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.     

The evolution of autosomal suppressors of sex-ratio drive in Drosophila simulans

Sex-ratio drive, which results in males siring female-biased progeny, has been reported in several Drosophila species, including D. simulans. It is caused by X-linked drivers that prevent the production of Y-bearing sperm. In natural populations of D. simulans, the drivers are usually cryptic, because their spread has elicited the evolution of drive suppressors. We investigated autosomal suppression in flies from Madagascar, Réunion and Kenya. Autosomal suppressors were found in all three places, indicating that they are a regular component of drive suppression over this geographic area, where strong Y-linked suppressors also occur. These suppressors were suspected of being polymorphic in Madagascar and Réunion and proved to be polymorphic in Kenya. We developed a model simulating the evolution of neutral autosomal suppressors in order to explore the effects of the number of suppressor genes, their relative strength and the co-occurrence of Y-linked suppressors. The most interesting prediction of the model is that when suppression is multigenic, suppressor loci can remain polymorphic despite the absence of balancing selection if an equal sex-ratio is restored in the population before the suppressor alleles become fixed at all loci. The model also emphasises the importance of the sterility of distorters sons in suppressor dynamics.     

Sex inheritance in gynodioecious species: a polygenic view

Gynodioecy is defined as the coexistence of two different sexual morphs in a population: females and hermaphrodites. This breeding system is found among many different families of angiosperms and is usually under nucleo-cytoplasmic inheritance, with maternally inherited genes causing male sterility and nuclear factors restoring male fertility. Numerous theoretical models have investigated the conditions for the stable coexistence of females and hermaphrodites. To date, all models rest on the assumption that restoration of a given male sterile genotype is controlled by a single Mendelian factor. Here, we review data bearing on the genetic determinism of sex inheritance in three gynodiecious plant species. We suggest that restoration of male fertility is probably best viewed as a quantitative trait controlled by many loci. We develop a threshold model that accommodates an underlying polygenic trait, which is resolved at the phenotypic level in discrete sexual morphs. We use this model to reanalyse data in Thymus vulgaris, Silene vulgaris and Plantago coronopus. A simple Mendelian inheritance of sex determinism is unlikely in all three species. We discuss how our model can shed additional light on the genetics of restoration and point towards future efforts in the modelling of gynodioecy.     

Pollen-stigma interference in two gynodioecious species of Lamiaceae with intermediate individuals

BACKGROUND AND AIMS: Intermediate individuals (perfect flowers with very high degree of pollen abortion) in a gynodioecious plant species are very rare. A study is made of male-female relationships in each flower type and how floral characters can enhance the avoidance of 'pollen discounting' and 'self-pollination' in two gynodioecious species, Teucrium capitatum and Origanum syriacum. METHODS: The relationship between stigma receptivity and pollen viability was studied in two gynodioecious protandrous species of Lamiaceae, in addition to measuring some floral morphological characters over the life span of the flowers. KEY RESULTS: Three plant types in each species were found: plants bearing hermaphrodite (or male fertile) flowers (MF), female (or male sterile) flowers (MS) and intermediate flowers (INT). Plant types differed in flower size, with MS types being shorter than the other two types. There was no difference in style length among plant types in T. capitatum. Stigma receptivity decayed with floral age and was negative and significantly correlated with pollen viability in the two species, and positive and significantly correlated with style length in O. syriacum but only in MS flowers of T. capitatum. CONCLUSIONS: Reduction in size of floral characters is associated with male sterility, except style length in T. capitatum. MF flowers have two successive reproductive impediments: self-pollination and pollen-stigma interference. In both species, self-pollination is avoided by dichogamy (negative correlation between stigma receptivity and pollen viability), and pollen-stigma interference shows two different patterns: (1) style elongation in O. syriacum is characterized by a significant length increase, final MF dimensions are greater than those of MS dimensions, and style length is positively and significantly correlated with stigma receptivity; and (2) style movement in T. capitatum is characterized by a non-significant increase in style length, final MF floral dimensions are similar to those of MS dimensions, and there is no correlation between style length and stigma receptivity.     

Brood-size and sex-ratio variation in field populations of three species of solitary aphid parasitoids (Hymenoptera: Braconidae, Aphidiinae)

Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.     

A field guide to the caecilians of the Western Ghats, India

Caecilians are legless amphibians quite characteristic of the Western Ghats. Fourteen out of 16 Indian species occur in the Western Ghats and all are endemic. The present paper deals with the biology of caecilians with reference to external morphology and general breeding behaviour. It consolidates information on 26 morphological parameters which are used in caecilian identification. Metric multidimensional scaling of 16 species of caecilians using pair-wise euclidian distances calculated on the basis of 11 important morphometric parameters clearly depicts morphological distances between different species and more so the genera, thereby validating the classification. A field guide has been developed for the identification of caecilians based on a survey made all over the Western Ghats, observation of holotypes at the Natural History Museum, London and review of the literature. The study also reveals the microhabitat requirements of the caecilians. Further, the localities of caecilian distribution in the Western Ghats are mapped. The taxonomy of Indian caecilians is discussed.     

Signature of selective sweep associated with the evolution of sex-ratio drive in Drosophila simulans

In several Drosophila species, the XY Mendelian ratio is disturbed by X-linked segregation distorters (sex-ratio drive). We used a collection of recombinants between a nondistorting chromosome and a distorting X chromosome originating from the Seychelles to map a candidate sex-ratio region in Drosophila simulans using molecular biallelic markers. Our data were compatible with the presence of a sex-ratio locus in the 7F cytological region. Using sequence polymorphism at the Nrg locus, we showed that sex-ratio has induced a strong selective sweep in populations from Madagascar and Réunion, where distorting chromosomes are close to a 50% frequency. The complete association between the marker and the sex-ratio phenotype and the near absence of mutations and recombination in the studied fragment after the sweep event indicate that this event is recent. Examples of selective sweeps are increasingly reported in a number of genomes. This case identifies the causal selective force. It illustrates that all selective sweeps are not necessarily indicative of an increase in the average fitness of populations.     

Selfing and resource allocation in Schiedea salicaria (Caryophyllaceae), a gynodioecious species

Abstract Levels of selfing and resource allocation patterns were investigated in Schiedea salicaria (Caryophyllaceae), a gynodioecious species with high levels of inbreeding depression and nuclear control of male sterility. Selfing levels were higher in hermaphrodites than females, especially when adjusted for early acting inbreeding depression. The sexes of S. salicaria were similar in most allocation patterns including number of flowers and capsules per inflorescence, seeds per flower, and seed mass. Seeds produced by females had higher levels of germination than seeds of hermaphrodites, a likely result of high selfing levels and the expression of inbreeding depression in the progeny of hermaphrodites. Invasion of females in populations of S. salicaria is probably related to the expression of inbreeding depression at germination and in later life history stages. Comparisons with related species of Schiedea that also have nuclear control of male sterility suggest that reallocation of resources in hermaphrodites to male function occurs as females increase in frequency, but that resource reallocation is not important for the success of females when they first invade populations.     

Reproductive capacities in the sexual forms of the gynodioecious species Thymus vulgaris L.

The reproductive capacities of the females (male-steriles or mS) of Thymus vulgaris L. have been investigated, in terms of size and weight of plants, seed production, germination ability and pollination. The data, collected during nine years under controlled conditions and in natural situations are discussed in comparison with the results known in the literature. It is suggested that the large diversity of characteristics promoting sexual polymorphism could justify the high and variable rates of mS met in natural conditions in T. vulgaris.     

Testing for sex differences in biparental inbreeding and its consequences in a gynodioecious species

In many gynodioecious species cross-pollinated seeds from females outperform those from hermaphrodites. Using the gynodioecious alpine perennial Silene acaulis, I investigated whether this was the result of greater biparental inbreeding among hermaphrodites leading to greater biparental inbreeding depression. I also determined the influence of relatedness on progeny fitness. Experiments were performed using individuals from a site whose population structure and coefficient of inbreeding was known. In the first experiment, crosses were made on plants in the field to determine the effect of seven different crossing distances, plus selfing, on germination and early seedling survival and growth. Although selfed seeds died more often and grew slower than crossed seeds, the effect of crossing distance was negligible for all measured fitness traits, refuting the biparental inbreeding hypothesis as a mechanism to explain why seeds from hermaphrodites die more often than those from females. Nonetheless, cross-pollinated seeds from hermaphrodites did die more, indicating that another mechanism must be responsible. In the second experiment, the effect of different levels of inbreeding on germination and seedling survival was determined by growing seeds from experimental matings varying in relatedness. Inbreeding depression for a multiplicative fitness estimate was significant for all levels of inbreeding, suggesting that inbred individuals are unlikely to become established in the population and providing insight into the results of the first experiment. Alternative hypotheses are discussed to explain why seeds from hermaphrodites die more often, which together with the results of this study, suggest that the restoration of male function in hermaphrodites comes with a correlated cost to seedling survival.     

A kin-selection approach to the resolution of sex-ratio conflict between mates

We investigate an instance of conflict between mates over the sex ratio of their brood. We construct a kin-selection model for the evolution of the sex ratio assuming local resource competition (LRC) among females. We explore two basic scenarios: (a) the case where parents make simultaneous sex-ratio decisions (the simultaneous allocation model); and (b) the case where parental sex-ratio decisions occur one after the other (the sequential allocation model). In the simultaneous investment model, resolution of the conflict between mates depends on the extent to which relative paternal contribution influences the brood sex ratio. In the sequential allocation model, fathers determine primary sex-ratio through fertilization bias; then mothers modify the paternal sex-ratio decision by adjusting the level of investment of some resource that contributes to offspring survival. Under the sequential model, a compromise is always achieved; however this compromise favours one perspective or the other, depending on the extent to which maternal investment influences offspring survival.     

Reproductive biology and female frequencies of two co-occurring gynodioecious Thymus species

Plant Ecology - Gynodioecy, the co-occurrence of female and hermaphrodite plants in the same population is relatively common in the genus Thymus, where a high and variable female frequency has been...