plants of the self-compatible, perennial herb Saxifraga granulata are compared using material derived from a gynodioecious population in northern England.
Females produced only 57% as many seeds as hermaphrodites, but their ovule offspring were 1.28 times as fit as those of hermaphrodites, and females were more vegetatively vigorous. The advantages to females in ovule offspring quality and in vegetative reproduction counteract their disadvantages in pollen and seed production and therefore probably play a role in the maintenance of the gynodioecious polymorphism. Pollination ecology, resource reallocation and inbreeding depression all appear to contribute to the observed sex differences in fitness.     

Floral dimorphism, pollination, and self-fertilization in gynodioecious GERANIUM RICHARDSONII (Geraniaceae)

The selective maintenance of gynodioecy depends on the relative fitness of the male-sterile (female) and hermaphroditic morphs. Females may compensate for their loss of male fitness by reallocating resources from male function (pollen production and pollinator attraction) to female function (seeds and fruits), thus increasing seed production. Females may also benefit from their inability to self-fertilize if selfing and inbreeding depression reduce seed quality in hermaphrodites. We investigated how differences in floral resource allocation (flower size) between female and hermaphroditic plants affect two measures of female reproductive success, pollinator visitation and pollen receipt, in gynodioecious populations of Geranium richardsonii in Colorado. Using emasculation treatments in natural populations, we further examined whether selfing by autogamy and geitonogamy comprises a significant proportion of pollen receipt by hermaphrodites. Flowers of female plants are significantly smaller than those of hermaphrodites. The reduction in allocation to pollinator-attracting structures (petals) is correlated with a significant reduction in pollinator visitation to female flowers in artificial arrays. The reduction in attractiveness is further manifested in significantly less pollen being deposited on the stigmas of female flowers in natural populations. Autogamy is rare in these protandrous flowers, and geitonogamy accounts for most of the difference in pollen receipt between hermaphrodites and females. Female success at receiving pollen was negatively frequency dependent on the relative frequency of females in populations. Thus, two of the prerequisites for the maintenance of females in gynodioecious populations, differences in resource allocation between floral morphs and high selfing rates in hermaphrodites, occur in G. richardsonii.     

Population Size,Pollination and Breeding System of Silene stockenii Chater (Caryophyllaceae), an Annual Gynodioecious Species of Southern Spain

The distribution area, phenology, sex polymorphism, floral characteristics and breeding system of Silene stockenii (Caryophyllaceae), a narrowly endemic annual species of southern Spain, were studied. Only five populations were found in a total area of 2 ha. Silene stockenii is a gynodioecious species with fully female, fully hermaphrodite and intermediate plants bearing hermaphrodite, male-sterile, and partially male-sterile flowers. Male-sterile flowers are typically smaller than hermaphrodites. Nectar production was significantly higher in hermaphrodite plants and during the female phase of hermaphrodite flowers. The red flowers appear during the spring (March-May) and are pollinated by long-tongued Bombyliidae. Hand pollinations revealed that the species is self-compatible; however, natural self-pollination is rare due to marked protandry. Hand pollination significantly increased the number of seeds per fruit and seed set, indicating limited pollination in the field. In controlled pollinations female plants of S. stockenii produced higher seed set than hermaphrodite plants, but in freely pollinated plants fruit set and seed production was similar in both morphs, indicating that pollinators do not discriminate in favour of hermaphrodite plants.     

Selection on floral and carbon uptake traits of Lobelia siphilitica is similar in females and hermaphrodites

Sexual dimorphism is common in plants and animals. Although this dimorphism is often assumed to be adaptive, natural selection has rarely been measured on sexually dimorphic traits of plants. We measured phenotypic selection via seed set on two floral and four carbon uptake traits of female and hermaphrodite Lobelia siphilitica. Because females can reproduce only via seeds, which are costlier than pollen, we predicted that females with smaller flowers and enhanced carbon uptake would have higher fitness, resulting in either sex morph-specific directional selection or stabilizing selection for different optimal trait values in females and hermaphrodites. We found that directional selection on one carbon uptake trait differed between females and hermaphrodites. We did not detect significant stabilizing selection on traits of either sex morph. Our results provide little support for the hypothesis that sexual dimorphism in gynodioecious plants evolved in response to sex morph-specific selection.     

ADVANTAGES OF BEING A SPECIALIST FEMALE IN NODIOECIOUS SILENE VULGARIS S.L. (CARYOPHYLLACEAE)

According to sex allocation theory, to maintain a mutant male-sterile plant in a population of hermaphrodites such a plant must compensate its loss of fitness caused by inhibition of pollen production with a higher reproductive success through its female function. In the present study of a gynodioecious population of Silene vulgaris (Caryophyllaceae) I show that hermaphrodites not only benefit from outcrossing, in that progeny from outcrossed flowers are more vigorous than those from selfed flowers within an individual plant, but they also suffer heavily from self-pollination between different flowers of the same individuals, which could be demonstrated in experimentally made male-sterile (emasculated) individuals. Seeds from the emasculation period were heavier and germinated better than when the same individual was an intact hermaphrodite. Naturally male-sterile (female) individuals produced more fruits due to flowers staying open longer for pollen to arrive via some vector. However, the higher seed number alone could not provide the fitness advantage needed for females to be maintained in the population, but females also produced heavier seeds as compared to the hermaphrodites. Differences in seed survival and seedling establishment in the field are expected to add the advantages necessary for female plants to be selectively plausible.