Natural selection and gynodioecy in Thymus vulgaris L.

Thymus vulgaris L. is a gynodioecious species with hermaphrodites (mF) and females (mS) widely distributed in southern France and in which reproduction of which is exclusively sexual. Numerous samples have been collected in the French Mediterranean region and the percentage of mS varies from 5 to 95% in different populations. By studying environmental conditions—the area covered at ground level by vegetation, stones and rocks—high percentages of mS are found to be related to a high vegetation cover and an absence or a scarcity of stones. Grasslands and old fields favour mS, while rocky sites favour mF. This relationship, which has been found both on the plains and on a mountain, is related to the most important reproductive characteristics of T. vulgaris —genetic control of male sterility, mode of pollination, percentage of selling and seed production. These relationships agree with the assumption that unstable environmental conditions such as grasslands or old fields in southern France favour maximum seed production and outcrossing. The greater stability of uncultivated areas such as rocky sites allows for the maintenance of autogamy. Consequently, it is natural selection which adjusts the percentage of male sterility in natural populations of Thymus vulgaris.     

Floral biology and reproductive ecology ofClusia nemorosa (Clusiaceae) in northeastern Brazil

Between March 1993 and May 1995 the floral biology and the phenology ofClusia nemorosa G. Mey. (Clusiaceae) were studied in two dioecious populations in Pernambuco State, and some observations were made in gynodioecious populations, that were encountered only in Bahia State. The species flowers from June to January/February. The flowers are bowl shaped, and last only one day. The floral reward is resin which is produced by staminodia in all types of flowers. The sex ratio was 1:1 for the studied populations. The pollen viability was approximately 98%. Apomixis was not observed. The rate of fruit set, in hermaphrodite individuals, under natural conditions and the sex ratio of the gynodioecious populations studied suggest that the hermaphrodite individuals may behave as males.Clusia nemorosa is melittophilous and was visited by 16 species of bees. The main pollinators wereEuplusia mussitans, E. iopyhrra, Eulaema cingulata, E. nigrita, Euglossa cordata andEuglossa sp.     

An association between chloroplast DNA haplotypeand gender in a plant metapopulation

Theoretical models have shown that metapopulation structure can influence the evolution of the sex ratio in gynodioecious plants when the fitness of females and/or hermaphrodites varies among populations with the local sex ratio. Such frequency-dependent fitness has been demonstrated previously in natural populations of Silene vulgaris, a gynodioecious plant in which sex determination is cytonuclear. We investigated whether there was an association between chloroplast DNA (cpDNA) haplotype and sex expression, with the assumption that cpDNA haplotypes could be associated with specific male sterility variants likely found in the mitochondrial DNA, owing to co-transmission of the two cytoplasmic genomes. Two cpDNA haplotypes were studied in a collection of 111 individuals of known sex taken from 20 natural populations found in Virginia, USA. The set of haplotype 1 individuals consisted of 71.4% hermaphrodites, where as only 7.7% of the haplotype 2 individuals were hermaphroditic; a highly statistically significant difference. By extension of a previous model it was also shown that the differential clustering of these two haplotypes into local populations contributes to among-population sex ratio variation and has the potential to lower the fitness of haplotype 2 by greater than 50%, relative to that expected with no population structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of herkogamy and gynodioecy in Moroccan species ofRomulea (Iridaceae)

Herkogamy and gynodioecy were studied in the Moroccan species ofRomulea. Several types of herkogamy are shown to occur in the different species, with each type corresponding to a characteristic perianth size. The degree of differentiation between female and hermaphrodite morphs varies among the different gynodioecious species. Herkogamy is considered to have evolved prior to the development of the gynodioecious condition. An evolutionary interpretation is proposed based on the degree of herkogamy and of gynodioecy in the different species.     

High variability and disomic segregation of microsatellites in the octoploid Fragaria virginiana Mill. (Rosaceae)

The objectives of the present study were to develop microsatellite markers for the wild strawberry, Fragaria virginiana, to evaluate segregation patterns of microsatellite alleles in this octoploid species, and assess genetic variability at microsatellite loci in a wild population. A genomic library was screened for microsatellite repeats and several PCR primers were designed and tested. We also tested the use of heterologous primers and found that F. virginiana primers amplified products in cultivated strawberry, Fragaria × ananassa Duch. and Fragaria chiloensis. Similarly, microsatellite loci developed from cultivated strawberry also successfully amplified F. virginiana loci. We investigated four microsatellite loci in detail, three developed from F. virginiana and one from cultivated strawberry. A survey of 100 individuals from a population of F. virginiana in Pennsylvania demonstrated high heterozygosities (H_e or gene diversity ranged from 0.80 to 0.88 per locus) and allelic diversity (12–17 alleles per locus), but individual plants had no more than two alleles per locus. Segregation patterns in parents and progeny of two controlled crosses at these four loci were consistent with disomic Mendelian inheritance. Together these findings suggest that the genome of F. virginiana is "highly diploidized" and at least a subset of microsatellite loci can be treated as codominant, diploid markers. Significant heterozygote deficiencies were found at three of the four loci for hermaphroditic individuals but for only one locus among females in this gynodioecious species.Communicated by J. Dvorak     

Outcrossing rates and male sterility in natural populations of Plantago coronopus

Summary Outcrossing rates were estimated in three populations of the gynodioecious species Plantago coronopus by means of electrophoresis of adult plants and their natural progenies. A multilocus estimation procedure was used. Heterogeneity among the pollen-pool allele frequencies did not exist either in space of in time. Differences between populations in mean outcrossing rates were large (range: 0.34–0.93), probably caused by differences in densities of flowering plants. In addition, there was considerable variability between individuals, which was at least partly caused by the presence of male sterility. Population density may, via its influence on outcrossing rates, be a factor influencing the maintenance of male sterile plants in the population. The level of outcrossing in hermaphrodites was not low enough to explain the maintenance of male steriles. Outcrossing rates in two populations, established via progeny analysis, were much lower than calculated with the fixation index, possibly indicating heterozygote advantage in these natural populations.Grassland Species Research Group Publication no. 134     

The Effect of Breeding System on Polymorphism in Mitochondrial Genes of Silene

Gynodioecy is a breeding system characterized by the co-occurrence of hermaphrodite and female individuals, generally as the result of nuclear–cytoplasmic interactions. The question remains whether the genetic factors controlling gynodioecy are maintained in species over long evolutionary timescales by balancing selection or are continually arising and being replaced in epidemic sweeps. If balancing selection maintains these factors, then neutral cytoplasmic diversity should be greater in gynodioecious than hermaphroditic species. In contrast, epidemic sweeps of factors controlling gynodioecy should decrease cytoplasmic diversity in gynodioecious relative to hermaphroditic species. We took a comparative approach in which we sequenced two mitochondrial genes, cytochrome b (cob) and cytochrome oxidase (cox1), for multiple populations of several hermaphroditic, gynodioecious, and dioecious species in the genus Silene. Breeding system was predictive of polymorphism. Gynodioecious species harbor many old haplotypes while hermaphroditic and dioecious species have little to no nucleotide diversity. The genealogical structure of neither gene departed from neutral expectations. Taken together, our results suggest that balancing selection acts on cytoplasmic male-sterility factors in several gynodioecious species in the genus.     

Reduced recombination in gynodioecious populations of a facultative apomictic orchid

Many plants combine sexual reproduction with some form of asexual reproduction to different degrees, and lower genetic diversity is expected with asexuality. Moreover, the ratios of sexual morphs in species with gender dimorphism are expected to vary in proportion to the reproductive success of the sexual process. Hence, sex ratios can directly influence the genetic structure and diversity of a population. We investigated genotypic diversity in 23 populations of a facultative, apomictic gynodioecious orchid, Satyrium ciliatum, to examine the effect on genotypic diversity of variation in the frequency of females and in the amount of sexual reproduction. The study involved one pure female, seven gynodioecious (both females and hermaphrodites present) and 15 hermaphroditic populations. Pollinia receipt was higher in hermaphroditic than in gynodioecious populations. Analyses of variation in ISSRs demonstrated that genotypic diversity was high in all populations and was not significantly different between hermaphroditic and gynodioecious populations. We used character compatibility analysis to determine the extent to which recombination by sexual reproduction contributed to genotypic diversity. The results indicate that the contribution of recombination to genotypic diversity is higher in hermaphroditic than in gynodioecious populations, consistent with the finding that hermaphroditic populations received higher amounts of pollinia. Our finding of reduced recombination in gynodioecious populations suggests that maintenance of sex in hermaphrodites plays an important role in generating genotypic diversity in this apomictic orchid.     

Disentangling the effects of mating systems and mutation rates on cytoplamic diversity in gynodioecious Silene nutans and dioecious Silene otites

Many flowering plant species exhibit a variety of distinct sexual morphs, the two most common cases being the co-occurrence of females and males (dioecy) or the co-occurrence of hermaphrodites and females (gynodioecy). In this study, we compared DNA sequence variability of the three genomes (nuclear, mitochondrial and chloroplastic) of a gynodioecious species, Silene nutans, with that of a closely related dioecious species, Silene otites. In the light of theoretical models, we expect cytoplasmic diversity to differ between the two species due to the selective dynamics that acts on cytoplasmic genomes in gynodioecious species: under an epidemic scenario, the gynodioecious species is expected to exhibit lower cytoplasmic diversity than the dioecious species, while the opposite is expected in the case of balancing selection maintaining sterility cytoplasms in the gynodioecious species. We found no difference between the species for nuclear gene diversity, but, for the cytoplasmic loci, the gynodioecious S. nutans had more haplotypes, and higher nucleotide diversity, than the dioecious relative, S. otites, even though the latter has a relatively high rate of mitochondrial synonymous substitutions, and therefore presumably a higher mutation rate. Therefore, as the mitochondrial mutation rate cannot account for the higher cytoplasmic diversity found in S. nutans, our findings support the hypothesis that gynodioecy in S. nutans has been maintained by balancing selection rather than by epidemic-like dynamics.     

Breeding system and seed set inEuonymus europaeus (Celastraceae)

The seed set and breeding system in a naturalized population ofEuonymus europaeus is described. The average seed set is lower (5.7%) and more variable in male than in female plants (9.3%). Some male plants set little or no seed. The results are compared with those ofDarwin, who describedE. europaeus as polygamous and trioecious; it is concluded that the species is better described as gynodioecious.     

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.     

A comparative study of cleaning activity of two reef fishes at Fernando de Noronha Archipelago,tropical West Atlantic

Cleaner fishes are usually classified as obligate or facultative cleaners according to their diet and the extent to which their nutritional requirements in the different ontogenetic stages are gained from cleaning. While obligate cleaners clean throughout their lives and ingest mainly food taken from the clients’ body surface, facultative cleaners clean only as juveniles and have a broader diet. In addition, some facultative cleaners may experience a relatively higher predation risk, and thus rarely interact with piscivorous fishes. Despite these acknowledged differences, there are very few studies that compare cleaning activity of obligate and facultative cleaners within the same area. Cleaning activity of the obligate cleaner goby Elacatinus cf. randalli and the facultative cleaner wrasse Thalassoma noronhanum were comparatively examined at Fernando de Noronha Archipelago, tropical West Atlantic. The client assemblage attended by the two cleaners differed, as the goby attended a slightly greater diversity of species (22), mostly piscivores and zoobenthivores, and the wrasse attended fewer species (19), mostly planktivores. Chromis multilineata was the most common client species of both cleaners, although body size (which is expected to be positively correlated to clients’ ectoparasite load) of C. multilineata individuals attended by the goby was larger than that of the individuals attended by the wrasse. Despite such differences, T. noronhanum showed a surprisingly species-rich client assemblage when compared with other cleaners of the genus Thalassoma. In addition, the frequency and time spent on cleaning interactions, as well as the number of client species attended per 10-min period, was similar for both cleaner species, which indicate that they have important yet complimentary ecological roles in the reef community at Fernando de Noronha Archipelago.     

Frequency‐dependent fitness in gynodioecious Lobelia siphilitica

Selection is frequency dependent when an individual's fitness depends on the frequency of its phenotype. Frequency‐dependent selection should be common in gynodioecious plants, where individuals are female or hermaphroditic; if the fitness of females is limited by the availability of pollen to fertilize their ovules, then they should have higher fitness when rare than when common. To test whether the fitness of females is frequency dependent, we manipulated the sex ratio in arrays of gynodioecious Lobelia siphilitica. To test whether fitness was frequency dependent because of variation in pollen availability, we compared open‐pollinated and supplemental hand‐pollinated plants. Open‐pollinated females produced more seeds when they were rare than when they were common, as expected if fitness is negatively frequency dependent. However, hand‐pollinated females also produced more seeds when they were rare, indicating that variation in pollen availability was not the cause of frequency‐dependent fitness. Instead, fitness was frequency dependent because both hand‐ and open‐pollinated females opened more flowers when they were rare than when they were common. This plasticity in the rate of anthesis could cause fitness to be frequency dependent even when reproduction is not pollen limited, and thus expand the conditions under which frequency‐dependent selection operates in gynodioecious species.     

FEMALE FREQUENCIES IN GYNODIOECIOUS POPULATIONS CORRELATED WITH SELFING RATES IN HERMAPHRODITES

Gynodioecious populations consist of separate hermaphroditic and female individuals. Females are at a selective disadvantage because they contribute genes to the next generation only through ovules, while hermaphrodites contribute genes through ovules and pollen. For females to be maintained in populations they must have some compensating selective advantage. The outcrossing hypothesis postulates that females are maintained because their progeny result from obligate outcrossing, whereas some of the progeny of hermaphrodites result from self-fertilization and are less fit because of inbreeding depression. If correct, the frequency of females should be positively correlated with selfing rates of hermaphrodites in populations. We found a strong positive correlation between female frequency and selfing rates of hermaphrodites (r = 0.91, P < 0.01) in eight gynodioecious populations of Hawaiian species of Bidens. Our results confirm that the obligate outcrossing of females is a major factor maintaining females in gynodioecious populations. However, the observed selfing rates are insufficient by themselves to account for the frequency of females in these populations.     

Evolution of male sterility mechanisms in gynodioecious and dioecious species ofCirsium (Cynareae,Compositae)

The genusCirsium comprises both gynodioecious and dioecious species. The observation of microsporogenesis in female plants ofC. montanum, C. oleraceum, C. palustre andC. spinosissimum shows that the male sterility is due to a degeneration of the tapetum. This degeneration occurs more or less early according to the species and, in the light of these results, a scheme of evolution in the male sterility mechanism is proposed. Furthermore, the male sterility mechanism inC. montanum is very similar to that previously found in female plants of the dioecious speciesC. arvense. This fact enhances the possibility of evolution of the dioecy ofC. arvense from the gynodioecy found in other species. According to these results, a general scheme of evolution of sexes in the genusCirsium is proposed.     

Effects of trait plasticity on aboveground biomass production depend on species identity in experimental grasslands

Experimental studies with grassland species found a positive relationship between species richness and community biomass production, however the response of individual species was highly variable. The mechanisms behind these patterns are poorly understood. Here we studied aboveground biomass production and plasticity of growth characteristics of four legumes with similar morphology (Lotus corniculatus, Medicago lupulina, Onobrychis viciifolia, Trifolium hybridum) in experimental grasslands varying in species richness (1, 2, 4, 8, 16 and 60) and composition. We identified O. viciifolia and T. hybridum as species that reached higher biomass production in mixtures than expected from monoculture yields, while L. corniculatus and M. lupulina mostly had lower yields than expected. Variation of morphological traits across the species-richness gradient was lowest in the highly competitive O. viciifolia, but increased in the smaller species. The tall-growing O. viciifolia achieved higher biomass production by both, a higher number of plant individuals and an increase in mean mass per individual. Mean shoot number per individual remained constant, but individuals produced heavier shoots. The medium-sized T. hybridum also increased the number of plant individuals, but mean mass per individual did not respond to community species richness. The average mass per shoot was increased in mixtures, but the species developed less shoots per individual. Shoot length and stem weight ratio of T. hybridum increased with community species richness. Morphological changes in the less successful L. corniculatus and M. lupulina with a smaller growth stature were similarly directed as those of T. hybridum. The observed morphological changes are known as typical shade-avoidance mechanisms in dense vegetation. Our study shows that stress responses to changes in resource availability may be a mechanism to enforce higher aboveground biomass production of individual species in mixtures, but it depends on species identity whether trait plasticity is large enough to exceed stress-induced growth limitations.     

Macroparasite community in molluscs of a tidal basin in the Wadden Sea

We provide a quantitative inventory of macroparasites in intertidal molluscs from a tidal basin in the Wadden Sea (eastern North Sea). Gastropods and bivalves contained a species rich macroparasite community consisting of trematodes (26 species), turbellarians (1), nematodes (1), copepods (2) and polychaetes (1) in 3,800 host individuals from 10 host species. Highest parasite burdens were observed in the gastropods Hydrobia ulvae and Littorina littorea and in the bivalves Cerastoderma edule and Mytilus edulis. In contrast, only one parasite species and no trematodes were found in Crepidula fornicata. The parasite community in the molluscs was similar to other Western European localities but some parasite species showed obvious differences, related to the large-scale distribution of intermediate and final hosts. Parasitism seems to be a common phenomenon in molluscs of the Wadden Sea and hence the detrimental effects observed in experiments can be expected to frequently happen in the field.     

INBREEDING DEPRESSION IN GYNODIOECIOUS LOBELIA SIPHILITICA: AMONG-FAMILY DIFFERENCES OVERRIDE BETWEEN-MORPH DIFFERENCES

If inbreeding depression is caused by deleterious recessive alleles, as suggested by the partial dominance hypothesis, a negative correlation between inbreeding and inbreeding depression is predicted. This hypothesis has been tested several times by comparisons of closely related species or comparisons of populations of the same species with different histories of inbreeding. However, if one is interested in whether this relationship contributes to mating-system evolution, which occurs within populations, comparisons among families within a population are needed; that is, inbreeding depression among individuals with genetically based differences in their rate of selfing should be compared. In gynodioecious species with self-compatible hermaphrodites, hermaphrodites will have a greater history of potential inbreeding via both selfing and biparental inbreeding as compared to females and may therefore express a lower level of inbreeding depression. We estimated the inbreeding depression of female and hermaphrodite lineages in gynodioecious Lobelia siphilitica in a greenhouse experiment by comparing the performance of selfed and outcrossed progeny, as well as sibling crosses and crosses among subpopulations. We did not find support for lower inbreeding depression in hermaphrodite lineages. Multiplicative inbreeding depression (based on seed germination, juvenile survival, survival to flowering, and flower production in the first growing season) was not significantly different between hermaphrodite lineages (δ = 0.30 ± 0.08) and female lineages (δ = 0.15 ± 0.18), although the trend was for higher inbreeding depression in the hermaphrodite lineages. The population-level estimate of inbreeding depression was relatively low for a gynodioecious species (δ = 0.25) and there was no significant inbreeding depression following biparental inbreeding (δ = 0.01). All measured traits showed significant variation among families, and there was a significant interaction between family and pollination treatment for four traits (germination date, date of first flowering, number of flowers, and aboveground biomass). Our results suggest that the families responded differently to selfing and outcrossing: Some families exhibited lower fitness following selfing whereas others seemed to benefit from selfing as compared to outcrossing. Our results support recent simulation results in that prior inbreeding of the lineages did not determine the level of inbreeding depression. These results also emphasize the importance of determining family-level estimates of inbreeding depression, relative to population-level estimates, for studies of mating-system evolution.     

Small but attractive: female‐biased nectar production and floral visitors in a dimorphic shrub

• In sexually dimorphic species, hermaphrodite flowers in gynodioecious species or male flowers in dioecious species are often larger and produce more nectar than their conspecific female flowers. As a consequence, hermaphrodite or male flowers frequently receive more pollinator visits.
• Sex ratio, flower size, floral display, nectar production and floral visits were evaluated in two natural populations of Fuchsia thymifolia, a morphologically gynodioecious but functionally subdioecious insect‐pollinated shrub.
• Sex ratio did not differ from the expected 1:1 in the two studied populations. As expected, hermaphrodite flowers were larger than female flowers, but in contrast to the general pattern, hermaphrodite flowers did not produce nectar or produced much less than female flowers. Flower visitors were flies (68%) and bumblebees (24%), both of which showed a preference for female flowers. No sex difference was detected in either flower longevity or floral display across the flowering season.
• Higher nectar production by females may attract more pollinators, and may be a strategy to enhance female reproductive success in this species. Finally, floral dimorphism and insect preferences did not seem to hamper the maintenance of sub‐dioecy or prevent the evolution of dioecy in F. thymifolia.