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 ratio, reproductive mode and genetic diversity in Triops cancriformis

1. Aquatic invertebrates display a wide array of alternative reproductive modes from apomixis to hermaphroditism and cyclical parthenogenesis. These have important effects on genetic diversity and population structure. Populations of the 'living fossil' Triops cancriformis display a range of sex ratios, and various reproductive modes are thought to underlie this variation. Using sex ratio information and histological analyses European populations have been inferred to be gonochoric (with separate males and females), selfing hermaphroditic and androdioecious, a rare reproductive mode in which selfing hermaphrodites coexist with variable proportions of males. In addition, some populations have been described as meiotic parthenogens.
2. Here we use population genetic analysis using microsatellite loci in populations with a range of sex ratios including a gonochoric population, and marker segregation patterns in heterozygote individuals reared in isolation, to clarify the reproductive mode in this species.
3. Our data show that populations in general have very low levels of genetic diversity. Non-gonochoric populations show lower genetic diversity, more heterozygote deficiencies, higher inbreeding coefficients and stronger linkage disequilibria than the gonochoric population. The maintenance of some heterozygosity in populations is consistent with some male influence in T. cancriformis populations, as would be expected from an androdioecious reproductive system. Results of marker segregation in eggs produced in isolation from non-gonochoric populations indicate that meiosis occurs and are consistent with two reproductive modes: selfing hermaphroditism and a type of ameiotic parthenogenesis.
4. Overall, our data indicate that androdioecy and selfing hermaphroditism are the most likely reproductive modes of non-gonochoric European Triops populations. Triops populations are strongly structured, suggesting high genetic drift and low levels of gene flow.     

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.     

Selection in plant populations of effectively infinite size. V. Biallelic models of trioecy

A one-locus two-allele model of trioecy (presence of hermaphrodites, males and females in one population) is considered, in order to study the conditions for the persistence of this system. All possible assignments of the three sex types to the three genotypes are considered. This leads to three different modes of inheritance of trioecy, namely (a) females heterozygous, (b) males heterozygous and (c) hermaphrodites heterozygous, where in each mode each of the remaining two sex types is homozygous for one of the alleles. For mode (c) trioecy is always persistent, and the dependence of the sex ratio (for the three sex types) on the ovule and pollen fertilities and on the hermaphrodite selfing rate is specified. For the other two modes, (a) and (b), trioecy is not protected, i.e., it may not persist for any fertilities, viabilities or selfing rates. Thus, in this situation it is important to study the conditions under which the "marginal" systems of sexuality of trioecy, i.e., hermaphroditism, dioecy and gynodioecy in mode (a), and hermaphroditism, dioecy and androdioecy in mode (b), may become established. The results show that each marginal system may evolve from each other via trioecy. The evolution of dioecy is easier in mode (a) than in (b), so that female heterogamety would be expected to occur more often than male heterogamety in the present model. Under some conditions the breeding system obtained in equilibrium populations may depend on the initial genotype frequencies.—The necessity of considering modes of inheritance for sexual polymorphisms is demonstrated by comparing our results with those obtained from an evolutionary stable strategy (ESS) analysis of a purely phenotypic model.     

Sexual systems and life history of barnacles: a theoretical perspective

Thoracican barnacles show one of the most diverse sexual systems in animals: hermaphroditism, dioecy (males and females), and androdioecy (males and hermaphrodites). In addition, when present, male barnacles are very small and are called "dwarf males". The diverse sexual systems and male dwarfism in this taxon have attracted both theoretical and empirical biologists. In this article, we review the theoretical studies on barnacles' sexual systems in the context of sex allocation and life history theories. We first introduce the sex allocation models by Charnov, especially in relation to the mating group size, and a new expansion of his models is also proposed. We then explain three studies by Yamaguchi et al., who have studied the interaction between sex allocation and life history in barnacles. These studies consistently showed that limited mating opportunity favors androdioecy and dioecy over hermaphroditism. In addition, other factors, such as rates of survival and availability of food, are also important. We discuss the importance of empirical studies testing these predictions and how empirical studies interact with theoretical constructs.     

Ancient androdioecy in the freshwater crustacean Eulimnadia

Among the variety of reproductive mechanisms exhibited by living systems, one permutation--androdioecy (mixtures of males and hermaphrodites)--is distinguished by its rarity. Models of mating system evolution predict that androdioecy should be a brief stage between hermaphroditism and dioecy (separate males and females), or vice versa. Herein we report evidence of widespread and ancient androdioecy in crustaceans in the genus Eulimnadia, based on observations of over 33,000 shrimp from 36 locations from every continent except Antarctica. Using phylogenetic, biogeographical and palaeontological evidence, we infer that androdioecy in Eulimnadia has persisted for 24-180 million years and has been maintained through multiple speciation events. These results suggest that androdioecy is a highly successful aspect of the life history of these freshwater crustaceans, and has persisted for orders of magnitude longer than predicted by current models of this rare breeding system.     

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.     

Evolutionary consequences of gender plasticity in genetically dimorphic breeding systems

A functional view of gender helps evolutionary biologists evaluate the mechanisms underlying breeding-system evolution. Evolutionary pathways from hermaphroditism to dioecy include the intermediate breeding systems of gynodioecy and androdioecy. These pathways start with the invasion of unisexual mutants, females or males, respectively, followed by alteration of the hermaphrodites to allocate more to the sexual function that the unisexuals lack. Eventually, hermaphrodites become unisexual and dioecy has evolved. Some species evolving along these pathways stop short of completing this second step, or even revert back from dioecy. We evaluate the hypothesis that gender plasticity is involved in these transitions to and from dioecy. Evidence from studies of subdioecious species that have evolved along the gynodioecy pathway suggests that gender plasticity occurs and stabilizes subdioecy by lowering the cost of producing seed. Factors influencing species evolving toward androdioecy, or reverting to androdioecy from dioecy, appear to be more varied and include reproductive assurance, herbivory and gender plasticity. In general, gender specialization appears to be favored in resource-poor environments regardless of which pathway is taken to dioecy.     

The differentiation and development of pistils of hermaphrodites and pistillodes of males in androdioecious Osmanthus fragrans L. and implications for the evolution to androdioecy

The evolutionary pathway between hermaphroditism and dioecy draws widespread interests, and androdioecy 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 Osmanthus fragrans L. were investigated by paraffin sectioning to elucidate the evolution to androdioecy. Results showed that the regularity and rhythm in flower bud differentiation between males and hermaphrodites were almost consistent and included six main stages. However, the hermaphrodites always lagged behind the males at each stage. The apical floret in the same inflorescence developed earlier than did the lateral ones in both hermaphrodites and males. The most significant difference between males and hermaphrodites was observed at the carpel differentiation stage. Two carpel primordia appeared inside the stamens of both males and hermaphrodites at the initial stage. These two carpels gradually fused with each other in hermaphrodites and eventually developed into a normal pistil with a stigma, a style, and an ovary. However, a cavity grew conspicuously over time between two carpels as developed in males. The two carpels eventually developed into a pistillode with two independent bracteal tissues. However, from the whole development process, the male retained the developmental residue of the hermaphrodite. Thus, the pistillodes of males could be traced to the pistils of hermaphrodites. This finding shows that males may be derived from hermaphrodites in O. fragrans. On the basis of this finding and previous studies on Oleaceae, androdioecy could be regarded as a transition from hermaphroditism to dioecy in this family.     

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.     

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.     

The Evolution of Hermaphroditism from Dioecy in Crustaceans: Selfing Hermaphroditism Described in a Fourth Spinicaudatan Genus

The evolution of reproductive systems has intrigued evolutionary biologists for well over a century. Recent empirical and theoretical work has elucidated the evolution of dioecy (separate males and females) from hermaphroditism in many plant species. The reverse transition, evolving hermaphroditism from dioecy, has occurred many times in animals, and yet is poorly studied relative to its reverse analog in plants. Crustaceans in the sub-order Spinicaudata have evolved hermaphroditism from dioecy three separate times, in some cases forming all-hermaphroditic species and in others forming androdioecious (males + hermaphrodites) species. Herein we report evidence of hermaphroditism in a fourth spinicaudatan genus: the newly described Calalimnadia. We present sex ratio and anatomical evidence that Calalimnadia mahei comprises selfing hermaphrodites, with no males being found in over 10,000 offspring reared. We combine these reproductive results with those of other Spinicaudata to estimate the evolution of hermaphroditism in this crustacean sub-order. We use these genetic data combined with anatomical evidence to suggest that C. mahei represents a fourth, independent derivation of hermaphroditism from dioecy in these reproductively labile crustaceans.     

The genetic basis and experimental evolution of inbreeding depression in Caenorhabditis elegans

Determining the genetic basis of inbreeding depression is important for understanding the role of selection in the evolution of mixed breeding systems. Here, we investigate how androdioecy (a breeding system characterized by partial selfing and outcrossing) and dioecy (characterized by obligatory outcrossing) influence the experimental evolution of inbreeding depression in Caenorhabditis elegans. We derived inbred lines from ancestral and evolved populations and found that the dioecious lineages underwent more extinction than androdioecious lineages. For both breeding systems, however, there was selection during inbreeding because the diversity patterns of 337 single-nucleotide polymorphisms (SNPs) among surviving inbred lines deviated from neutral expectations. In parallel, we also followed the evolution of embryo to adult viability, which revealed similar starting levels of inbreeding depression in both breeding systems, but also outbreeding depression. Under androdioecy, diversity at a neutral subset of 134 SNPs correlated well with the viability trajectories, showing that the population genetic structure imposed by partial selfing affected the opportunity for different forms of selection. Our findings suggest that the interplay between the disruptions of coevolved sets of loci by outcrossing, the efficient purging of deleterious recessive alleles with selfing and overdominant selection with outcrossing can help explain mixed breeding systems.     

Repeated evolution of dioecy from androdioecy in Acer

The evolution of breeding systems was studied in the genus Acer, with special attention to the origin of androdioecy and dioecy, using a phylogenetic approach. Parsimony and maximum-likelihood techniques were used to infer the ancestral character state and trends in the evolution of breeding systems. Information on breeding systems was obtained from the literature, and phylogenetic relationships were taken from three published phylogenies. Although a general trend from duodichogamy to dioecy through heterodichogamy has been proposed for the genus Acer, our results show that a general trend is not detected when phylogenetic relationships are taken into account. Dioecy appeared as a derived state that evolved at least three times and never reversed towards other states. Three different paths to dioecy have been followed in the genus Acer: from heterodichogamous androdioecy; from heterodichogamous trioecy; and from dichogamous subdioecy. Therefore, although the best documented cases of evolution of androdioecy indicate that this breeding system evolves from dioecy, in the genus Acer the opposite situation occurs (androdioecy leading to dioecy). Here we discuss the role of inbreeding avoidance and sexual specialization as selective forces driving the evolution of dioecy in the genus Acer.     

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.     

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.     

Polyploidy and the sexual system: what can we learn from Mercurialis annua?

The evolutionary success of polyploidy most directly requires the ability of polyploid individuals to reproduce and transmit their genes to subsequent generations. As a result, the sexual system (i.e. the mating system and the sex allocation of a species) will necessarily play a key role in determining the fate of a new polyploid lineage. The effects of the sexual system on the evolution of polyploidy are complex and interactive. They include both aspects of the genetic system, the genetic load maintained in a population and the ecological context in which selection takes place. Here, we explore these complexities and review the empirical evidence for several potentially important genetic and ecological interactions between ploidy and the sexual system in plants. We place particular emphasis on work in our laboratory on the European annual plant Mercurialis annua , which offers promising scope for detailed investigations on this topic. M. annua forms a polyploid complex that varies in its sexual system from dioecy (separate sexes) through androdioecy (males and hermaphrodites) to functional hermaphroditism.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 547–560.     

How does breeding system variation modulate sexual antagonism?

The study of sexually antagonistic (SA) traits remains largely limited to dioecious (separate sex), mobile animals. However, the occurrence of sexual conflict is restricted neither by breeding system (the mode of sexual reproduction, e.g. dioecy or hermaphroditism) nor by sessility. Here, we synthesize how variation in breeding system can affect the evolution and expression of intra- and inter-locus sexual conflicts in plants and animals. We predict that, in hermaphrodites, SA traits will (i) display lower levels of polymorphism; (ii) respond more quickly to selection; and (iii) involve unique forms of interlocus conflict over sex allocation, mating roles and selfing rates. Explicit modelling and empirical tests in a broader range of breeding systems are necessary to obtain a general understanding of the evolution of SA traits.     

Mating group size and evolutionarily stable pattern of sexuality in barnacles

Barnacles, marine crustaceans, have various patterns of sexuality depending on species including simultaneous hermaphroditism, androdioecy (hermaphrodites and dwarf males), and dioecy (females and dwarf males). We develop a model that predicts the pattern of sexuality in barnacles by two key environmental factors: (i) food availability and (ii) the fraction of larvae that settle on the sea floor. Populations in the model consist of small individuals and large ones. We calculate the optimal resource allocation toward male function, female function and growth for small and large barnacles that maximizes each barnacle's lifetime reproductive success using dynamic programming. The pattern of sexuality is defined by the combination of the optimal resource allocations. In our model, the mating group size is a dependent variable and we found that sexuality pattern changes with the food availability through the mating group size: simultaneous hermaphroditism appears in food-rich environments, where the mating group size is large, protandric simultaneous hermaphroditism appears in intermediate food environments, where the mating group size also takes intermediate value, the other sexuality patterns, androdioecy, dioecy, and sex change are observed in food-poor environments, where the mating group size is small. Our model is the first one where small males can control their growth to large individuals, and hence has ability to explain a rich spectrum of sexual patterns found in barnacles.     

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.