The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp

The nematode worm Caenorhabditis elegans and the clam shrimp Eulimnadia texana are two well‐studied androdioecious species consisting mostly of self‐fertilizing hermaphrodites and few males. To understand how androdioecy can evolve, a simple two‐step mathematical model of the evolutionary pathway from a male–female species to a selfing‐hermaphrodite species is constructed. First, the frequency of mutant females capable of facultative self‐fertilization increases if the benefits of reproductive assurance exceed the cost. Second, hermaphrodites become obligate self‐fertilizers if the fitness of selfed offspring exceeds one‐half the fitness of outcrossed offspring. Genetic considerations specific to C. elegans and E. texana show that males may endure as descendants of the ancestral male–female species. These models combined with an extensive literature review suggest a sexual conflict over mating in these androdioecious species: selection favours hermaphrodites that self and males that outcross. The strength of selection on hermaphrodites and males differs, however. Males that fail to outcross suffer a genetic death. Hermaphrodites may never encounter a rare male, and those that do and outcross only bear less fecund offspring. This asymmetric sexual conflict results in an evolutionary stand‐off: rare, but persistent males occasionally fertilize common, but reluctant hermaphrodites. A consequence of this stand‐off may be an increase in the longevity of the androdioecious mating system.     

Adaptive evolution of sexual systems in pedunculate barnacles

How and why diverse sexual systems evolve are fascinating evolutionary questions, but few empirical studies have dealt with these questions in animals. Pedunculate (gooseneck) barnacles show such diversity, including simultaneous hermaphroditism, coexistence of dwarf males and hermaphrodites (androdioecy), and coexistence of dwarf males and females (dioecy). Here, we report the first phylogenetically controlled test of the hypothesis that the ultimate cause of the diverse sexual systems and presence of dwarf males in this group is limited mating opportunities for non-dwarf individuals, owing to mating in small groups. Within the pedunculate barnacle phylogeny, dwarf males and females have evolved repeatedly. Females are more likely to evolve in androdioecious than hermaphroditic populations, suggesting that evolution of dwarf males has preceded that of females in pedunculates. Both dwarf males and females are associated with a higher proportion of solitary individuals in the population, corroborating the hypothesis that limited mating opportunities have favoured evolution of these diverse sexual systems, which have puzzled biologists since Darwin.     

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.     

A one‐locus model of androdioecy with two homomorphic self‐incompatibility groups: expected vs. observed male frequencies

Androdioecy, the occurrence of males and hermaphrodites in a single population, is a rare breeding system because the conditions for maintenance of males are restrictive. In the androdioecious shrub Phillyrea angustifolia, high male frequencies are observed in some populations. The species has a sporophytic self‐incompatibility (SI) system with two self‐incompatibility groups, which ensures that two groups of hermaphrodites can each mate only with the other group, whereas males can fertilize hermaphrodites of both groups. Here, we analyse a population genetic model to investigate the dynamics of such an androdioecious species, assuming that self‐incompatibility and sex phenotypes are determined by a single locus. Our model confirms a previous prediction that a slight reproductive advantage of males relative to hermaphrodites allows the maintenance of males at high equilibrium frequencies. The model predicts different equilibria between hermaphrodites of the two SI groups and males, depending on the male advantage, the initial composition of the population and the population size, whose effect is studied through stochastic simulations. Although the model can generate high male frequencies, observed frequencies are considerably higher than the model predicts. We finally discuss how this model may help explain the large male frequency variation observed in other androdioecious species of Oleaceae: some species show only androdioecious populations, as P. angustifolia, whereas others show populations either completely hermaphrodite or androdioecious.     

Intersexual conflict in androdioecious clam shrimp: Do androdioecious hermaphrodites evolve to avoid mating with males?

A recent sexual conflict model posits that a form of intersexual conflict may explain the persistence of males in androdioecious (males + hermaphrodites) populations of animals that are being selected to transition from dioecious (gonochoristic) mating to self‐compatible hermaphroditism. During the evolutionary spread of a self‐compatible hermaphrodite to replace females, the selective pressures on males to outcross are in conflict with the selective pressures on hermaphrodites to self. According to this model, the unresolved conflict interferes with the evolutionary trajectory from dioecy to hermaphroditism, slowing or halting that transition and strengthening the otherwise “transitory” breeding system of androdioecy into a potentially stable breeding strategy. Herein, we assess this model using two dioecious and two androdioecious clam shrimp (freshwater crustaceans) to ask two questions: (1) Have hermaphrodites evolved so that males cannot effectively recognize them?; and (2) Do androdioecious hermaphrodites avoid males? Androdioecious males made more mistakes than dioecious males when guarding potential mates suggesting that androdioecious males were less effective at finding hermaphrodites than dioecious males were at finding females. Similarly, in a three‐chambered experiment, focal hermaphrodites chose to aggregate with their same sex, whereas focal dioecious males chose to aggregate with the alternate sex. Together, these two experiments support the sexual conflict model of the maintenance of androdioecy and suggest that hermaphrodites are indeed evolving to avoid and evade males.     

When males and hermaphrodites coexist: a review of androdioecy in animals

Androdioecy (populations consisting of males and hermaphrodites)is a rare mating system in plants and animals: up to 50 plantsand only 36 animals have been described as being androdioecious,with most of the latter being crustaceans. To date, a thoroughcomparative analysis of androdioecy in animals has not beenundertaken. Herein we present such an analysis. Androdioecyhas only been extensively surveyed in 2 animal taxa: the nematodeCaenorhabditis and the clam shrimp Eulimnadia. The other majortaxon having androdioecious species is the Cirripedia (barnacles),but there are only limited studies on androdioecy in this group.In animals, androdioecy is found either in species that havemorphologically and ecologically distinct sexes (that is, hermaphroditesand small, "complemental" males) that are derived from hermaphroditicancestors (that is, the barnacles) or in species that have similarly-sizedmales and hermaphrodites that have been derived from dioeciousancestors (the remaining androdioecious species). We suggestthat the barnacles have evolved a sexual specialization in theform of these complemental males that can more efficiently usethe constrained habitats that these barnacles often experience.For the remaining species, we suggest that androdioecy has evolvedas a response to reproductive assurance in species that experienceepisodic low densities. Additionally, we hypothesize that thedevelopment of mechanisms allowing reproductive assurance inspecies with a number of sexually differentiated traits is mostlikely to result in androdioecy rather than gynodioecy (mixturesof females and hermaphrodites), and that these species may bedevelopmentally constrained to stay androdioecious rather thanbeing capable of evolving into populations solely consistingof efficient, self-compatible hermaphrodites. We conclude bysuggesting several areas in need of further study to understandmore completely the evolution and distribution of this interestingmating system in animals.     

Functional androdioecy in the rare endemic tree Tapiscia sinensis

Tapiscia sinensis, a rare endemic woody plant with both male and hermaphrodite individuals, is distributed in southern China. Whether T. sinensis is functionally androdioecious is unknown. In this study, we compare the male fitness between male and hermaphrodite individuals and perform pollination experiments in different habitats, identify the ability of actual siring of pollen from hermaphrodites and males under natural pollination, and discuss the evolution and maintenance of androdioecy in T. sinensis. Research suggests that flowers and fruits grow synchronously on hermaphrodite plants of T. sinensis from April to June. The males of T. sinensis had more than twice the genetic contribution of hermaphrodites through their male function and the fruit set from male pollination and cross‐pollination was the highest in all of the treatments, whereas that from self‐pollination was the lowest. Additionally, paternity analysis showed that the hermaphroditic pollen could result in siring success under natural pollination. The results showed that T. sinensis is a functionally androdioecious tree, that male individuals might evolve from a hermaphroditic ancestor and that the synchronous growth of flowers and fruit in hermaphrodites might facilitate the evolution and maintenance of androdioecy in T. sinensis.     

Mating behavior and time budget of an androdioecious crustacean,Eulimnadia texana (Crustacea: Conchostraca)

The clam shrimp,Eulimnadia texana (Crustacea, Conchostraca), is found in freshwater ephemeral environments throughtout the United States. Individual clam shrimp of this species are either hermaphroditic or male, a relatively rare mating system for animals known as androdioecy. Comparison of sex ratios between four neighboring populations ofE. texana in Southern New Mexico showed wide variation in the ratio of males to hermaphrodites with males making up as much as 42% of some populations and not occurring at all within others. Since little is known about the behavior of this species, an ethogram and time budget were prepared based on observations of laboratory populations. Males attempt to clasp hermaphrodites prior to mating. Precopulatory mate guarding occurs in this species. Outcrossing generally occurs during mate guarding and after the hermaphrodite molts. Hermaphrodites, however, seem to control the mating process. Successful mating by males never occured if the hermaphrodite struggled with him; hermaphrodite will self in the presence of males.     

Implications for the Maintenance of Androdioecy in the Freshwater Shrimp, Eulimnadia texana Packard: Encounters between Males and Hermaphrodites are not Random

Androdioecy is a mixed‐mating system in which there are males and hermaphrodites but no pure females. Few species exhibit such a mating system. Eulimnadia texana is a branchiopod crustacean that has recently been identified as an androdioecious species. This system is ideal for testing questions related to the evolution of sexual reproduction. We are testing a model that predicts androdioecy to be a stable mixed‐mating system under certain conditions. Specifically, we investigated whether encounters between males and hermaphrodites are random or if either sex seeks out the other for mating. Focal male or hermaphrodite clam shrimp were presented with stimulus shrimp of the other sex or kept alone. Swimming speed and time spent within different areas of a test chamber were recorded. Males did not alter mean swimming speed or spend more time than expected by chance near partitioned hermaphrodites. Hermaphrodites, however, decreased mean swimming speed in the presence of males and also spent more time than expected by chance near partitioned males, suggesting that hermaphrodites respond to male chemical and/or visual stimuli. Modified swimming behaviour probably facilitates inter‐sexual contact, thereby increasing opportunities for out‐crossing above that expected by random encounters.     

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.     

Do inbreeding depression and relative male fitness explain the maintenance of androdioecy in white mangrove, Laguncularia racemosa (Combretaceae)?

Mathematical models predict that to maintain androdioecious populations, males must have at least twice the fitness of male function in hermaphrodites. To understand how androdioecy is maintained in Laguncularia racemosa (white mangrove), outcrossing, inbreeding depression, and relative male fitness were estimated in two androdioecious populations and one hermaphroditic population. Outcrossing was estimated based on length of pollinator foraging bout and pollen carryover assumptions. Inbreeding depression was measured at three life stages: fruit set, seedling emergence, and seedling survivorship. The relative fitnesses of males and the male component of hermaphrodites were compared at these three stages and at the pollen production stage. Male frequency predictions generated by Lloyd's model were compared with observed frequencies in two androdioecious subpopulations. Outcrossing estimates were moderate for all populations (0.29-0.66). Inbreeding depression varied among populations (-0.03-0.86), but the strength of inbreeding depression did not increase with male frequency. Males produced significantly more flowers/inflorescence than hermaphrodites, but pollen production/flower did not differ. Male and hermaphroditic progeny did not differ significantly at other life stages. Populations of white mangrove with male plants were functionally androdioecious. Lloyd's model accurately predicted male frequency in one androdioecious subpopulation, but underestimated male frequency in the second subpopulation.     

Floral development in the androdioecious tree Tapiscia sinensis: Implications for the evolution to androdioecy

The evolutionary pathway between hermaphroditism and dioecy (females and males in a single population) draws widespread interests, and androdioecy (bisexuals and males in a single population) is rarely achieved as an intermediate state between the two breeding systems. Flower bud differentiations in the pistils of hermaphrodites and the pistillodes of males in androdioecious Tapiscia sinensis Oliv. are investigated by routine paraffin section technology, light microscopy, and scanning electron microscopy. A phylogenetic approach is used to analyze the origin of androdioecy. In T. sinensis, hermaphroditic flowers (HF) and male flowers (MF) experienced a similar development pattern in early flower bud differentiation, including the initiation of tepals and stamens. However, the carpel differentiation of MF and HF proceed in different patterns. In HF, the central zone bulges out and produces a ring meristem on which two to three carpel primordia emerge, which eventually developed into a normal pistil with a stigma, a style, and an ovary. However, in most MF, vestigial pistils are stem‐like (type I), and very few have an empty ovary (type II) or a sterile ovule (type III). Moreover, the evolution of sexual systems within the Huerteales indicates that hermaphroditism is the primitive character of T. sinensis. Tapiscia sinensis shows different degrees of reduction between male flowers and bisexual ones in the evolution to dioecy. Functional androdioecy originated from a hermaphroditic ancestor in T. sinensis and, as an intermediate sexual system, involves evolution from hermaphrodites to dioecy.     

A field test of a model for the stability of androdioecy in the freshwater shrimp,Eulimnadia texana

The evolution of hermaphroditism from dioecy is a poorly studied transition. Androdioecy (the coexistence of males and hermaphrodites) has been suggested as an intermediate step in this evolutionary transition or could be a stable reproductive mode. Freshwater crustaceans in the genus Eulimnadia have reproduced via androdioecy for 24+ million years and thus are excellent organisms to test models of the stability of androdioecy. Two related models that allow for the stable maintenance of males and hermaphrodites rely on the counterbalancing of three life history parameters. We tested these models in the field over three field seasons and compared the results to previous laboratory estimates of these three parameters. Male and hermaphroditic ratios within years were not well predicted using either the simpler original model or a version of this model updated to account for differences between hermaphroditic types (‘monogenic’ and ‘amphigenic’ hermaphrodites). Using parameter estimates of the previous year to predict the next year's sex ratios revealed a much better fit to the original relative to the updated version of the model. Therefore, counter to expectations, accounting for differences between the two hermaphroditic types did not improve the fit of these models. At the moment, we lack strong evidence that the long‐term maintenance of androdioecy in these crustaceans is the result of a balancing of life history parameters; other factors, such as metapopulation dynamics or evolutionary constraints, may better explain the 24+ million year maintenance of androdioecy in clam shrimp.     

MUTATIONAL MELTDOWN IN SELFING ARABIDOPSIS LYRATA

The majority of plant species and many animals are hermaphrodites, with individuals expressing both female and male function. Although hermaphrodites can potentially reproduce by self‐fertilization, they have a high prevalence of outcrossing. The genetic advantages of outcrossing are described by two hypotheses: avoidance of inbreeding depression because selfing leads to immediate expression of recessive deleterious mutations, and release from drift load because self‐fertilization leads to long‐term accumulation of deleterious mutations due to genetic drift and, eventually, to extinction. I tested both hypotheses by experimentally crossing Arabidopsis lyrata plants (self‐pollinated, cross‐pollinated within the population, or cross‐pollinated between populations) and measuring offspring performance over 3 years. There were 18 source populations, each of which was either predominantly outcrossing, mixed mating, or predominantly selfing. Contrary to predictions, outcrossing populations had low inbreeding depression, which equaled that of selfing populations, challenging the central role of inbreeding depression in mating system shifts. However, plants from selfing populations showed the greatest increase in fitness when crossed with plants from other populations, reflecting higher drift load. The results support the hypothesis that extinction by mutational meltdown is why selfing hermaphroditic taxa are rare, despite their frequent appearance over evolutionary time.     

Consequences of inbreeding depression due to sex-linked loci for the maintenance of males and outcrossing in branchiopod crustaceans.

Androdioecy, where males co-occur with hermaphrodites, is a rare sexual system in plants and animals. It has a scattered phylogenetic distribution, but it is common and has persisted for long periods of evolutionary time in branchiopod crustaceans. An earlier model of the maintenance of males with hermaphrodites in this group, by Otto et al. (1993), considered the importance of male-hermaphrodite encounter rates, sperm limitation, male versus hermaphrodite viability and inbreeding depression suffered by selfed progeny. Here I advance this model in two ways: (1) by exploring the conditions that would allow the invasion of hermaphrodites into a dioecious population and that of females into an androdioecious population; and (2) by incorporating a term that accounts for the potential effects of genetic load linked to a dominant hermaphrodite-determining allele in androdioecious populations. The new model makes plausible sense of observations made in populations of the species Eulimnadia texana, one of a number of related species whose common ancestor evolved hermaphroditism (and androdioecy) from dioecy. In particular, it offers an explanation for the long evolutionary persistence of androdioecy in branchiopods and suggests reasons for why dioecy has not re-evolved in the clade. Finally, it provides a rather unusual illustration of the implications of the degeneration of loci linked to a sex-determining locus.     

Pollen Production in the Androdioecious Datisca glomerata (Datiscaceae): Implications for Breeding System Equilibrium

Abstract Androdioecy (the presence of males and hermaphrodites in a breeding population) is a rare reproductive system in plants, with Datisca glomerata (Datiscaceae) representing the only well-documented example. Recent reports of high outcrossing rates, inbreeding depression, and high male pollen production satisfy theoretical predictions for the continued maintenance of androdioecy in populations of this species. However, in prior studies pollen production was measured indirectly in terms of numbers of anthers per flower—based on the assumption that male and hermaphroditic plant have equal numbers of flowers and that anthers from the two sexual morphs produce equivalent amounts of pollen. Herein, we demonstrate that male and hermaphrodite plants do not differ significantly in terms of flower number, but that pollen production in anthers from hermaphroditic plants is 12.6% higher than in anthers from male plants, thus refining the estimate of relative pollen fecundity of male versus hermaphrodite plants. The differential lowers the frequency of males predicted by theory, but is still consistent with the maintenance of androdioecy in this species.     

THE ROLE OF ANDRODIOECY AND GYNODIOECY IN MEDIATING EVOLUTIONARY TRANSITIONS BETWEEN DIOECY AND HERMAPHRODITISM IN THE ANIMALIA

Dioecy (gonochorism) is dominant within the Animalia, although a recent review suggests hermaphroditism is also common. Evolutionary transitions from dioecy to hermaphroditism (or vice versa) have occurred frequently in animals, but few studies suggest the advantage of such transitions. In particular, few studies assess how hermaphroditism evolves from dioecy or whether androdioecy or gynodioecy should be an “intermediate” stage, as noted in plants. Herein, these transitions are assessed by documenting the numbers of androdioecious and gynodioecious animals and inferring their ancestral reproductive mode. Both systems are rare, but androdioecy was an order of magnitude more common than gynodioecy. Transitions from dioecious ancestors were commonly to androdioecy rather than gynodioecy. Hermaphrodites evolving from sexually dimorphic dioecious ancestors appear to be constrained to those with female‐biased sex allocation; such hermaphrodites replace females to coexist with males. Hermaphrodites evolving from sexually monomorphic dioecious ancestors were not similarly constrained. Species transitioning from hermaphroditic ancestors were more commonly androdioecious than gynodioecious, contrasting with similar transitions in plants. In animals, such transitions were associated with size specialization between the sexes, whereas in plants these transitions were to avoid inbreeding depression. Further research should frame these reproductive transitions in a theoretical context, similar to botanical studies.     

Maintenance of androdioecy in the freshwater shrimp, Eulimnadia texana: do hermaphrodites need males for complete fertilization?

Androdioecy (populations comprised of mixtures of males and hermaphrodites) is a rare mating system, found only in a few plants and animals. The rarity of this system stems from the limited benefits to males in an otherwise all-hermaphroditic population. One of the potential benefits to males is typified by the nematode Caenorhabditis elegans, in which hermaphrodites do not produce sufficient sperm to fertilize all of their eggs. Here we explore the possibility that males are needed for complete fertilization of hermaphrodites' eggs in a second androdioecious animal, the clam shrimp Eulimnadia texana. We compare the fertilization rate of outcrossed to selfed eggs to test whether the latter exhibit lower fertilization due to sperm limitation (as in C. elegans). Because this comparison confounds differences in egg fertilization due to sperm limitation with the potential for early inbreeding depression, we also used a third mating treatment, a brother/sister cross, to allow separation of sperm limitation from inbreeding depression. In both populations examined, the proportion of eggs that were fertilized decreased linearly with increasing relatedness: comparing eggs produced by outcrossing, brother/sister, and selfed matings, respectively. This pattern suggests that differences in fertilization among these three treatments were caused solely by inbreeding depression, and therefore that hermaphrodites are not sperm limited. These results are combined with previous data on this species to test whether the maintenance of males can be explained using a population genetics model specifically designed for this species.     

Do tiny males grow up? Sperm competition and optimal resource allocation schedule of dwarf males of barnacles

Barnacles, marine crustaceans, have three sexual patterns: simultaneous hermaphroditism, dioecy and androdioecy. In dioecy and androdioecy, large individuals (females and hermaphrodites, respectively) are attached by dwarf males. Depending on species, some dwarf males grow up, others do not in their life time. To investigate which environmental conditions affect growth patterns of dwarf males of barnacles, we investigate the evolutionarily stable life history strategy of dwarf males using Pontryagin's maximum principle. Sperm competition among dwarf males and that among dwarf males and large hermaphrodites is taken into account. Dwarf males grow up in food-rich environments, while they do not grow at all in food-poor environments. ESS of the resource allocation schedule between reproduction and growth follows an "intermediate growth strategy" (simultaneous growth and reproduction) for dioecious species, in which sperm competition is not severe. On the other hand, it approaches "bang-bang control" (switching from allocating all resources toward growth then to reproduction), as sperm competition against surrounding large hermaphrodites becomes severe in androdioecious species.