Clonal genetic structure and diversity in populations of an aquatic plant with combined vs. separate sexes

Clonality is often implicated in models of the evolution of dioecy, but few studies have explicitly compared clonal structure between plant sexual systems, or between the sexes in dioecious populations. Here, we exploit the occurrence of monoecy and dioecy in clonal Sagittaria latifola (Alismataceae) to evaluate two main hypotheses: (i) clone sizes are smaller in monoecious than dioecious populations, because of constraints imposed on clone size by costs associated with geitonogamy; (ii) in dioecious populations, male clones are larger and flower more often than female clones because of sex‐differential reproductive costs. Differences in clone size and flowering could result in discordance between ramet‐ and genet‐based sex ratios. We used spatially explicit sampling to address these hypotheses in 10 monoecious and 11 dioecious populations of S. latifolia at the northern range limit in Eastern North America. In contrast to our predictions, monoecious clones were significantly larger than dioecious clones, probably due to their higher rates of vegetative growth and corm production, and in dioecious populations, there was no difference in clone size between females and males; ramet‐ and genet‐based sex ratios were therefore highly correlated. Genotypic diversity declined with latitude for both sexual systems, but monoecious populations exhibited lower genotypic richness. Differences in life history between the sexual systems of S. latifolia appear to be the most important determinants of clonal structure and diversity.     

Differences in female reproductive success between female and hermaphrodite individuals in the subdioecious shrub Eurya japonica (Theaceae)

Subdioecy is thought to occupy a transitional position in the gynodioecy–dioecy pathway, explaining one of the evolutionary routes from hermaphroditism to dioecy. Quantifying any female reproductive advantage of females versus hermaphrodites is fundamental to examining the spectrum between subdioecy and dioecy; however, this is challenging, as multiple interacting factors, such as pollen limitation and resource availability, affect plant reproduction. We compared the female reproductive success of females and hermaphrodites via a field experiment in which we hand‐pollinated individuals of the subdioecious shrub Eurya japonica of similar size growing under similar light conditions. Effects of pollen limitation and seed quality were also evaluated through comparing the results of hand‐ and natural‐pollination treatments and performing additional laboratory and greenhouse experiments. Overall, females had higher fruit set and produced heavier fruit and more seeds than hermaphrodites, and these results were more pronounced for hand‐pollinated than for natural‐pollinated plants of both sexes. We also found that seeds naturally produced by females had a higher mean germination rate. These results indicate that females had a pronounced advantage in female reproductive success under conditions of no pollen limitation. The sexual difference in the degree of pollen limitation suggests a pollinator‐mediated interaction, whereas the higher female reproductive success of females even under natural conditions implies that E. japonica is a good model species for elucidating the later stages of the gynodioecy–dioecy pathway.     

Sniffing out patterns of sexual dimorphism in floral scent

1.   A major transition in flowering plants has been the evolution of separate sexes (dioecy) from combined sexes (hermaphroditism). This transition is often, but not always, accompanied by the evolution of sexual dimorphism in attractive traits, and floral scent is no exception.
2.   In this review I aim to improve our understanding of variation in sexual dimorphism in floral scent characteristics by first explicating the relevant hypotheses, and then deriving explicit predictions for the pattern of floral scent from each.
3.   Next, I synthesize and qualitatively review published data on floral volatile emission rate and composition in 33 gender dimorphic species to identify emerging patterns, and evaluate these in light of predictions derived from the hypotheses.
4.   Although conclusions must be viewed as preliminary, a handful of strong patterns were revealed: (1) in the majority of the species studied males emit more volatiles per flower than females, (2) in over half of the species studied the sexes differed in at least one aspect of scent composition, (3) sexual dimorphism in scent composition was less common in species with rewardless females and/or brood site pollination than those that offered nectar and/or pollen, (4) a one-to-one correspondence between sex differences in overall scent composition and male organ-specific scent production is largely not found.
5.   This review has highlighted gaps in our understanding of the genesis of patterns of sexual dimorphism in floral scent, and makes clear that to move the field forward we need to shift our focus from pattern to process, and this will be best achieved by simultaneously testing alternative hypotheses at the same level of analysis.     

Selfish male‐determining element favors the transition from hermaphroditism to androdioecy

According to the current, widely accepted paradigm, the evolutionary transition from hermaphroditism toward separate sexes occurs in two successive steps: an initial, intermediate step in which unisexual individuals, male or female, sterility mutants coexist with hermaphrodites and a final step that definitively establishes dioecy. Two nonexclusive processes can drive this transition: inbreeding avoidance and reallocation of resources from one sexual function to the other. Here, we report results of controlled crosses between males and hermaphrodites in Phillyrea angustifolia, an androdioecious species with two mutually intercompatible, but intraincompatible groups of hermaphrodites. We observed different segregation patterns that can be explained by: (1) epistatic interactions between two unlinked diallelic loci, determining sex and mating compatibility, and (2) a mutation with pleiotropic effects: female sterility, full compatibility of males with both hermaphrodite incompatibility groups, and complete male‐biased sex‐ratio distortion in one of the two groups. Modeling shows that these mechanisms can explain the high frequency of males in populations of P. angustifolia and can promote the maintenance of androdioecy without requiring inbreeding depression or resource reallocation. We thus argue that segregation distortion establishes the right conditions for the evolution of cryptic dioecy and potentially initiates the evolution toward separate sexes.     

The transmission of genes via pollen and ovules in gynodioecious angiosperms

Gynodioecy is defined as the class of sexual dimorphism in which females are constant (functionally unisexual) and males are inconstant (bisexual). Quantitative measures describe the sexual performance of males in gynodioecious populations. The relative (male/female) ovule contribution compares the ovule contributions of the sexes to the next generation. Male constancy and inconstancy, the proportions of male genes transmitted via pollen and ovules, are direct measure of maleness and femaleness. Equations permitting the values of the sex parameters to be estimated are derived. The values obtained for twelve species show that gynodioecy is not a single breeding system, but merges into hermaphroditism and dioecy at opposite extremes. The distribution of inconstancies among individual males is described for six species. In most species, seed counts overestimate the ovule contributions of males.     

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.     

On the rarity of dioecy in flowering plants

Dioecy, the coexistence of separate male and female individuals in a population, is a rare but phylogenetically widespread sexual system in flowering plants. While research has concentrated on why and how dioecy evolves from hermaphroditism, the question of why dioecy is rare, despite repeated transitions to it, has received much less attention. Previous phylogenetic and theoretical studies have suggested that dioecy might be an evolutionary dead end. However, recent research indicates that the phylogenetic support for this hypothesis is attributable to a methodological bias and that there is no evidence for reduced diversification in dioecious angiosperms. The relative rarity of dioecy thus remains a puzzle. Here, we review evidence for the hypothesis that dioecy might be rare not because it is an evolutionary dead end, but rather because it easily reverts to hermaphroditism. We review what is known about transitions between hermaphroditism and dioecy, and conclude that there is an important need to consider more widely the possibility of transitions away from dioecy, both from an empirical and a theoretical point of view, and by combining tools from molecular evolution and insights from ecology.     

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.     

RECURRENT EVOLUTION OF DIOECY IN BRYOPHYTES

The origin and maintenance of separate sexes (dioecy) is an enduring evolutionary puzzle. Although both hermaphroditism and dioecy occur in many diverse clades, we know little about the long‐term evolutionary consequences of changing sexual system. Here we find evidence for at least 133 transitions between sexual systems in mosses, representing an almost unparalleled lability in the evolution of their sexual systems. Furthermore, in contrast to predictions, the transition rate from hermaphroditism to dioecy was approximately twice as high as the reverse transition. Our results also suggest that hermaphrodites may have higher rates of diversification than dioecious mosses. These results illustrate the utility of mosses for understanding the genomic and macroevolutionary consequences of hermaphroditism and dioecy.     

The role of hybridization in the evolution of sexual system diversity in a clonal,aquatic plant

The stable coexistence within populations of females, males, and hermaphrodites (subdioecy) is enigmatic because theoretical models indicate that maintenance of this sexual system involves highly restricted conditions. Subdioecy is more commonly interpreted as a transitory stage along the gynodioecious pathway from hermaphroditism to dioecy. The widespread, North American, aquatic plant Sagittaria latifolia is largely composed of monoecious or dioecious populations; however, subdioecious populations with high frequencies of hermaphrodites (mean frequency = 0.50) characterize the northern range boundary of dioecy in eastern North America. We investigated two hypotheses for the origin of subdioecy in this region. Using polymorphic microsatellite loci, we evaluated whether subdioecy arises through selection on standing genetic variation for male sex inconstancy in dioecious populations, or results from hybridization between monoecious and dioecious populations. We found evidence for both pathways to subdioecy, although hybridization was the more common mechanism, with genetic evidence of admixture in nine of 14 subdioecious populations examined. Hybridization has also played a role in the origin of androdioecious populations in S. latifolia, a mechanism not often considered in the evolution of this rare sexual system. Our study demonstrates how hybridization has the potential to play a role in the diversification of plant sexual systems.     

GENETIC EVIDENCE FOR MULTIPLE ORIGINS OF DIOECY IN THE HAWAIIAN SHRUB WIKSTROEMIA (THYMELAEACEAE)

Dioecy is unusually common in the Hawaiian Islands, yet little is known about the evolutionary biology of this breeding system. A native shrub, Wikstroemia, has an unusually diverse array of breeding systems: two forms of dioecy, cryptic and morphological dioecy, as well as hermaphroditism (perfect flowers). The existence of two forms of dioecy is significant for three reasons: 1) the presence of cryptic unisexuals that are functionally unisexual, but retain the appearance of hermaphroditism in both sexes, is strong evidence for the ancestral status of hermaphroditism; 2) the production of nonfunctional pollen, by female cryptic unisexuals, is a new instance of a phenomenon which has previously been reported for a few other species; 3) the two forms of dioecy are morphological markers which are useful in hybridization studies for tracing the genetic basis of their inheritance. Crosses were made between cryptically unisexual individuals (C), between morphologically unisexual individuals (M), and between the two types of unisexuality. The offspring of crosses between individuals with the same sex type usually resulted in offspring with that sex type, but most of the progeny of between-sex type crosses were, unexpectedly, perfect-flowered hermaphrodites. These results show that genetic control of sex determination is not homologous in all populations, suggesting that dioecy has evolved at least twice in Hawaiian Wikstroemia. The genetic data further suggest that males are the heterozygous sex.     

Maytenus obtusifolia Mart. (Celastraceae): a tropical woody species in a transitional evolutionary stage of the gynodioecy–dioecy pathway

‘Gynodioecy–dioecy’ is one of the pathways by which dioecy can arise from hermaphroditism. Studies on sex determination and development of gynodioecious systems have focused on temperate and/or annual species. Little is known about the evolutionary dynamics of gynodioecy and dioecy in perennial tropical species, where these systems have more frequently evolved. Maytenus obtusifolia is an abundant species in restingas in southeastern Brazil. The sexual system of M. obtusifolia was investigated by studying the floral structure and reproductive biology. We considered the sexual system as an intermediate step in the pathway gynodioecy-dioecy. The characterization of the hermaphrodite morph was complex, because of a gradient of variation in floral morphology and reproductive characteristics (sizes of the style and stigmatic surface, pollen viability, embryo sacs containing hypertrophied synergids, and fruit set). This variation leads to different proportions of functional male and female flowers among hermaphrodite plants and is responsible for the different levels of reproductive success. Female reproductive success and pollen viability were negatively correlated with the hermaphrodite morph (r = ?0.67). The higher fruiting intensity and fruit/flower ratio of females (41 %) compared to hermaphrodites (2 %) and the fact that female plants produce more and better-quality seeds support the female compensation. We suggest that female sterility may be linked to the set of changes in the carpels. The differences in the quality and quantity of pollen grains of hermaphrodite plants, and the similar individual rates of pollen viability observed for three consecutive flowering events, may indicate a relationship with nuclear cytoplasmic sex determination.     

Dioecy,more than monoecy,affects plant spatial genetic structure: the case study of Ficus

In this analysis, we attempt to understand how monoecy and dioecy drive spatial genetic structure (SGS) in plant populations. For this purpose, plants of the genus Ficus were used as a comparative model due to their particular characteristics, including high species diversity, variation in life histories, and sexual systems. One of the main issues we assessed is whether dioecious fig tree populations are more spatially genetically structured than monoecious populations. Using the Sp statistic, which allows for quantitative comparisons among different studies, we compared the extent of SGS between monoecious and dioecious Ficus species. To broaden our conclusions we used published data on an additional 27 monoecious and dioecious plant species. Furthermore, genetic diversity analyses were performed for two monoecious Ficus species using 12 microsatellite markers in order to strengthen our conclusions about SGS. Our results show that dioecy, more than monoecy, significantly contributes to SGS in plant populations. On average, the estimate of Sp was six times higher for dioecious Ficus species than monoecious Ficus species and it was two times higher in dioecious than monoecious plant species. Considering these results, we emphasize that the long‐distance pollen dispersal mechanism in monoecious Ficus species seems to be the dominant factor in determining weak spatial genetic structure, high levels of genetic diversity, and lack of inbreeding. Although Ficus constitute a model species to study SGS, a more general comparison encompassing a wider range of plants is required in order to better understand how sexual systems affect genetic structure.     

Sexual dimorphism and the genetic potential for evolution of sex allocation in the gynodioecious plant, Schiedea salicaria

Sex allocation theory addresses how separate sexes can evolve from hermaphroditism but little is known about the genetic potential for shifts in sex allocation in flowering plants. We tested assumptions of this theory using the common currency of biomass and measurements of narrow-sense heritabilities and genetic correlations in Schiedea salicaria, a gynodioecious species under selection for greater differentiation of the sexes. Female (carpel) biomass showed heritable variation in both sexes. Male (stamen) biomass in hermaphrodites also had significant heritability, suggesting the potential for further evolution of dioecy. Significant positive genetic correlations between females and hermaphrodites in carpel mass may slow differentiation between the sexes. Within hermaphrodites, there were no negative genetic correlations between male and female biomass as assumed by models for the evolution of dioecy, suggesting that S. salicaria is capable of further changes in biomass allocation to male and female functions and evolution toward dioecy.     

Male sterility, fitness gain curves and the evolution of gender specialization from distyly in Erythroxylum havanense

The evolution of dioecy from a monomorphic hermaphroditic condition requires two mutations, one producing females and one producing males. Conversely, a single mutation sterilizing one sexual function in one morph of distylous species would result in functional dioecy because such a mutation also affects the complementary function in the other morph. In this study, we tested these ideas with Erythroxylum havanense, a distylous species with morph-biased male sterility. Based on sex allocation theory we evaluated whether the invasion of thrum females is favoured over the maintenance of this morph cosexuals. Completely male sterile thrum plants obtained higher fitness returns than hermaphrodites or partial male sterile individuals of the same morph, thus favouring the invasion of female thrum plants. We concluded that because fruit production of pin individuals depends on the pollen produced by thrum plants, the invasion of thrum females would result on the evolution of functional dioecy.     

Phenological match drives pollen‐mediated gene flow in a temporally dimorphic tree

• Variation in flowering phenology is common in natural populations, and is expected to be, together with inter‐mate distance, an important driver of effective pollen dispersal. In populations composed of plants with temporally separated sexual phases (i.e. dichogamous or heterodichogamous populations), pollen‐mediated gene flow is assumed to reflect phenological overlap between complementary sexual phases. In this study, we conducted paternity analyses to test this hypothesis in the temporally dimorphic tree Acer opalus.
• We performed spatially explicit analyses based on categorical and fractional paternity assignment, and included tree size, pair‐wise genetic relatedness and morph type as additional predictors. Because differences between morphs in flowering phenology may also influence pollination distances, we modelled separate pollen dispersal kernels for the two morphs.
• Extended phenological overlap between male and female phases (mainly associated with inter‐morph crosses) resulted in higher siring success after accounting for the effects of genetic relatedness, morph type and tree size, while reduced phenological overlap (mainly associated with intra‐morph crosses) resulted in longer pollination distances achieved. Siring success also increased in larger trees.
• Mating patterns could not be predicted by phenology alone. However, as heterogeneity in flowering phenology was the single morph‐specific predictor of siring success, it is expected to be key in maintaining the temporal dimorphism in A. opalus, by promoting not only a prevalent pattern of inter‐morph mating, but also long‐distance pollination resulting from intra‐morph mating events.

     

Optimal sex allocation under pollen limitation

Most flowering plants are simultaneous hermaphrodites. Within species and even within local populations, sex allocation is usually highly plastic. Here, we link pollen sufficiency to the size of pollen-exchanging groups (i.e., pollen neighborhoods) and to pollen transfer efficiency, using an individual-based game-theoretic framework to determine the stable distribution of sex allocation that does not require the unrealistic assumption of infinitely large, panmictic populations. In the absence of selfing, we obtain the novel result that pollen limitation destabilizes hermaphroditism and favors separate sexes, whereas hermaphroditism remains stable without pollen limitation. With mixed mating, hermaphroditism is stable except when the fitness value of selfed offspring is less than half that of outcrossed offspring (i.e., strong inbreeding depression). In that case, the size of pollen neighborhoods, pollen transfer efficiencies, and the relative fitness of selfed offspring determine whether separate sexes or hermaphroditism is the stable outcome. The model thus predicts that separate sexes can derive from either of two ancestral states: obligate outcrossing under pollen limitation, or mixed mating (competing self-fertilization) under severe inbreeding depression. It also predicts conditions under which variance in sex-allocation among hermaphrodites within pollen exchanging groups along a gradient of pollen limitation can range from high (dioecy) to near zero (equal proportions of male and female investment).     

Evolutionary transitions among dioecy,androdioecy and hermaphroditism in limnadiid clam shrimp (Branchiopoda: Spinicaudata)

Examinations of breeding system transitions have primarily concentrated on the transition from hermaphroditism to dioecy, likely because of the preponderance of this transition within flowering plants. Fewer studies have considered the reverse transition: dioecy to hermaphroditism. A fruitful approach to studying this latter transition can be sought by studying clades in which transitions between dioecy and hermaphroditism have occurred multiple times. Freshwater crustaceans in the family Limnadiidae comprise dioecious, hermaphroditic and androdioecious (males + hermaphrodites) species, and thus this family represents an excellent model system for the assessment of the evolutionary transitions between these related breeding systems. Herein we report a phylogenetic assessment of breeding system transitions within the family using a total evidence comparative approach. We find that dioecy is the ancestral breeding system for the Limnadiidae and that a minimum of two independent transitions from dioecy to hermaphroditism occurred within this family, leading to (1) a Holarctic, all‐hermaphrodite species, Limnadia lenticularis and (2) mixtures of hermaphrodites and males in the genus Eulimnadia. Both hermaphroditic derivatives are essentially females with only a small amount of energy allocated to male function. Within Eulimnadia, we find several all‐hermaphrodite populations/species that have been independently derived at least twice from androdioecious progenitors within this genus. We discuss two adaptive (based on the notion of ‘reproductive assurance’) and one nonadaptive explanations for the derivation of all‐hermaphroditism from androdioecy. We propose that L. lenticularis likely represents an all‐hermaphrodite species that was derived from an androdioecious ancestor, much like the all‐hermaphrodite populations derived from androdioecy currently observed within the Eulimnadia. Finally, we note that the proposed hypotheses for the dioecy to hermaphroditism transition are unable to explain the derivation of a fully functional, outcrossing hermaphroditic species from a dioecious progenitor.     

Sex‐biased oviposition by a nursery pollinator on a gynodioecious host plant: Implications for breeding system evolution and evolution of mutualism

Dioecy, a breeding system where individual plants are exclusively male or female, has evolved repeatedly. Extensive theory describes when dioecy should arise from hermaphroditism, frequently through gynodioecy, where females and hermaphrodites coexist, and when gynodioecy should be stable. Both pollinators and herbivores often prefer the pollen‐bearing sex, with sex‐specific fitness effects that can affect breeding system evolution. Nursery pollination, where adult insects pollinate flowers but their larvae feed on plant reproductive tissues, is a model for understanding mutualism evolution but could also yield insights into plant breeding system evolution. We studied a recently established nursery pollination interaction between native Hadena ectypa moths and introduced gynodioecious Silene vulgaris plants in North America to assess whether oviposition was biased toward females or hermaphrodites, which traits were associated with oviposition, and the effect of oviposition on host plant fitness. Oviposition was hermaphrodite‐biased and associated with deeper flowers and more stems. Sexual dimorphism in flower depth, a trait also associated with oviposition on the native host plant (Silene stellata), explained the hermaphrodite bias. Egg‐receiving plants experienced more fruit predation than plants that received no eggs, but relatively few fruits were lost, and egg receipt did not significantly alter total fruit production at the plant level. Oviposition did not enhance pollination; egg‐receiving flowers usually failed to expand and produce seeds. Together, our results suggest that H. ectypa oviposition does not exert a large fitness cost on host plants, sex‐biased interactions can emerge from preferences developed on a hermaphroditic host species, and new nursery pollination interactions can arise as negative or neutral rather than as mutualistic for the plant.     

Complex sex determination in the stinging nettle Urtica dioica

The dioecious species Urtica dioica harbours wide variation in sex ratio of seeds. We conducted a series of crosses to analyse the genetic basis of sex determination in this species. Dutch populations of U. dioica contain low proportions of monoecious individuals beside male and female plants. Self-pollination of monoecious plants always yielded female, male and monoecious plants, generally in a ratio of one female to three male/monoecious individuals. This motivated us to write down a simple model in which gender is determined by one major sex-determination locus with four alleles. In the model males and monoecious plants have distinct genotypes but are both heterozygous at the sex-determination locus. We first made crosses among progeny obtained after self-pollination of monoecious plants. These crosses showed that the monoecious trait generally showed Mendelian inheritance and was passed on to the next generation via both pollen and seeds. Further crosses between monoecious plants and plants from dioecious system indicated that alleles from the dioecious system are often dominant. However, many exceptions to our genetic model are observed which suggest that dominance is incomplete and/or that more genes are involved in sex determination. We discuss to what extent sex determination genes explain the strongly biased seed sex ratios and argue that additional genes, for instance genes for female choice, must also be involved.