THE EFFECT OF POLLEN VERSUS SEED FLOW ON THE MAINTENANCE OF NUCLEAR‐CYTOPLASMIC GYNODIOECY

Gynodioecy, where females co‐occur with hermaphrodites, is a relatively common sexual system in plants that is often the result of a genetic conflict between maternally inherited male sterility genes in the mitochondrial genome and the biparentally inherited male fertility restorer genes in the nucleus. Previous models have shown that nuclear‐cytoplasmic gynodioecy can be maintained under certain conditions by negative frequency‐dependent selection, but the effect of other evolutionary processes such as genetic drift and population subdivision is only partially understood. Here, we investigate the joint effects of frequency‐dependent selection, drift, and migration through either pollen or seeds on the maintenance of nuclear‐cytoplasmic gynodioecy in a subdivided population. We find that the combination of drift and selection causes the loss of gynodioecy under scenarios that would maintain it under the influence of selection alone, and that both seed and, more surprisingly, pollen flow can maintain the polymorphism. In particular, although pollen flow could not avoid the loss of cytoplasmic polymorphism within demes, it allowed the maintenance of nuclear‐cytoplasmic polymorphism at the metapopulation level.     

Mathematical Study of the Evolution of Gynodioecy with Cytoplasmic Inheritance under the Effect of a Nuclear Restorer Gene

A study is described of the influence of the introduction of a dominant nuclear restorer gene into a cytoplasmic gynodioecious plant population. This study includes the consideration of separate effects on the relative female fertility of nuclear, cytoplasmic and sex (phenotypic) factors. Under these assumptions, the introduction of a dominant nuclear restorer gene into a cytoplasmic gynodioecious population can lead to several different situations: persistence of cytoplasmic gynodioecy, appearance of a nuclear-cytoplasmic gynodioecy, appearance of a nuclear gynodioecy or complete restoration of male fertility. The development of stable nuclear-cytoplasmic gynodioecy in a mathematical model is new and is possible because of the consideration of the separate relative female fertilities. The possibility of a transformation of cytoplasmic gynodioecy into a nuclear one has never been obtained before. It could constitute a route for the appearance of this latter kind of gynodioecy in plant populations. Finally, the possibilities of evolution of gynodioecy from one kind to the other, and towards dioecy, are discussed, as are some theoretical schemes that seem to correspond to observed actual situations.     

Modeling gynodioecy: novel scenarios for maintaining polymorphism

Nuclear-cytoplasmic gynodioecy is a breeding system of plants in which females and hermaphrodites co-occur in populations, and gender is jointly determined by cytoplasmic male sterility (CMS) genes and nuclear restorers of male fertility. Persistent polymorphism at both CMS and nuclear-restorer loci is necessary to maintain this breeding system. Theoretical models have explained how nuclear-cytoplasmic gynodioecy can be stable for certain assumptions. However, recent advances in our understanding of the genetics, population biology, and molecular mechanisms of sex determination in nuclear-cytoplasmic gynodioecious species suggest the utility of new models with different underlying assumptions. In this article, we examine different negative pleiotropic fitness effects of nuclear restorers (costs of restoration) using genetic and population assumptions based on recent literature. Specifically, we model populations with two CMS types and separate nuclear restorer loci for each CMS type. Under these assumptions, both overdominance for fitness and frequency-dependent selection at nuclear-restorer loci can support nuclear-cytoplasmic gynodioecy. Costs of restoration can be either dependent or independent of the cytoplasmic background. Seed fitness costs are more vulnerable to fixation of CMS types than pollen costs. Survivorship costs are effective at maintaining polymorphism even when total reproductive effects are low. Overall, our models display differences in the stability of nuclear-cytoplasmic gynodioecy and predicted population sex ratios that should be informative to researchers studying gynodioecy in the wild.     

The evolution of gynodioecy on a lattice

Gynodioecy is a breeding system in plants where populations consist of hermaphrodites and females. The females result from a genetic mutation which impairs pollen production in hermaphrodite plants. Most previous models for the evolution of gynodioecy do not take into account any spatial detail, which might be expected to play an important role in populations with short range interactions caused by poor or no locomotion.In this article we present a generalised mean-field analysis (which ignores any spatial detail), together with stochastic spatial simulations, to investigate the spatial effect on the evolution of gynodioecy. We show that, in a population of hermaphrodites where male sterility is caused by a dominant allele in a nuclear gene, mean-field calculations greatly underestimate the reproductive advantage females require to become viable under spatial constraints. This suggests that gynodioecy is less likely to evolve in plants with more localised pollination and seed setting. This may have implications for the evolution of dioecy, a breeding system in plants where the population consists of males and females, as gynodioecy is thought to be a route to dioecy. Our results also demonstrate that a lower frequency of females should be expected for gynodioecious populations when interactions are local. This is relevant when comparing the results of breeding experiments with observations of female frequency in the wild.     

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.     

Parental effects and gender specialization in a tropical heterostylous shrub

Abstract.— Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense , thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene.     

HOW SELFING,INBREEDING DEPRESSION,AND POLLEN LIMITATION IMPACT NUCLEAR‐CYTOPLASMIC GYNODIOECY: A MODEL

Gynodioecy, the co‐occurrence of females and hermaphrodites, is often due to conflicting interactions between cytoplasmic male sterility genes and nuclear restorers. Although gynodioecy often occurs in self‐compatible species, the effect of self‐pollination, inbreeding depression, and pollen limitation acting differently on females and hermaphrodites remains poorly known in the case of nuclear‐cytoplasmic gynodioecy (NCG). In this study, we model NCG in an infinite population and we study the effect of selfing rate, inbreeding depression, and pollen limitation on the maintenance of gynodioecy and on sex ratios at equilibrium. We found that selfing and inbreeding depression have a strong impact, which depends on whether restorer cost acts on male or female fitness. When cost affects male fitness, the strength of cost has no effect, whereas selfing and inbreeding depression only impact gynodioecy by modifying the value of the female advantage. When cost affects female fitness, selfing facilitates NCG and reduces the role of strength of the cost, even when no inbreeding depression occurs, whereas inbreeding depression globally restricts the maintenance of the polymorphism. Finally, we found that pollen limitation could additionally strongly modify the dynamic of gynodioecy. We discuss our findings in the light of empirical data available in gynodioecious species.     

Sex‐specific fitness variation in gynodioecious Beta vulgaris ssp. maritima: do empirical observations fit theoretical predictions?

In gynodioecious species, in which hermaphroditic and female plants co-occur, the maintenance of sexual polymorphism relies on the genetic determination of sex and on the relative fitness of the different phenotypes. Flower production, components of male fitness (pollen quantity and pollen quality) and female fitness (fruit and seed set) were measured in gynodioecious Beta vulgaris spp. maritima, in which sex is determined by interactions between cytoplasmic male sterility (CMS) genes and nuclear restorers of male fertility. The results suggested that (i) female had a marginal advantage over hermaphrodites in terms of flower production only, (ii) restored CMS hermaphrodites (carrying both CMS genes and nuclear restorers) suffered a slight decrease in fruit production compared to non-CMS hermaphrodites and (iii) restored CMS hermaphrodites were poor pollen producers compared to non-CMS hermaphrodites, probably as a consequence of complex determination of restoration. These observations potentially have important consequences for the conditions of maintenance of sexual polymorphism in B. vulgaris and are discussed in the light of existing theory on evolutionary dynamics of gynodioecy.     

TESTING MODELS OF SEX RATIO EVOLUTION IN A GYNODIOECIOUS PLANT: FEMALE FREQUENCY COVARIES WITH THE COST OF MALE FERTILITY RESTORATION

In many gynodioecious species, cytoplasmic male sterility genes (CMS) and nuclear male fertility restorers (Rf) jointly determine whether a plant is female or hermaphrodite. Equilibrium models of cytonuclear gynodioecy, which describe the effect of natural selection within populations on the sex ratio, predict that the frequency of females in a population will primarily depend on the cost of male fertility restoration, a negative pleiotropic effect of Rf alleles on hermaphrodite fitness. Specifically, when the cost of restoration is higher, the frequency of females at equilibrium is predicted to be higher. To test this prediction, we estimated variation in the cost of restoration across 26 populations of Lobelia siphilitica, a species in which Rf alleles can have negative pleiotropic effects on pollen viability. We found that L. siphilitica populations with many females were more likely to contain hermaphrodites with low pollen viability. This is consistent with the prediction that the cost of restoration is a key determinant of variation in female frequency. Our results suggest that equilibrium models can explain variation in sex ratio among natural populations of gynodioecious species.     

Disentangling the causes of heterogeneity in male fecundity in gynodioecious Beta vulgaris ssp. maritima

Variation among individuals in reproductive success is advocated as a major process driving evolution of sexual polymorphisms in plants, such as gynodioecy where females and hermaphrodites coexist. In gynodioecious Beta vulgaris ssp. maritima, sex determination involves cytoplasmic male sterility (CMS) genes and nuclear restorers of male fertility. Both restored CMS and non-CMS hermaphrodites co-occur. Genotype-specific differences in male fitness are theoretically expected to explain the maintenance of cytonuclear polymorphism. Using genotypic information on seedlings and flowering plants within two metapopulations, we investigated whether male fecundity was influenced by ecological, phenotypic and genetic factors, while taking into account the shape and scale of pollen dispersal. Along with spatially restricted pollen flow, we showed that male fecundity was affected by flowering synchrony, investment in reproduction, pollen production and cytoplasmic identity of potential fathers. Siring success of non-CMS hermaphrodites was higher than that of restored CMS hermaphrodites. However, the magnitude of the difference in fecundity depended on the likelihood of carrying restorer alleles for non-CMS hermaphrodites. Our results suggest the occurrence of a cost of silent restorers, a condition supported by scarce empirical evidence, but theoretically required to maintain a stable sexual polymorphism in gynodioecious species.     

Variation in pollen production and pollen viability in natural populations of gynodioecious Beta vulgaris ssp. maritima: evidence for a cost of restoration of male function?

Gynodioecious species are defined by the co-occurrence of two clearly separated categories of plants: females and hermaphrodites. The hermaphroditic category may, however, not be homogeneous, as male fitness may vary among hermaphrodites as a result of many biological factors. In this study, we analysed estimates of pollen quantity and viability in the gynodioecious Beta vulgaris ssp. maritima, comparing hermaphrodites bearing a male-fertile cytotype and hermaphrodites bearing cytoplasmic male sterility (CMS) genes, which are counteracted by nuclear restoration factors. We show that: (i) pollen quantity continuously varies among restored hermaphrodites, suggesting a complex genetic determination of nuclear restoration; (ii) pollen viability was lower in restored (CMS) hermaphrodites than in non-CMS hermaphrodites, probably because of incomplete restoration in some of these plants; and (iii) pollen quantity and viability also varied among hermaphrodites with male-fertile cytotypes, possibly a result of a silent cost of restoration. Finally, we discuss the consequences of these results for pollen flow and the dynamics of gynodioecy.     

The nucleo-mitochondrial conflict in cytoplasmic male sterilities revisited

Cytoplasmic male sterility (CMS) in plants is a classical example of genomic conflict, opposing maternally-inherited cytoplasmic genes (mitochondrial genes in most cases), which induce male sterility, and nuclear genes, which restore male fertility. In natural populations, this type of sex control leads to gynodioecy, that is, the co-occurrence of female and hermaphroditic individuals within a population. According to theoretical models, two conditions may maintain male sterility in a natural population: (1) female advantage (female plants are reproductively more successful than hermaphrodites on account of their global seed production); (2) the counter-selection of nuclear fertility restorers when the corresponding male-sterility-inducing cytoplasm is lacking. In this review, we re-examine the model of nuclear-mitochondrial conflict in the light of recent experimental results from naturally occurring CMS, alloplasmic CMS (appearing after interspecific crosses resulting from the association of nuclear and cytoplasmic genomes from different species), and CMS plants obtained in the laboratory and carrying mitochondrial mutations. We raise new hypotheses and discuss experimental models that would take physiological interactions between cytoplasmic and nuclear genomes into account.     

Pollen limitation and the evolution of androdioecy from dioecy

Androdioecy is an unusual breeding system in which populations consist of separate male and hermaphrodite individuals. The evolution of androdioecy is still poorly understood; however, there is evidence from several androdioecious species that the breeding system may have evolved from dioecy (males and females). This article presents a simple deterministic model showing that androdioecy can evolve from dioecy under a broad range of realistic conditions. For the evolution of androdioecy from dioecy, hermaphrodites must be able to invade the dioecious population. Then, males must be maintained, while females are eliminated. Hermaphrodite invasion is favored when females are pollen limited and hermaphrodites have high overall fertility and are self-fertile. Male maintenance is favored when hermaphrodites resemble females, having high seed production and low pollen fitness, and when the selfing rate is not too high. These conditions were satisfied over a broad and realistic range of parameter values, suggesting that the evolution of androdioecy from dioecy is highly plausible.     

Effective population sizes for cytoplasmic and nuclear genes in a gynodioecious species. The role of the sex determination system

Equations are derived for the effective sizes of gynodioecious populations with respect to both nuclear and cytoplasmic genes (N(ec) and N(en), respectively). Compared to hermaphroditism, gynodioecy generally reduces effective population sizes for both kinds of loci to an extent depending on the frequency of females, the sex determination system, and the selfing rate of hermaphrodites. This reduction is due to fitness differences between the sexes and is highly influenced by the mode of inheritance of this fitness. In absence of selfing, nuclear gynodioecy results in a reduction of N(ec) that depends strongly on the dominance of male sterility alleles, while N(en) remains equal to the census number (N). With cytonuclear gynodioecy, both cytoplasmic and nuclear effective sizes are reduced, and at the extreme, dioecy results in the minimum N(ec) values and either minimum or maximum N(en) values (for low or high frequency of females, respectively). When selfing occurs, gynodioecy either increases or decreases N(en) as compared to hermaphroditism with the same selfing rate of hermaphrodites. Unexpectedly, N(ec) also varies with the selfing rate. Thus the genetic sex-determination system appears as a major factor for the nuclear and cytoplasmic genetic diversities of gynodioecious species.     

Nuclear androdioecy and gynodioecy

We formulate two single-locus Mendelian models, one for androdioecy and the other one for gynodioecy, each with 3 parameters: t the male (female) fertility rate of males (females) to hermaphrodites, s the fraction of the progeny derived from selfing; and g the fitness of inbreeders. Each model is expressed as a transformation of a 3 dimensional zygotic algebra, which we interpret as a rational map of the projective plane. We then study the dynamics for the evolution of each reproductive system; and compare our results with similar published models. In this process, we introduce a general concept of fitness and list some of its properties, obtaining a relative measure of population growth, computable as an eigenvalue of a mixed mating transformation for a population in equilibrium. Our results concur with previous models of the evolution of androdioecy and gynodioecy regarding the threshold values above which the sexual polymophism is stable, although the previous models assume constant the fraction of ovules from hermaphrodites that are self pollinated, while we assume constant the fraction of the progeny derived from selfing. A stable androdioecy requires more stringent conditions than a stable gynodioecy if the amount of pollen used for selfing is negligible in comparison with the total amount of pollen produced by hermaphrodites. Otherwise, both models are identical. We show explicitly that the genotype fitnesses depend linearly on their frequencies. Simulations show that any population not at equilibrium always converges to the equilibrium point of higher fitness. However, at intermediate steps, the fitness function occasionally decreases.     

Inbreeding effect on male and female fertility and inheritance of male sterility in Nemophila menziesii (Hydrophyllaceae)

Models of the evolution of gynodioecy assume that inbreeding affects male and female fertility equally and ignore quantitative variation in sex expression. The objectives of this study were to assess inbreeding effects, genetic background, and plant maturity on male and female fertility and the mechanism of male sterility inheritance for Nemophila menziesii (Hydrophyllaceae). Frequency of male-sterile flowers, number of anthers and ovules, and percentage of viable pollen were measured on plants from different pedigrees and five inbreeding levels (F = 0, 0.0625, 0.25, 0.5, and 0.75). Quantitative variation in male sterility was evident. As inbreeding increased, anther and ovule number decreased; the effect on anther number was greater than on ovule number. Pedigrees varied in number of male-sterile flowers and inbreeding effects. Frequency of male-sterile flowers was greatest among first flowers. No trade-off between male and female fertility was detected. A model attributing male sterility to a cytoplasmic locus and restoration to male fertility to a nuclear locus accounted for the distribution of complete sterility and hermaphroditism over the pedigrees. This study suggests that models of the evolution and maintenance of gynodioecy should allow for quantitative variation in male and female fertility components due to inbreeding, pedigree, and plant maturity.     

Androdioecy and the evolution of dioecy

The likelihood that dioecy could evolve via androdioecy is examined. It is concluded that female-sterility mutations are unlikely to be able to invade populations of self-compatible hermaphrodite species, even if the resources that an hermaphrodite devotes to seed production can be diverted to yield increased survival and also to increase male fertility. These findings are in agreement with the great rarity of androdioecy. Claimed cases of androdioecy are reviewed. All of the species in question appear to be functionally dioecious, with females retaining substantial anther vestiges. It is argued that this morphological androdioecy is in no way indicative of a previous functionally androdioecious state. The details of the reproductive biology of many of these species seem rather to be consistent with their having evolved dioecy via gynodioecy.
The rarity of androdioecy, as a route to the evolution of dioecy, suggests that re-allocation of reproductive resources is unlikely to be the sole factor of importance, and supports an important role for inbreeding avoidance. The fact that females in some dioecious species retain anthers of substantial size, containing considerable quantities of pollen, gives further support to the view that male-sterility mutations can sometimes be favoured even when little or no resources are re-allocated to male functions. This is impossible without substantial selfing and inbreeding. It is therefore concluded that inbreeding avoidance is generally important in the evolution of dioecy, though reallocation of reproductive resources is also necessary.     

Evidence of restoration cost in the annual gynodioecious Phacelia dubia

A negative pleiotropic effect on fitness of nuclear sex‐determining genes (cost of restoration) could explain nuclear–cytoplasmic gynodioecy but rarely has been demonstrated empirically. In a gynodioecious Phacelia dubia population, maternal lineages produce only hermaphroditic progenies irrespective of the pollen parent (N) or can segregate females (S). Natural progenies of N maternal plants had lower seed viability than that of S. Full‐sib progenies of unrelated hermaphrodites from all possible matings between N and S lineages had similar pollen filling but differed in sporophyte performance, mainly at seed germination stage. A discrete multivariate analysis reveals that the performance of N_♀ × S_♂ progeny at early stages of development was significantly lower than that of the other three types of mating in agreement with the silent‐cost‐of‐restoration hypothesis, affecting the sporophyte. The restoration cost and male sterility appear to be dominant and consequence of nuclear–cytoplasmic incompatibilities that may maintain nuclear–cytoplasmic polymorphism by frequency‐dependent selection.     

Paternal leakage sustains the cytoplasmic polymorphism underlying gynodioecy but remains invasible by nuclear restorers

Cytoplasmic male sterility (CMS) in plants often results in gynodioecious populations, composed of hermaphrodites and male-sterile females. All models of gynodioecy assume maternal inheritance of the cytoplasmic alleles and postulate a variety of negatively frequency-dependent mechanisms to maintain the cytoplasmic polymorphisms observed in many natural populations. However, in some plant species, mitochondria are transmitted at least occasionally by pollen, a process called paternal leakage. We show that even a small amount of paternal leakage is sufficient to sustain a permanent, stable cytoplasmic polymorphism. Because only hermaphrodites provide pollen in gynodioecious species, the effects of paternal leakage are biased and occur more often from the non-CMS male-fertile haplotype to the CMS male-sterile haplotype. We also show that a nuclear restorer disrupts the polymorphic cytoplasmic equilibrium, leading to fixation of both the CMS allele and the restorer. Although a dominant nuclear restorer fixes, it fixes much more slowly than in the standard CMS models. Although a stable cytonuclear polymorphism is possible with "matching alleles" nuclear restoration, oscillations to low frequencies present a risk of loss by drift. Paternal leakage enhances the stability of joint cytonuclear polymorphism by reducing the chance that a CMS allele is lost by drift.     

Merging theory and mechanism in studies of gynodioecy

In gynodioecious species, females and hermaphrodites coexist and the genetics of sex determination is usually nuclear cytoplasmic. Maintaining nuclear-cytoplasmic gynodioecy requires polymorphism for the feminizing genes (contained in the mitochondria) and the genes that restore male fertility (contained in the nucleus). This complex polymorphism depends, in part, on there being negative pleiotropic effects (i.e. costs) of the nuclear restorer alleles. Here, we combine information from theoretical studies and studies on the molecular action of restorer alleles in crops to interpret the probable costs of such alleles, and suggest how various aspects of the theoretical models could be tested. In doing so, we highlight how crops can be used to address evolutionary questions about the maintenance of nuclear-cytoplasmic gynodioecy.