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.     

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.     

Pollen and resource limitation in a gynodioecious species

Differences between plant sex morphs in pollen or resource availability may affect their relative fitness and thereby the sex ratio of dimorphic species. In gynodioecious species, in which hermaphroditic and female plants coexist, a variety of factors (e.g., hermaphrodite self-fertility or rarity or pollinator discrimination against females) might be expected to lead to stronger pollen limitation in females than in hermaphrodites. On the other hand, females have been found to be superior compared to hermaphrodites in low-nutrient conditions. The effects of supplemental hand-pollination and resource addition on the reproductive output of the self-fertile gynodioecious perennial Geranium sylvaticum (Geraniaceae) were tested for several populations that differ in their female frequency (4.4-23.0%). Both pollen and resource availability limited fruit set and the number of seeds produced per plant; however, seed set (i.e., the number of seeds produced per fruit) was limited only by resources. Because pollen limitation in females did not correlate with female frequency, our results suggest that pollen limitation in females does not depend on the frequency of the pollen-producing hermaphrodites. Furthermore, because pollen and resource availability limited reproductive output of both sex morphs, these factors may not contribute significantly to maintenance and evolution of gynodioecy in G. sylvaticum.     

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.     

Dry environments promote the establishment of females in monomorphic populations of <Emphasis Type="Italic">Wurmbea biglandulosa</Emphasis> (Colchicaceae)

In flowering plants, the evolution of dimorphic breeding systems from monomorphic ancestors can be associated with dry environments. One hypothesis to explain this pattern is that seed fertility of hermaphrodites decreases more than seed fertility of females under dry conditions, so that females have greater relative fitness. This could occur if seed production of hermaphrodites is more resource-limited than that of females, or shifts in pollination increase levels of selfing and inbreeding depression in hermaphrodites. Here we assess the role of dry environments in promoting a female fitness advantage in Wurmbea biglandulosa by focusing on monomorphic and dimorphic populations that occur along a longitudinal gradient of decreasing rainfall. Dimorphic populations occurred in sites with higher temperatures, lower rainfall and lower soil moisture. Overall, females had greater seed fertility than did hermaphrodites from monomorphic populations, which in turn had greater seed fertility than hermaphrodites from dimorphic populations. Ovuliferous flower and ovule production by the three gender morphs and seed fertility of females and hermaphrodites in monomorphic populations did not vary with soil moisture. By contrast, seed fertility of hermaphrodites in dimorphic populations was positively related to soil moisture. Accordingly, female frequency was higher in those sites where hermaphrodites produced relatively fewer seeds. Taken together our results indicate that dry environments promote the establishment of females by decreasing the relative seed fitness of hermaphrodites. Moreover, because seed fertility of hermaphrodites in monomorphic populations did not vary with soil moisture, resource limitation of female function may play only a minor role in the establishment of females. Other factors such as shifts in pollination and mating patterns of hermaphrodites could be involved. Key words:breeding system evolution, environmental stress, gender dimorphism, gynodioecy, sex ratio variationCo-ordinating editor: J.F. Stuefer     

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.     

Evidence for effects of restorer genes on male and female reproductive functions of hermaphrodites in the gynodioecious species Thymus vulgaris L.

In Thymus vulgaris L., sex determination involves both the nuclear and the cytoplasmic genomes: the cytoplasm is responsible for male-sterility (the female phenotype) while specific nuclear genes may restore male fertility (the hermaphrodite phenotype). Previous observations have shown high variation among hermaphrodites for pollen and seed production. In order to investigate the origin of this variation, 12 female plants, four from each of three populations, were hand-pollinated with pollen from hermaphrodites from three different paternal populations. The sex-ratio (i.e. the frequency of hermaphrodites) produced and the reproductive functions of these offspring were measured. A strong positive correlation was observed between the sex-ratio within a family and both female and male reproductive functions of its hermaphrodites. No such correlation was found for females. This result suggests that restorer genes may be directly or indirectly involved both in sex determination and in the efficiency of resource allocation to reproductive functions. As a consequence, female advantage, i.e. the relative fecundity of females to hermaphrodites, is larger in families with low sex-ratio, and this might affect the evolution of this gynodioecious breeding system.     

Soil moisture and sex ratio in a plant with nuclear-cytoplasmic sex inheritance

I investigated whether soil moisture affects relative fitness of females and hermaphrodites and sex ratio in a gynodioecious plant with nuclear-cytoplasmic sex inheritance. I contrast these results with those from species with strictly nuclear sex inheritance. I performed a manipulative watering experiment on seed fitness of the two sexes, and field studies measuring seed fitness and sex ratio as a function of soil moisture. In the dry site, watered hermaphrodites produced approximately twice as many seeds as unwatered hermaphrodites, with little treatment effect on female seed production. Over a natural soil moisture gradient, the ratio of female to hermaphrodite seed production was higher in dry than in wet sites. These data show that the seed fitness advantage of females is a function of soil moisture. Despite this, regression of soil moisture on the sex ratio of 23 populations was not significant. These results indicate a sex-dependent effect of soil moisture on resource allocation to seeds that does not translate into a strong effect on sex ratio. This is consistent with theory based on genomic conflict in which sex ratios are predicted to be only partly determined by fitness differences of the sexes.     

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.     

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.     

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.     

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     

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.     

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.     

Mating system, sex ratio, and persistence of females in the gynodioecious shrub Daphne laureola L. (Thymelaeaceae)

Although in gynodioecious populations male steriles require a fecundity advantage to compensate for their gametic disadvantage, southern Spanish populations of the long-lived shrub Daphne laureola do not show any fecundity advantage over hermaphrodites in terms of seed production and early seedling establishment. By using allozyme markers, we assess the mating system of this species in five populations differing in sex ratio, and infer levels of inbreeding depression over the whole life cycle by comparing the inbreeding coefficients at the seed and adult plant stages. Extremely low outcrossing rates (0.001相似文献   

RESOURCE COMPENSATION AND THE EVOLUTION OF GYNODIOECY IN PHACELIA LINEARIS (HYDROPHYLLACEAE)

Gynodioecy is a dimorphic breeding system in which hermaphrodite and female individuals coexist in populations. Theoretical models have shown that if nuclear genes control sex expression, then gynodioecy can evolve only when females have large advantages in one or more fitness components. These female advantages must be large enough that females' expected lifetime production of viable seeds is more than twice that of hermaphrodites. Previous studies have found that cytoplasmic inheritance and/or a large offspring-vigor advantage of females (caused by hermaphrodite self-pollination and inbreeding depression of selfed seeds) account for this breeding system's evolution. This paper reports studies of gynodioecy in Phacelia linearis, an insect-pollinated annual plant in which gender inheritance appears to be nuclear. Twenty-six P. linearis populations surveyed in northern Utah, USA, contain a majority of perfect-flowered hermaphrodites, but most (22) also contain male-sterile individuals (females), at frequencies of up to 0.16. The hermaphrodite selfing rate is low (0.00–0.20 in four populations). Maternal gender does not consistently affect components of offspring vigor, such as seed size, germination rate, seedling survivorship, and vegetative size. Plants of the two genders do not differ in number of seeds per fruit or mean seed mass. Females produce significantly more fruits and seeds than hermaphrodites in natural populations. The ratio of the mean lifetime seed production of females to the mean lifetime seed production of hermaphrodites ranged from 1.31 to 2.52 in six natural populations. Females have greater shoot biomass than hermaphrodites and produce more seeds at any given shoot biomass than hermaphrodites, suggesting that their seed-production advantage arises from gender-specific patterns of resource allocation to growth and reproduction. The gender difference in plant size varies across environments and across genetic backgrounds. In this species nuclear gynodioecy appears to be evolutionarily stable mainly because of resource compensation by females, without a large outcrossing advantage of females.     

Male‐biased hermaphrodites in a gynodioecious shrub,Daphne jezoensis

Gynodioecy, a state where female and hermaphrodite plants coexist in populations, has been widely proposed an intermediate stage in the evolutionary pathway from hermaphroditism to dioecy. In the gynodioecy–dioecy pathway, hermaphrodites may gain most of their fitness through male function once females invade populations. To test this prediction, comprehensive studies on sex ratio variation across populations and reproductive characteristics of hermaphrodite and female phenotypes are necessary. This study examined the variation in sex ratio, sex expression, flower and fruit production and sexual dimorphism of morphological traits in a gynodioecious shrub, Daphne jezoensis, over multiple populations and years. Population sex ratio (hermaphrodite:female) was close to 1:1 or slightly hermaphrodite‐biased. Sex type of individual plants was largely fixed, but 15% of plants changed their sex during a 6‐year census. Hermaphrodite plants produced larger flowers and invested 2.5 times more resources in flower production than female plants, but they exhibited remarkably low fruit set (proportion of flowers setting fruits). Female plants produced six times more fruits than hermaphrodite plants. Low fruiting ability of hermaphrodite plants was retained even when hand‐pollination was performed. Fruit production of female plants was restricted by pollen limitation under natural conditions, irrespective of high potential fecundity, and this minimised the difference in resources allocated to reproduction between the sexes. Negative effects of previous flower and fruit production on current reproduction were not apparent in both sexes. This study suggests that gynodioecy in this species is functionally close to a dioecious mating system: smaller flower production with larger fruiting ability in female plants, and larger flower production with little fruiting ability in hermaphrodite plants.     

Density-dependent regulation of the sex ratio in an annual plant

Sex ratios are subject to strong frequency-dependent selection regulated by the mating system and the relative male versus female investment. In androdioecious plant populations, where males co-occur with hermaphrodites, the sex ratio depends on the rate of self-fertilization by hermaphrodites and on the relative pollen production of males versus hermaphrodites. Here, we report evolutionary changes in the sex ratio from experimental mating arrays of the androdioecious plant Mercurialis annua. We found that the progeny sex ratio depended strongly on density, with fewer males in the progeny of plants grown under low density. This occurred in part because of a plastic adjustment in pollen production by hermaphrodites, which produced more pollen when grown at low density than at high density. Our results provide support for the prediction that environmental conditions govern sex ratios through their effects on the relative fertility of unisexual versus hermaphrodite individuals.     

Associations among cytoplasmic molecular markers,gender, and components of fitness in Silene vulgaris,a gynodioecious plant

It has been suggested that the dynamics of chloroplast DNA (cpDNA) or mitochondrial DNA (mtDNA) genetic markers used in studies of plant populations could be influenced by natural selection acting elsewhere in the genome. This could be particularly true in gynodioecious plants if cpDNA or mtDNA genetic markers are in gametic disequilibrium with genes responsible for sex expression. In order to investigate this possibility, a natural population of the gynodioecious plant Silene vulgaris was used to study associations among mtDNA haplotype, cpDNA haplotype, sex and some components of fitness through seed. Individuals were sampled for mtDNA and cpDNA haplotype as determined using restriction fragment length polymorphism (RFLP) methods, sex (female or hermaphrodite), fruit number, fruit set, seeds/fruit and seed germination. The sex of surviving germinating seeds was also noted. All individuals in the population fell into one of two cytoplasmic categories, designated haplotypes f and g by a unique electrophoretic signature in both the mtDNA and cpDNA. The subset of the population carrying haplotype g included a significantly higher proportion of females when compared with the sex ratio of the subset carrying the f haplotype. Haplotype g had a significantly higher fitness when measured by fruit number, fruit set and seeds/fruit, whereas haplotype f had significantly higher fitness when measured by seed germination. Offspring of individuals carrying haplotype g included a significantly greater proportion of females when compared with offspring of individuals carrying the f haplotype. Other studies of gynodioecious plants have shown that females generally have higher fitness through seed than hermaphrodites, but in this study not all fitness differences between haplotypes could be predicted from differences in haplotype-specific sex ratio alone. Rather, some differences in haplotype-specific fitness were due to differences in fitness between individuals of the same sex, but carrying different haplotypes. The results are discussed with regard to the potential for hitchhiking selection to influence the dynamics of the noncoding regions used to designate the cpDNA and mtDNA haplotypes.     

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.