Inbreeding effects on progeny sex ratio and gender variation in the gynodioecious Silene vulgaris (Caryophyllaceae)

The patterns of inbreeding depression in food-deceptive Dactylorhiza orchids

Introduction

Inbreeding depression (ID) in food-deceptive plants have been reported previously, however, it has not been often proven that selfed seeds germinate better than outbred ones or that selfing affects ID. To resolved these issues, food-deceptive related Dactylorhiza majalis, D. incarnata var. incarnata and D. fuchsii orchids were investigated.

Methods

Hand pollination treatments and control pollination were conducted. Fruit set, number of seeds per fruit, seed length, number of well-developed seeds per fruit, and proportion of in vitro asymbiotic germination seeds, were analyzed in relation to inflorescence levels and used as fitness indicators for these orchids. The ID and pollen limitation were measured.

Results

The lowest ID (δ = -1.000) was in D. majalis, and present in combination with a high pollen limitation in its populations. D. fuchsii showed higher ID (δ = 0.366), and D. incarnata var. incarnata weak ID (δ = 0.065), although ID varied between its fitness components. The seed number per fruit differed significantly between the treatments and the inflorescence levels in treatments.

Discussion

This study emphasizes that the breeding system rather than the flower position on the inflorescence shaped the quality and quantity of reproductive output. The ID and its effect on germination of food-deceptive orchid seeds undoubtedly played an important role.

Scale dependence of sex ratio in wild plant populations: implications for social selection

Social context refers to the composition of an individual's social interactants, including potential mates. In spatially structured populations, social context can vary among individuals within populations, generating the opportunity for social selection to drive differences in fitness functions among individuals at a fine spatial scale. In sexually polymorphic plants, the local sex ratio varies at a fine scale and thus has the potential to generate this opportunity. We measured the spatial distribution of two wild populations of the gynodioecious plant Silene vulgaris and show that there is fine‐scale heterogeneity in the local distribution of the sexes within these populations. We demonstrate that the largest variance in sex ratio is among nearest neighbors. This variance is greatly reduced as the spatial scale of social interactions increases. These patterns suggest the sex of neighbors has the potential to generate fine‐scale differences in selection differentials among individuals. One of the most important determinants of social interactions in plants is the behavior of pollinators. These results suggest that the potential for selection arising from sex ratio will be greatest when pollen is shared among nearest neighbors. Future studies incorporating the movement of pollinators may reveal whether and how this fine‐scale variance in sex ratio affects the fitness of individuals in these populations.

Diversity of sexual systems within different lineages of the genus Silene

Species and populations can be categorized by their sexual systems, depending on the spatial distribution of female and male reproductive structures within and among plants. Although a high diversity of sexual systems exists in Silene, their relative frequency at the genus and infrageneric level is unknown. Here, we carried out an extensive literature search for direct or indirect descriptions of sexual systems in Silene species. We found descriptions of sexual systems for 98 Silene species, where 63 and 35 correspond to the phylogenetically supported subgenera Silene and Behenantha, respectively. Hermaphroditism was the commonest sexual system (58.2 %), followed by dioecy (14.3 %), gynodioecy (13.3 %) and gynodioecy-gynomonoecy (i.e. hermaphroditic, female and gynomonoecious plants coexisting in the same population; 12.2 %). The presence of these sexual systems in both subgenera suggests their multiple origins. In 17 species, the description of sexual systems varied, and in most cases these differences corresponded to variations within or among populations. Interestingly, the poorly studied gynodioecy-gynomonoecy sexual system showed similar frequency to dioecy and gynodioecy in both subgenera. In addition, the incidence of gynodioecy-gynomonoecy was analysed in the species of section Psammophilae (Silene littorea, S. psammitis, S. adscendens and S. cambessedesii), in a survey of 26 populations across the distribution area of the species. The four species showed gynomonoecy-gynodioecy in most populations. Hermaphrodites were the most frequent morph, with a low number of females and gynomonoecious plants in all populations. The frequency of sexual morphs varied significantly among the studied populations but not among species. Female plants generally produced smaller numbers of flowers than hermaphroditic or gynomonoecious plants, and the percentages of female flowers per population were low. All these findings suggest that the gynodioecious-gynomonoecious sexual system in section Psammophilae is closer to hermaphroditism or gynomonoecy than gynodioecy.

Gynodioecy to dioecy: are we there yet?

BACKGROUND

The 'gynodioecy-dioecy pathway' is considered to be one of the most important evolutionary routes from hermaphroditism to separate sexes (dioecy). Despite a large accumulation of evidence for female seed fertility advantages in gynodioecious species (females and hermaphrodites coexist) in support of the first step in the gynodioecy-dioecy pathway, we still have very little evidence for the second step, i.e. the transition from gynodioecy to dioecy.

SCOPE

We review the literature to evaluate whether basic predictions by theory are supported. To establish whether females' seed fertility advantage and frequencies are sufficient to favour the invasion of males, we review these for species along the gynodioecy-dioecy pathway published in the last 5 years. We then review the empirical evidence for predictions deriving from the second step, i.e. hermaphrodites' male fertility increases with female frequency, selection favours greater male fertility in hermaphrodites in gynodioecious species, and, where males and hermaphrodites coexist with females (subdioecy), males have greater male fertility than hermaphrodites. We review how genetic control and certain ecological features (pollen limitation, selfing, plasticity in sex expression and antagonists) influence the trajectory of a population along the gynodioecy-dioecy pathway.

CONCLUSIONS

Females tend to have greater seed fertility advantages over hermaphrodites where the two coexist, and this advantage is positively correlated with female frequency across species, as predicted by theory. A limited number of studies in subdioecious species have demonstrated that males have an advantage over hermaphrodites, as also predicted by theory. However, less evidence exists for phenotypic selection to increase male traits of hermaphrodites or for increasing male function of hermaphrodites in populations with high female frequency. A few key case studies underline the importance of examining multiple components of male fertility and the roles of pollen limitation, selfing and plasticity, when evaluating advantages. We conclude that we do not yet have a full understanding of the transition from gynodioecy to dioecy.

Genetic determination of male sterility in gynodioecious Silene nutans

Gynodioecy, the coexistence of female and hermaphrodite plants within a species, is often under nuclear-cytoplasmic sex determination, involving cytoplasmic male sterility (CMS) genes and nuclear restorers. A good knowledge of CMS and restorer polymorphism is essential for understanding the evolution and maintenance of gynodioecy, but reciprocal crossing studies remain scarce. Although mitochondrial diversity has been studied in a few gynodioecious species, the relationship between mitotype diversity and CMS status is poorly known. From a French sample of Silene nutans, a gynodioecious species whose sex determination remains unknown, we chose the four most divergent mitotypes that we had sampled at the cytochrome b gene and tested by reciprocal crosses whether they carry distinct CMS genes. We show that gynodioecy in S. nutans is under nuclear-cytoplasmic control, with at least two different CMSs and up to four restorers with epistatic interactions. Female occurrence and frequency were highly dependent on the mitotype, suggesting that the level of restoration varies greatly among CMSs. Two of the mitotypes, which have broad geographic distributions, represent different CMSs and are very unequally restored. We discuss the dynamics of gynodioecy at the large-scale meta-population level.

A novel approach to estimating the cost of male fertility restoration in gynodioecious plants

*In many gynodioecious plants, sex is determined by cytoplasmic male sterility genes (CMS) and nuclear male fertility restorers (Rf). Models predict that the costs of restoration are important determinants of population sex ratios. However, current approaches to the estimation of these costs require prior identification of CMS genotypes, information that is available for few species. *We tested a novel approach to estimating the cost of restoration in natural populations without determining CMS or Rf genotypes. We used estimates of pollen viability and offspring sex ratios from open- and hand-pollinated families of Lobelia siphilitica to test whether the cost of restoration, expressed as low pollen viability, is higher in populations with more females. *Among populations with CMS, we found that variation in pollen viability was higher in small populations with more females, as expected if the proportion of females within populations increases with the maximum cost of restoration. In controlled crosses, families with low pollen viability also produced fewer females, suggesting that variation in viability is primarily determined by the number and frequency of Rf alleles carried. *This approach to estimating the cost of restoration can be applied to other cytonuclear gynodioecious species, offering new opportunities for testing gynodioecy models in the wild.

Recent advances in the study of gynodioecy: the interface of theory and empiricism

BACKGROUND

In this review we report on recent literature concerned with studies of gynodioecy, or the co-occurrence of female and hermaphrodite individuals in natural plant populations. Rather than review this literature in its entirety, our focus is on the interplay between theoretical and empirical approaches to the study of gynodioecy.

SCOPE

Five areas of active inquiry are considered. These are the cost of restoration, the influence of population structure on spatial sex-ratio variation, the influence of inbreeding on sex expression, the signature of cyto-nuclear coevolution on the mitochondrial genome, and the consequences of mitochondrial paternal leakage.

CONCLUSIONS

Recent advances in the study of gynodioecy have been made by considering both the ecology of female:hermaphrodite fitness differences and the genetics of sex expression. Indeed theory has guided empiricism and empiricism has guided theory. Future advances will require that some of the methods currently available only for model organisms be applied to a wider range of species.

Experimental evidence for frequency dependent self-fertilization in the gynodioecious plant, Silene vulgaris

After over a half century of empirical and theoretical research regarding the evolution and maintenance of gynodioecy in plants, unexplored factors influencing the relative fitnesses of females and hermaphrodites remain. Theoretical studies suggest that hermaphrodite self-fertilization (selfing) rate influences the maintenance of gynodioecy and we hypothesized that population sex ratio may influence hermaphrodite selfing rate. An experimental test for frequency-dependent self-fertilization was conducted using replicated populations constructed with different sex ratios of the gynodioecious plant Silene vulgaris. We found that hermaphrodite selfing increased with decreased hermaphrodite frequency, whereas evidence for increased inbreeding depression was equivocal. We argue that incorporation of context dependent inbreeding into future models of the evolution of gynodioecy is likely to yield novel insights into sex ratio evolution.

Paternal effects on seed germination: a barrier to the genetic assimilation of an endemic plant taxon?

We used a crossing experiment to investigate post-zygotic barriers that might limit introgression between a pair of closely-related, gynodioecious plant species--the widespread weed Silene vulgaris and the local Swedish endemic S. uniflora ssp. petraea. The study not only considered the effects of hybridization on conventionally-used (primary) fitness components such as seed set and progeny survival, but also provided a test for the effects of interspecific hybridization on characters with more subtle or habitat-specific effects on fitness. We detected highly significant paternal effects on seed germination properties, with the germination characteristics of hybrid seed resembling those of the species that served as the pollen donor. These paternal effects on germination represent a potentially strong barrier to interspecific introgression in the two species' natural habitats, where an inappropriate germination response in the habitat of the maternal parent may lead to the failure of seedling establishment. Interspecific crosses had weak or variable effects on progeny survival, flowering and sex ratio, but these effects could not be interpreted in terms of barriers to introgression. Our results indicate that nuclear restorers in S. vulgaris have the capacity to suppress cytoplasmic male-sterility genes in its endemic congener.

Do recent findings in plant mitochondrial molecular and population genetics have implications for the study of gynodioecy and cytonuclear conflict?

The coexistence of females and hermaphrodites in plant populations, or gynodioecy, is a puzzle recognized by Darwin. Correns identified cytoplasmic inheritance of one component of sex expression, now known as cytoplasmic male sterility (CMS). Lewis established cytonuclear inheritance of gynodioecy as an example of genetic conflict. Although biologists have since developed an understanding of the mechanisms allowing the joint maintenance of CMS and nuclear male fertility restorer genes, puzzles remain concerning the inheritance of sex expression and mechanisms governing the origination of CMS. Much of the theory of gynodioecy rests on the assumption of maternal inheritance of the mitochondrial genome. Here we review recent studies of the genetics of plant mitochondria, and their implications for the evolution and transmission of CMS. New studies of intragenomic recombination provide a plausible origin for the chimeric ORFs that characterize CMS. Moreover, evidence suggests that nonmaternal inheritance of mitochondria may be more common than once believed. These findings may have consequences for the maintenance of cytonuclear polymorphism, mitochondrial recombination, generation of gynomonoecious phenotypes, and interpretation of experimental crosses. Finally we point out that CMS can alter the nature of the cytonuclear conflict that may have originally selected for uniparental inheritance.

Effects of inbred/outbred crosses on progeny sex ratio in Silene latifolia (Caryophyllaceae)

Sex ratio polymorphism has been extensively studied in Silene latifolia, but it is neither known whether inbreeding (which is likely to occur under field conditions) affects it, nor which of the proposed mechanisms (Y degeneration, X-linked drive) is more important. Both mechanisms predict reduced pollen performance. In this study, females were crossed with pollen from related and unrelated males in single-donor and two-donor crosses, and the sex ratio of offspring (n = 866, 60 crosses), sons'in vitro pollen germination and sex ratios in parental families were scored. Flowers receiving only unrelated pollen produced a significant excess of sons. Sex ratios were not significantly correlated between generations. Sons'in vitro pollen germination was significantly negatively correlated with the 'sex-ratio phenotype' of maternal grandfathers, but not of fathers. This generation leap may be consistent with X-linked determinants because Y-linked determinants alone cannot explain it (grandfathers, fathers and sons share the same Y chromosome). Further work is required, but inbreeding and limited dispersal may lead to local accumulation of biasing factors, a process potentially countered by conditional shifts to produce more sons in pure outbred crosses.

Inheritance of chloroplast DNA is not strictly maternal in Silene vulgaris (Caryophyllaceae): evidence from experimental crosses and natural populations

Chloroplast DNA (cpDNA) is maternally inherited in the majority, but not all, of angiosperm species. The mode of inheritance of cpDNA is a critical determinant of its molecular evolution and of its population genetic structure. Here, we present the results of investigations of the inheritance of cpDNA in Silene vulgaris, a plant used in a variety of studies in which cpDNA is an important component. PCR/RFLP markers were used to compare mother and offspring cpDNA genotypes sampled from two natural populations, and mother, father, and offspring genotypes obtained from controlled greenhouse crosses. Ten of 215 offspring cpDNA genotypes studied in the controlled crosses and three of 156 offspring from natural populations did not match that of the mother, demonstrating rare nonmaternal inheritance. That the chloroplast genome is occasionally transmitted through pollen is discussed in the context of using S. vulgaris cpDNA as a marker in studies of seed dispersal and when considering the joint evolution of the chloroplast and mitochondrial genomes.