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1.
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.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
The importance of ecological factors such as sex lability, spatial segregation, and resource allocation in the evolution of dioecy were examined in Schiedea globosa. S. globosa is a subdioecious species with equal numbers of plants possessing strictly male or female function and a small proportion of hermaphrodites. The propensity for labile sex expression was under both environmental and genetic control; some plants with male function became hermaphroditic (by producing female flowers) under better growing conditions in the field and in the greenhouse. There was some spatial segregation of the sexes. Because of sex lability, more hermaphrodites than males occurred on moister slopes. Although there were not measurable sex-related differences in mortality within or between two flowering seasons, more females than males and hermaphrodites occurred at the bottom of slopes. Males and females produced the same number of ramets and inflorescences, but females had a greater number of flowers per inflorescence. Males and females had the same number of ovules (vestigial in males), but females had larger ovules and longer stigmas. Hermaphrodites and males had the same amount of pollen per flower despite the production of fruit by the hermaphrodites. In hermaphrodites, there was no apparent tradeoff within flowers between pollen production and ovule production. These results indicate that spatial segregation, sex lability, and environmental conditions influence allocation patterns of S. globosa, and in combination with high inbreeding depression and selling rates, may promote the further evolution of dioecy in S. globosa.  相似文献   

8.
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.  相似文献   

9.
Field and laboratory studies of 19 diclinous species endemic to Australia help to clarify the nature and evolution of andromonoecy, androdioecy, and dioecy in the genus Solanum. Ten species are andromonoecious; typically these species bear inflorescences with a single, large basal hermaphroditic flower and 12–60 distal, smaller staminate flowers. We suggest that the andromonoecious condition was derived from hermaphroditic-flowered ancestors in part by hemisterilization of flowers but largely by addition of staminate flowers. The resultant larger inflorescences are hypothesized to serve both to attract and to entrain pollinators, yielding more or higher-quality seed set in hermaphroditic flowers and/or greater dispersion of pollen from staminate flowers. We suggest that andromonoecy may also serve to reduce selling. Nine other species are morphologically androdioecious but functionally dioecious. In these species, staminate flowers, like those of the andromonoecious species, bear anthers with copious tricolporate pollen and a highly reduced gynoecium. The morphologically hermaphroditic flowers are functionally pistillate and borne singly in inflorescences, and they bear anthers with inaperturate pollen. The inaperturate pollen, although viable, never germinates and is hypothesized to be retained in pistillate flowers as a reward to pollinators in the nectarless Solanum flowers. All other species of Solanum studied with pollen dimorphism in which one pollen morph is inaperturate are also best treated as functionally dioecious. We conclude that there is no evidence for androdioecy in Solanum. A review of other families suggests that there is little support for this unusual breeding system in any other angiosperm group either. Preliminary analyses suggest that andromonoecy and dioecy are polyphyletic in Solanum. Furthermore, dioecy is as likely to have arisen from hermaphroditic as from andromonoecious ancestors.  相似文献   

10.
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.  相似文献   

11.
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.  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
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.  相似文献   

15.
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.  相似文献   

16.
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.  相似文献   

17.
Abstract. Crustaceans in the class Branchiopoda exhibit a wide range of breeding systems, including dioecy (gonochorism), androdioecy, parthenogenesis, cyclic parthenogenesis, and hermaphroditism. The largest subgroup of the Branchiopods, the Diplostraca, is reported to encompass all five of these breeding systems. However, many of these reports are based primarily on simple observations of sex ratios in natural populations. Herein we report the beginnings of a more rigorous approach to breeding system determination in the Diplostraca, starting with the family Limnadiidae. We combine measurements of sex ratio, offspring rearings, and behavior to identify three breeding systems within the Limnadiidae: dioecy, androdioecy, and selfing hermaphroditism. To date, no instances of parthenogenetic reproduction have been identified in this family. Comparisons of breeding system determination via simple population sex ratios with our more controlled studies show that simple sex ratios can be useful when these sex ratios are ∼50% males (=dioecy) or 5–30% males (androdioecy). However, population sex ratios of 0–5% males or 35–45% males necessitate further investigation because estimates in these ranges cannot distinguish selfing hermaphroditism from androdioecy or androdioecy from dioecy, respectively. We conclude by noting that the genetic sex-determining system outlined for one of these limnadiid species, Eulimnadia texana , provides a parsimonious framework to describe the evolution of the three breeding systems observed within the Limnadiidae.  相似文献   

18.
Crustaceans in the order Spinicaudata display a broad range of reproductive strategies, ranging from pure hermaphroditism to pure dioecy (separate males and females), and intermediate combinations. One particularly interesting genus of these “clam shrimps” is Eulimnadia. Based on offspring sex ratios, it has been suggested that all members of the genus are androdioecious: populations consist of mixtures of males and hermaphrodites. However, only two of the ~40 species in this genus have been examined histologically to confirm the presence of ovotestes in the purported hermaphrodites of this group. Here, we report both sex ratio and histological evidence showing that populations of five additional Eulimnadia species from India and Thailand are indeed mixes of males and hermaphrodites (four species) or hermaphrodite only (one species). Sex ratios of adults and offspring from isolated hermaphrodites are in accordance with those previously reported for 15 Eulimnadia species, and histological assays of four of the five species show the presence of both testicular and ovarian tissue in these hermaphrodites. As has been previously reported, the testicular tissue in members of these Eulimnadia spp. is located in a small section at the distal end of the gonad. In addition, the sperm produced in these hermaphrodites forms distinct plaques of compacted chromatin. Overall, these data are consistent with a single origin of hermaphroditism in Eulimnadia, and support the notion that all members of the genus are either androdioecious or all‐hermaphroditic.  相似文献   

19.
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.  相似文献   

20.
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