An experimental test of the effects of resources and sex ratio on maternal fitness and phenotypic selection in gynodioecious Fragaria virginiana

Resources, sex ratio, and seed production by hermaphrodites covary among natural populations of many gynodioecious plant species, such that they are functionally "more dioecious" as resources become more limiting. Strong correlations among these three factors confound our understanding of their relative roles in maintaining polymorphic sexual systems. We manipulated resource availability and sex ratio and measured their effects on relative fertility and phenotypic selection through the maternal fitness of females and hermaphrodites of Fragaria virginiana. Two results were particularly surprising. First, hermaphrodites showed little variability in fecundity across resource treatments and showed strong positive and context-dependent selection for fruit set. This suggests that variation in hermaphrodite seed production along resource gradients in nature may result from adaptation rather than plasticity. Second, although females increased their fecundity with higher resources, their fertility was unaffected by sex ratio, which is predicted to mediate pollen limitation of females in natural populations where they are common. Selection on petal size of females was also weak, indicating a minimal effect of pollinator attraction on variation in the fertility of female plants. Hence, we found no mechanistic explanation for the complete absence of high-resource high female populations in nature. Despite strong selection for increased fruit set of hermaphrodites, both the strength of selection and its contribution to the maintenance of gynodioecy are severely reduced under conditions where females have high relative fecundity (i.e., low resources and high-female sex ratios). High relative fertility plus high female frequency means that the evolution of phenotypic traits in hermaphrodites (i.e., response to selection via seed function) should be manifested through females because most hermaphrodites will have female mothers. Fruit set was never under strong selection in females; hence, selection to increase fruit set hermaphrodites will be less effective in maintaining their fruiting ability in natural populations with low resources and high female frequency. In sum, both sex ratio and resource availability influence trait evolution indirectly-through their effects on relative fertility of the sexes and patterns of selection. Sex ratio did not impose strong pollen limitation on females but did directly moderate the outcome of natural selection by biasing the maternal sex of the next generation. This direct effect of sex ratio on the manifestation of natural selection is expected to have far greater impact on the evolution of traits, such as seed-producing ability in hermaphrodites and the maintenance of sexual polymorphisms in nature, compared to indirect effects of sex ratio on relative fertility of the sexes.     

Female frequency and relative fitness of females and hermaphrodites in gynodioecious Geranium sylvaticum (Geraniaceae)

We determined female frequency of 23 populations of the gynodioecious Geranium sylvaticum (Geraniaceae) in Finland. We compared our results to previous results on this species from the 1960s in order to reveal putative changes in female frequencies. Because females may be maintained in gynodioecious populations if their seed production or offspring quality is higher than that of hermaphrodites, we explored reproductive success of females and hermaphrodites in detail in 11 populations for two consecutive years. Female frequencies varied from 0.4 to 27.2%; this variation is similar to that observed in the 1960s. Contrary to previous results that indicated lower seed production in females, females produced 1.2 and 1.7 times more seeds per flower than hermaphrodites in 2000 and 2001, respectively. Females also had higher fruit set than hermaphrodites. Thus, higher seed production of females partly explains the maintenance of gynodioecy in this species. Furthermore, female frequency correlated negatively with relative seed fitness of hermaphrodites suggesting that relative seed fitness is related to population sex ratio. Female frequency and the distance of the population from the most southern population also tended to correlate positively, suggesting that harsher environmental conditions in the north may benefit female plants. Given the observed yearly variation, our results also highlight the importance of temporal variation for the relative seed fitness of females and hermaphrodites.     

Floral sex ratios, disease and seed set in dioecious Silene dioica

1 In the dioecious, perennial herb Silene dioica , the density of pollen donors in a population is determined by overall plant density, the sex ratio and the proportion of plants infected with the anther-smut fungus Microbotryum violaceum , which results in permanent sterility of both male and female plants.
2 Pollinators ( Bombus spp.) were found to prefer male flowers and to avoid diseased flowers. This may result in an overall lower visitation frequency and increased risk for pollen limitation in populations with a low density of males or a high incidence of disease.
3 Compared with open-pollinated flowers, hand pollination resulted in a significant increase in the number of seeds produced per fruit in populations with an experimentally reduced proportion of males (25% and 50% male flowers) but not in a naturally male-dominated population (75% male flowers). Seed production per plant was increased by hand pollination only in the most female-dominated population. Because the floral sex ratio is often male-biased, resources rather than pollen availability are likely to set the upper limit for total seed production per individual in most healthy populations of S. dioica.
4 There was a negative relationship between seed set and incidence of disease across 22 populations in both years of a field study. However, there was no consistent difference between the responses of highly diseased populations (incidence 30–56%) and populations with a low disease incidence (incidence 0–8%) to hand pollination.
5 In a greenhouse experiment with cloned hand-pollinated females, the presence of spores on healthy flowers was found to reduce seed set significantly. In highly diseased populations, therefore, the frequent deposition of spores by flower visitors onto remaining healthy plants may decrease seed production below the potential level determined by resources or pollen availability.     

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.     

Population size, female fecundity, and sex ratio variation in gynodioecious Plantago maritima

Theory predicts that the sex ratio of gynodioecious populations (in which hermaphrodites and females coexist) will be affected by the relative female fitness of females and hermaphrodites, and by founder events and genetic drift in small populations. We documented the sex ratio and size of 104 populations of the gynodioecious, perennial herb Plantago maritima in four archipelagos in eastern Sweden and western Finland (from latitude 53 to 64 degrees N). The sex ratio varied significantly both among and within archipelagos (range 0-70% females, median 6.3% females). The frequency of females was highest in the northernmost archipelago and lowest in the southernmost archipelago. As predicted, females were more frequently missing from small than from large populations, and the variance in sex ratio increased with decreasing population size. The relative fecundity of female plants (mean seed output per female/mean seed output per hermaphrodite) ranged from 0.43 to 2.16 (median 1.01, n = 12 populations). Among the 12 populations sampled for seed production (four in each of three archipelagos), the frequency of females was positively related to relative fecundity of females and negatively related to population size. The results suggest that the local sex ratio is influenced both by the relative fecundity of females and hermaphrodites and by stochastic processes in small populations.     

Founder effects and sex ratio in the gynodioecious Thymus vulgaris L.

Thymus vulgaris is a gynodioecious species (in which females and hermaphrodites coexist) with a highly variable frequency of females among natural populations (5–95%) and a high average female frequency (60%). Sex determination involves both cytoplasmic genes responsible for male sterility, i.e. the female phenotype, and specific nuclear factors responsible for the restoration of male fertility, and thus a hermaphrodite phenotype. In this study, molecular markers of the mitochondrial genome have been used to quantify the cytoplasmic diversity in 11 clumps of individuals observed in four recently founded populations. The very low diversity within patches in conjunction with the strong diversity among patches strongly suggests that clumps of individuals are the result of single matrilinear families. In clumps that contain mainly females, all the analysed females showed the same cytoplasmic pattern. This pattern differed from that shown by neighbouring hermaphrodites, indicating that the determination of sex is locally cytoplasmic. A comparison of genetic diversity before and after fire in one population showed that disturbances may cause a reduction in genetic diversity and a concurrent induction of local cytoplasmic determination of sex. Such cytoplasmic determination of sex in colonizing populations, together with the greater seed set of females, may largely improve the colonizing ability of the species.     

Exploring the genetic basis and proximate causes of female fertility advantage in gynodioecious Thymus vulgaris

In many gynodioecous species, females produce more viable seeds than hermaphrodites. Knowledge of the relative contribution of inbreeding depression in hermaphrodites and maternal sex effects to the female fertility advantage and the genetic basis of variation in female fertility advantage is central to our understanding of the evolution of gender specialization. In this study we examine the relative contribution of inbreeding and maternal sex to the female fertility advantage in gynodioecious Thymus vulgaris and quantify whether there is genetically based variation in female fertility advantage for plants from four populations. Following controlled self and outcross (sib, within-population, and between-population) pollination, females had a more than twofold fertility advantage (based on the number of germinating seeds per fruit), regardless of the population of origin and the type of pollination. Inbreeding depression on viable seed production by hermaphrodites occurred in two populations, where inbreeding had been previously detected. Biparental inbreeding depression on viable seed production occurred in three of four populations for females, but in only one population for hermaphrodites. Whereas the maternal sex effect may consistently enhance female fertility advantage, inbreeding effects may be limited to particular population contexts where inbreeding may occur. A significant family x maternal sex interaction effect on viable seed production was observed, illustrating that the extent of female fertility advantage varies significantly among families. This result is due to greater variation in hermaphrodite (relative to female) seed fertility between families. Despite this genetic variation in female fertility advantage and the highly female biased sex ratios in populations of T. vulgaris, gynodioecy is a stable polymorphism, suggesting that strong genetic and/or ecological constraints influence the stability of this polymorphism.     

Two's company, three's a crowd: experimental evaluation of the evolutionary maintenance of trioecy in Mercurialis annua (Euphorbiaceae)

Trioecy is an uncommon sexual system in which males, females, and hermaphrodites co-occur as three clearly different gender classes. The evolutionary stability of trioecy is unclear, but would depend on factors such as hermaphroditic sex allocation and rates of outcrossing vs. selfing. Here, trioecious populations of Mercurialis annua are described for the first time. We examined the frequencies of females, males and hermaphrodites across ten natural populations and evaluated the association between the frequency of females and plant densities. Previous studies have shown that selfing rates in this species are density-dependent and are reduced in the presence of males, which produce substantially more pollen than hermaphrodites. Accordingly, we examined the evolutionary stability of trioecy using an experiment in which we (a) indirectly manipulated selfing rates by altering plant densities and the frequency of males in a fully factorial manner across 20 experimental plots and (b) examined the effect of these manipulations on the frequency of the three sex phenotypes in the next generation of plants. In the parental generation, we measured the seed and pollen allocations of hermaphrodites and compared them with allocations by unisexual plants. In natural populations, females occurred at higher frequencies in denser patches, a finding consistent with our expectations. Under our experimental conditions, however, no combination of plant densities and male frequencies was associated with increased frequencies of females. Our results suggest that the factors that regulate female frequencies in trioecious populations of M. annua are independent of those regulating male frequencies (density), and that the stable co-existence of all three sex phenotypes within populations is unlikely.     

Effect of ascochyta blight (Mycosphaerella pinodes) on yield components of single pea (Pisum sativum) plants under field conditions

Field studies were made in 1992 and 1993 to examine the yield components of pea inoculated with Mycosphaerella pinodes and those of healthy pea (sprayed with a mixture of flutriafol + chlorothalonil), in a split-plot design with the cv. Solara sown at different plant densities. Ascochyta blight was severe on leaves and on internodes of the basal part of the plants; pods had few lesions. The number and length of stems per plant were the same for diseased and healthy plants. The number of reproductive nodes and pods per stem were affected by disease only in 1993. In 1992 and 1993 respectively, disease caused reductions in the number of seeds per stem of 18% and 25%, and in seed size of 13.5% and 16.7%, compared with healthy plants. The harvest index and total biomass were lower in diseased than in healthy plants and seed yield was reduced by 40% in diseased plots. These results show a high relationship between the disease parameters (disease mean on stipules/nodes 8–18/ and on internodes/nodes 5–15/, percentages of stipules or internodes with a disease score 4, and percentage of stems encircled by lesions), plant density and yield reduction.     

GENDER VARIATION AND SEXUAL DIFFERENCES IN REPRODUCTIVE CHARACTERS AND SEED PRODUCTION IN GYNODIOECIOUS GERANIUM MACULATUM

Variation in sex expression, flowering pattern, and seed production was studied in the self-compatible perennial herb Geranium maculatum in Illinois and Indiana. In a survey of eight populations, female (male-sterile) plants were found in seven (frequencies ranging from 0.5% to 24.3% [median 4.2%]), and intermediate plants (with partly reduced male function) were found in all populations. Gender variation and sexual differences in reproductive characters were studied in detail in two populations. One population consisted of 5% female, 27% intermediate, and 68% hermaphrodite plants; the other consisted of 1% female, 20% intermediate, and 79% hermaphrodite plants. Females produced smaller flowers and began flowering earlier than hermaphrodites. Intermediates produced flowers of an intermediate size and began flowering as early as females. Females and hermaphrodites did not differ in flower number, vegetative size, flowering frequency, survival, or seed size. However, females produced 1.6 times more seeds than hermaphrodites. Intermediates produced 1.3–1.6 times more seeds than hermaphrodites. Some between-year variation in sex expression was observed. Hand-pollination with outcross pollen produced two to four times as many seeds as hand-pollination with self-pollen. A lower outcrossing rate in hermaphrodites than in females may at least partly explain the lower seed set in hermaphrodites. The higher seed production of females, and possibly the high fecundity of the intermediates, should contribute to the maintenance of this sexual polymorphism.     

Sex allocation of females and hermaphrodites in the gynodioecious Geranium sylvaticum

Seed production and patterns of sex allocation were studied in female and hermaphroditic plants in two gynodioecious populations of Geranium sylvaticum (Geraniaceae). Females produced more flower buds and seeds than hermaphrodites in one of the two study populations. The other female traits measured (pistil biomass, seed number per fruit, individual seed mass) did not differ between the gender morphs. The relative seed fitness of hermaphrodites differed between the study populations, with hermaphrodites gaining less of their fitness through female function in the population with a high frequency of females. However, the amount and size of pollen produced by hermaphrodites did not differ between populations. The number of flower buds was positively correlated with seed production in females, whereas in hermaphrodites a positive correlation between number of buds and seed production was found in only one of the two study populations. These results suggest that fitness gain through female function is labile in hermaphrodites of this species, and is probably affected by environmental factors such as the sex ratio of the population.     

ECOLOGICAL GENETICS OF GYNODIOECY IN SILENE VULGARIS: RELATIVE FITNESS OF FEMALES AND HERMAPHRODITES DURING THE COLONIZATION PROCESS

Recent theoretical models have addressed the influence of metapopulation dynamics on the fitness of females and hermaphrodites in gynodioecious plants. In particular, selection is thought to favor hermaphrodites during population establishment because that sex should be less prone to pollen limitation, especially if self-fertilization is possible. However, inbreeding depression could limit this advantage. In this experimental study of Silene vulgaris, a weedy gynodioecious plant, the fitness of females and hermaphrodites was estimated from seed production in both mixed-sex populations and for individuals isolated from these populations by 20, 40, 80, or 160 m. In mixed populations females display statistically significant greater per capita seed production owing to higher capsule production and higher rates of seed germination. The fitness of both sexes declines with increasing isolation, but at different rates, such that in the 160-m treatment hermaphrodites are by far the more fit sex. Allozyme studies suggest that this differential decline is because the selfing rate in hermaphrodites increases as a function of isolation, at least partially compensating for a decline in the availability of outcross pollen. Overall, the negative effects of pollen limitation on females far outweighs the negative effects of inbreeding depression following selfing in hermaphrodites. Thus, extinction/recolonization dynamics would appear to favor hermaphrodites as long as seed dispersal events exceed some critical distance.     

Fine scale spatial structuring of sex and mitochondria in Silene vulgaris

Fine scale spatial structure (FSSS) of cytoplasmic genes in plants is thought to be generated via founder events and can be amplified when seeds germinate close to their mother. In gynodioecious species these processes are expected to generate FSSS in sex ratio because maternally inherited cytoplasmic male sterility genes partially influence sex expression. Here we document a striking example of FSSS in both mitochondrial genetic markers and sex in roadside populations of Silene vulgaris. We show that in one population FSSS of sexes influences relative fruit production of females compared to hermaphrodites. Furthermore, FSSS in sex ratio is expected to persist into future generations because offspring sex ratios from females are female-biased whereas offspring sex ratios from hermaphrodites are hermaphrodite-biased. Earlier studies indicated that pollen limitation is the most likely mechanism underlying negative frequency dependent fitness of females. Our results support the theoretical predictions that FSSS in sex ratio can reduce female fitness by decreasing the frequency at which females experience hermaphrodites. We argue that the influence of FSSS on female fitness is complementary to the influence of larger scale population structure on female fitness, and that population structure at both scales will act to decrease female frequencies in gynodioecious species. Better comprehension of the spatial structure of genders and genes controlling sex expression at a local scale is required for future progress toward understanding sex ratio evolution in gynodioecious plants.     

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     

Sex expression,female frequency,and reproductive output in a gynodioecious clonal herb, <Emphasis Type="Italic">Glechoma longituba</Emphasis> (Lamiaceae)

We investigated the sex morphs and frequency of females in 24 natural populations of the clonal, gynodioecious species Glechoma longituba. In addition to the presence of hermaphrodites and females, there were some individuals with intermediate sex expression that could not be categorized as either morph. The majority of populations contained a predominance of hermaphrodites, although female frequency varied from 0 to 100%. The sexual expression of individual ramets did not change over three consecutive years or when ramets were transplanted to different populations, but it did change over the course of a flowering season. Seeds from females weighed more than those from hermaphrodites. The seed set of hermaphrodites was relatively constant across populations; the seed set of females was higher than that of hermaphrodites in most populations, but lower in populations in which females occurred at a very high frequency because of pollen limitation. This led to a negative correlation being detected between the seed set of females and the frequency of females in a population. We postulated that the adjustment of sex expression within seasons could be an adaptive mechanism of reproductive assurance in this system.     

EVOLUTION OF UNISEXUALITY IN THE HAWAIIAN FLORA: A TEST OF MICROEVOLUTIONARY THEORY

The evolution of separate sexes as a means of avoiding self-fertilization requires the controversial coexistence of large inbreeding depression and high selfing rate in the ancestral hermaphrodite population. Fitness components of adult females and hermaphrodites in nature, of their open-pollinated progeny, and of experimental selfs and outcrosses onto hermaphrodites were compared in endemic Hawaiian Bidens sandvicensis, all of whose known populations are gynodioecious, consisting of a mixture of females and hermaphrodites. Multilocus selfing rates of hermaphrodites were also estimated, and sex morph ratio monitored over four seasons in three populations of B. sandvicensis and one population of gynodioecious B. cervicata. Total mean inbreeding depression in seed set (in the glasshouse), germination rate (in an open-air nursery on Kauai), and first year survivorship and fecundity in the field were estimated as 0.94 (SE 0.04), and occurred primarily in drought months. Lower survivorship and fecundity of selfs were partially explained by their consistently smaller size. Open-pollinated seed of females had significantly lower germination rate, proportion flowering, and fecundity than outcrossed progeny of hermaphrodites, suggesting moderate biparental inbreeding in females and a lack of any non-outcrossing advantage to progeny of females. In all fitness components, open-pollinated progeny of hermaphrodites were inferior to those of females and to outcrosses, and in most components were superior to selfs. Total performance of open-pollinated progeny of females relative to those of hermaphrodites was calculated as 2.3 (SE = 0.4), but since inflorescences of females also set 20% to 50% more seed than those of hermaphrodites, their total relative ovule success was estimated as 3.2 (SE = 0.5). If inheritance of male sterility is nuclear, this superiority is sufficient to maintain females in frequencies over 20% in populations, whose actual frequencies ranged from 14% to 33%. In four populations, selfing rates of hermaphrodites, assayed in seedlings, were 0.50, 0.45, 0.25, and 0.30, but since substantial inbreeding depression occurred prior to germination, the mean selfing rate of hermaphrodite ovules exceeded 0.57. Female frequencies were significantly higher in the two populations with higher hermaphrodite selfing rate. These results suggest that inbreeding depression can exert a profound influence on the mating system of self-compatible plants on Hawaii and perhaps other oceanic islands, and can be sufficiently strong to electively favor the elimination of the male function.     

Pollination success across an elevation and sex ratio gradient in gynodioecious Daphne laureola

Gynodioecy is a dimorphic breeding system in which hermaphrodite and female individuals coexist in populations. Sex ratio and gender-relative lifetime seed production determine the stability of gynodioecy, and both genetic and ecological factors may influence these parameters. I analyzed the consequences of variation in population sex ratio and site elevation for the relative pollination success of female and hermaphrodite individuals of Daphne laureola in southern Spain, where previous studies failed to detect female fecundity advantages at two mid-elevation sites. Pollination success, estimated as stigmatic pollen loads, number of pollen tubes per style, and percentage of fertilized flowers, was higher for hermaphrodites than females in populations with 20-56% females. Furthermore, female quantitative disadvantage in pollination success increased with elevation, suggesting that the higher availability of pollen due to the increased proportion of hermaphrodites could not mitigate the negative effect that other factors associated with elevation apparently had on pollination. Supplemental hand pollinations showed that female seed production was pollen limited in populations with a proportion of females >50%, although both pollination success and natural fruit set of females in these sites were the highest recorded.     

Determinants of sex allocation in a gynodioecious wild strawberry: implications for the evolution of dioecy and sexual dimorphism

One evolutionary pathway from plants with combined male and female functions (hermaphroditism) to those with separate sexes (dioecy) involves females coexisting with hermaphrodites (gynodioecy). The research presented here explores sex allocation in Fragaria virginiana (a gynodioecious wild strawberry), within the context of theory on the gynodioecy–dioecy transition. By growing clonally replicated plants in the greenhouse and surveying six populations in situ, I evaluated the effects of plant size, genotype, sexual identity, population of origin and female frequency on sex allocation. I found significant positive effects of plant size on most sex allocation traits studied. In addition to strong sex-specific allocation patterns, I found significant broad-sense heritabilities for all traits, suggesting that plants could respond to selection. Moreover, there was a negative genetic correlation between pollen production and fruit set per flower within hermaphrodites, lending support to a basic assumption of sex allocation theory. On the other hand, several sex allocation traits, namely pollen and ovules per flower in hermaphrodites, were positively genetically correlated, suggesting that they may act to constrain the evolution of sexual dimorphism. Populations differed in the frequency of females, and females were more prevalent on sites with lower soil moisture and where hermaphrodites were least likely to produce fruit, suggesting that females’ seed fitness relative to that of hermaphrodites may be strongly environment-dependent in this species.     

When is it worth being a self-compatible hermaphrodite? Context-dependent effects of self-pollination on female advantage in gynodioecious Silene nutans

In gynodioecious plant species with nuclear‐cytoplasmic sex determination, females and hermaphrodites plants can coexist whenever female have higher seed fitness than hermaphrodites. Although the effect of self fertilization on seed fitness in hermaphrodites has been considered theoretically, this effect is far from intuitive, because it can either increase the relative seed fitness of the females (if it leads hermaphrodites to produce inbred, low quality offspring) or decrease it (if it provides reproductive assurance to hermaphrodites). Hence, empirical investigation is needed to document whether relative seed fitness varies with whether pollen is or is not limiting to seed production. In the current study, we measured fruit set and seed production in both females and hermaphrodites and the selfing rate in hermaphrodites in two experimental patches that differed in sex ratios in the gynodioecious plant Silene nutans. We found an impact of plant gender, patch, and their interaction, with females suffering from stronger pollen limitation when locally frequent. In the most pollen‐limited situation, the selfing rate of hermaphrodites increased and provided hermaphrodites with a type of reproductive assurance that is not available to females. By integrating both the beneficial (reproductive assurance) and costly effects (through inbreeding depression) of self‐pollination, we showed that whether females did or did not exhibit higher seed fitness depended on the degree of pollen limitation on seed production.     

Sex‐specific reproductive components and pollination ecology in the subdioecious shrub Fuchsia microphylla

In subdioecious populations, functional female, male and hermaphrodite individuals coexist. Subdioecy may be a transitional state towards dioecy or a breakdown of dioecy, although lability in sex expression may maintain subdioecy as a stable condition. To better understand the ecological aspects involved in sex ratio dynamics and breeding system evolution, we studied the pollination and female fitness components of female and hermaphrodite individuals of the subdioecious shrub Fuchsia microphylla. In two natural populations at the Trans‐Mexican Volcanic Belt we estimated female frequency and several reproductive components of female and hermaphrodite plants under natural pollination and experimental pollination treatments. Average female frequency was 42%, and on average, 42.5% of hermaphrodites produced fruits. Female plants showed a 17‐fold female fertility advantage over hermaphrodites through increased fruit production, as the number of seeds and germination rates did not differ between morphs. Hermaphrodite flowers were larger, with similar nectar production and concentration to female flowers, and pollinators did not show consistent morph preferences. Some hermaphrodites produced fruits autonomously, and female flowers excluded from pollinators produced fruits putatively by apomixis. Fruit production in hermaphrodites, but not in females, was related to height, suggesting increased investment of hermaphrodites in the female function at higher resource status. For sex ratios to be at equilibrium, the female fertility advantage should be reduced about eightfold. However, it may be that hermaphrodites are maintained by producing fruits at no cost to the male function at higher resource status, as the gender plasticity hypothesis proposes.