Rare sexual reproduction events in the clonal reproduction system of introduced populations of the little fire ant

A unique reproductive system has previously been described in Wasmannia auropunctata, a widespread invasive ant species, where males are produced clonally, female queens are parthenogens, and female workers are produced sexually. However, these findings were mostly based on samples originating from only a limited part of the native range of the species in South America. We used microsatellite markers to uncover the reproductive modes displayed by a large number of nests collected in various invasive W. auropunctata populations introduced 40 years ago into New Caledonia, where the species now forms a single 450-km-long supercolony. Although the main reproduction system in New Caledonia remained clonality for both male and female reproductives, we found evidence of rare sexual reproduction events that led to the production of both new queen and male clonal lineages. All clonal lineages observed in New Caledonia potentially derived from sexual reproduction, recombination, and mutation events from a single female and a single male genotype. Hence, the male and female gene pools are not strictly separated in New Caledonia and the two sexes do not follow independent evolutionary trajectories. Our results also suggest genetic determination for both parthenogenesis and caste. We discuss the evolutionary implications of the emergence of sex in the clonal reproduction system of introduced populations of W. auropunctata.     

Meiotic recombination dramatically decreased in thelytokous queens of the little fire ant and their sexually produced workers

The little fire ant, Wasmannia auropunctata, displays a peculiar breeding system polymorphism. Classical haplo-diploid sexual reproduction between reproductive individuals occurs in some populations, whereas, in others, queens and males reproduce clonally. Workers are produced sexually and are sterile in both clonal and sexual populations. The evolutionary fate of the clonal lineages depends strongly on the underlying mechanisms allowing reproductive individuals to transmit their genomes to subsequent generations. We used several queen-offspring data sets to estimate the rate of transition from heterozygosity to homozygosity associated with recombination events at 33 microsatellite loci in thelytokous parthenogenetic queen lineages and compared these rates with theoretical expectations under various parthenogenesis mechanisms. We then used sexually produced worker families to define linkage groups for these 33 loci and to compare meiotic recombination rates in sexual and parthenogenetic queens. Our results demonstrate that queens from clonal populations reproduce by automictic parthenogenesis with central fusion. These same parthenogenetic queens produce normally segregating meiotic oocytes for workers, which display much lower rates of recombination (by a factor of 45) than workers produced by sexual queens. These low recombination rates also concern the parthenogenetic production of queen offspring, as indicated by the very low rates of transition from heterozygosity to homozygosity observed (from 0% to 2.8%). We suggest that the combination of automixis with central fusion and a major decrease in recombination rates allows clonal queens to benefit from thelytoky while avoiding the potential inbreeding depression resulting from the loss of heterozygosity during automixis. In sterile workers, the strong decrease of recombination rates may also facilitate the conservation over time of some coadapted allelic interactions within chromosomes that might confer an adaptive advantage in habitats disturbed by human activity, where clonal populations of W. auropunctata are mostly found.     

Repeated evolution of queen parthenogenesis and social hybridogenesis in Cataglyphis desert ants

Over the last decade, genetic studies on social insects have revealed a remarkable diversity of unusual reproductive strategies, such as male clonality, female clonality, and social hybridogenesis. In this context, Cataglyphis desert ants are useful models because of their unique reproductive systems. In several species, queens conditionally use sexual reproduction and parthenogenesis to produce sterile workers and reproductive queens, respectively. In social hybridogenesis, two distinct genetic lineages coexist within a population, and workers result from mating between partners of different lineages; in contrast, queens and males are both produced asexually by parthenogenesis. Consequently, nonreproductive workers are all interlineage hybrids, whereas reproductives are all pure lineage individuals. Here, we characterized the reproductive systems of 11 species to investigate the distribution of the conditional use of sex and social hybridogenesis in Cataglyphis. We identified one new case in which sexual reproduction was conditionally used in the absence of dependent‐lineage reproduction. We also discovered five new instances of social hybridogenesis. Based on our phylogenetic analyses, we inferred that both the conditional use of sex and social hybridogenesis independently evolved multiple times in the genus Cataglyphis.     

Appearance of males in a thelytokous strain of Milnesium cf. tardigradum (Tardigrada)

Tardigrades are generally gonochoristic. Many moss-dwelling species propagate by parthenogenesis, but heterogony has not yet been found. Milnesium tardigradum, a carnivorous tardigrade, also has both sexes, but males are usually rare and many populations appear to have only parthenogenetic reproduction. Since 2000, I have maintained a thelytokous strain of Milnesium cf. tardigradum that originated from one female. Individuals of this strain were thought to be all females, but here I report that males have emerged in this strain at a very low frequency. This is the first report of the appearance of males in parthenogenetic tardigrades. On the first pair of legs of some individuals, I observed the modified claws characteristic of males of this species. It is unknown whether these males can actually function in sexual reproduction; however, they might allow some possibility of genetic exchange among clonal populations. No environmental factors that generate males were determined.     

Evolutionary genetics and ecology of sperm-dependent parthenogenesis

Sperm-dependent (or pseudogamous) forms of parthenogenetic reproduction occur in a wide variety of animals. Inheritance is typically clonal and matroclinous (of female descent), but sperm are needed to initiate normal development. As opposed to true parthenogenesis (i.e., sperm-independent reproduction), pseudogamous parthenogenetic lineages must coexist with a ‘sperm donor’— e.g., males from a conspecific sexual lineage, conspecific hermaphrodites, or males from a closely related sexual species. Such sperm donors do not contribute genetically to the next generation. The parasitic nature of sperm-dependent parthenogenesis raises numerous ecological and evolutionary questions. How do they arise? What factors help stabilize coexistence between the pseudogamous parthenogens and their sperm donors (i.e., ‘sexual hosts’)? Why do males waste sperm on the asexual females? Why does true parthenogenesis not evolve in pseudogamous lineages and free them from their dependency on sperm donors? Does pseudogamous parthenogenesis provide compensatory benefits that outweigh the constraints of sperm-dependence? Herein, we consider some genetic, ecological, and geographical consequences of sperm-dependent parthenogenesis in animals.     

Facultative sexual reproduction in the parthenogenetic ant Platythyrea punctata

Summary. In a few, scattered species of social Hymenoptera, unmated workers are capable of producing female offspring from unfertilized eggs through thelytokous parthenogenesis. Regular thelytoky has previously been demonstrated in a number of populations of the neotropical ant Platythyrea punctata. Nevertheless, the finding of males and inseminated queens and workers suggested the sporadic occurrence of sex. In this study we investigated the genetic structure of colonies from Puerto Rico and Costa Rica in order to detect traces of occasional sexual reproduction. Most Puerto Rican colonies had a clonal structure with all nestmates sharing the same multilocus genotype, indicating that thelytoky is the predominant mode of reproduction. Genetic variability was detected in six of 18 colonies and might have arisen from adoption of alien workers in one colony and from the adoption of alien workers, recombination during parthenogenesis, or sexual reproduction in the other colonies. The reproductive of one of these latter colonies was found to be an inseminated worker (gamergate), and the genotypes of its nestmates definitively suggested recombination and sexual reproduction. Three gamergates were found in a single colony collected in Costa Rica, and all produced offspring from fertilized eggs, while uninseminated workers were apparently incapable of reproducing by thelytoky.Received 10 August 2004; revised 20 October 2004; accepted 3 November 2004.     

Synthesis of clonality and polyploidy in vertebrate animals by hybridization between two sexual species

Because most clonal vertebrates have hybrid genomic constitutions, tight linkages are assumed among hybridization, clonality, and polyploidy. However, predictions about how these processes mechanistically relate during the switch from sexual to clonal reproduction have not been validated. Therefore, we performed a crossing experiment to test the hypothesis that interspecific hybridization per se initiated clonal diploid and triploid spined loaches (Cobitis) and their gynogenetic reproduction. We reared two F1 families resulting from the crossing of 14 pairs of two sexual species, and found their diploid hybrid constitution and a 1:1 sex ratio. While males were infertile, females produced unreduced nonrecombinant eggs (100%). Synthetic triploid females and males (96.3%) resulted in each of nine backcrossed families from eggs of synthesized diploid F1s fertilized by haploid sperm from sexual males. Five individuals (3.7%) from one backcross family were genetically identical to the somatic cells of the mother and originated via gynogenesis; the sperm of the sexual male only triggered clonal development of the egg. Our reconstruction of the evolutionary route from sexuality to clonality and polyploidy in these fish shows that clonality and gynogenesis may have been directly triggered by interspecific hybridization and that polyploidy is a consequence, not a cause, of clonality.     

Sex-specific pattern of spatial genetic structure in dioecious and clonal tree species, <Emphasis Type="Italic">Populus alba</Emphasis> L.

Fine-scale spatial genetic structure (SGS) has profound ecological and genetic consequences for plant populations, and some studies indicate that clonal reproduction may significantly enhance SGS. Clonality is widespread among dioecious species, but little is known about the relationship between clonal reproduction and SGS in the frame of sexual dimorphism. We asked the following questions: (1) Is there a sexually dependent pattern of SGS in white poplar population? (2) What is the relationship between clonal reproduction and SGS? and (3) Does this relationship have a sex-specific component? Using 16 microsatellite markers, genetic structure including fine-scale SGS and clonality of females and males of white poplar were investigated. Significant SGS was noted for both sexes at the ramet and genet levels. At the genet level, males had 2.7-fold higher SGS than that of females. Clonality significantly contributed to SGS only in females. A sibship structure revealed with pedigree analysis and clustering-based methods among males was likely the major factor of the observed SGS. The sexes differed in their clonal growth strategies. Spatial positioning of ramets in female clones suggested foraging behavior and/or avoidance of competition, while for male clones it indicated more expansion and space colonization. The obtained results led us to conclude that sexual dimorphism in life history traits may affect the course and rate of demo-genetic processes acting in natural populations of dioecious species. To our knowledge, this is the first study demonstrating a sex-specific pattern of SGS in natural populations of dioecious species.     

Ant queens adjust egg fertilization to benefit from both sexual and asexual reproduction

An enduring problem in evolutionary biology is the near ubiquity of sexual reproduction despite the inherent cost of transmitting only half the parent's genes to progeny. Queens of some ant species circumvent this cost by using selectively both sexual reproduction and parthenogenesis: workers arise from fertilized eggs, while new queens are produced by parthenogenesis. We show that queens of the ant Cataglyphis cursor maximize the transmission rate of their genes by regulating the proportion of fertilized and parthenogenetic eggs laid over time. Parthenogenetic offspring are produced in early spring, when workers raise the brood into sexuals. After the mating period, queens lay mostly fertilized eggs that will be reared as the non-reproductive caste.     

Unisexual reproduction among vertebrates

The past decade has seen a remarkable revision of perspectives on unisexual reproduction in vertebrates. One can no longer view it as a rare curiosity far outside the mainstream of evolution. More than 80 taxa of fish, amphibians, and reptiles are now known to reproduce by parthenogenesis (Greek for 'virgin birth') or its variants, and they persist in nature as all-female lineages. Other lower vertebrates that ordinarily rely on sexual reproduction can resort to facultative parthenogenesis under extenuating circumstances that isolate females from males. Molecular tools have now been applied to the study of unisexual organisms, and fascinating insights have emerged regarding the molecular mechanisms that preserve heterozygosity and increase genetic diversity in all-female populations. A deeper understanding of the underlying genetics increasingly calls into question the assumption that unisexuality in vertebrates is an evolutionary dead-end.     

Genetic determination of female castes in a hybridogenetic desert ant

In most social insects, the brood is totipotent and environmental factors determine whether a female egg will develop into a reproductive queen or a functionally sterile worker. However, genetic factors have been shown to affect the female's caste fate in a few ant species. The desert ant Cataglyphis hispanica reproduces by social hybridogenesis. All populations are characterized by the coexistence of two distinct genetic lineages. Queens are almost always found mated with a male of the alternate lineage than their own. Workers develop from hybrid crosses between the genetic lineages, whereas daughter queens are produced asexually via parthenogenesis. Here, we show that the association between genotype and caste in this species is maintained by a ‘hard‐wired’ genetic caste determination system, whereby nonhybrid genomes have lost the ability to develop as workers. Genetic analyses reveal that, in a rare population with multiple‐queen colonies, a significant proportion of nestmate queens are mated with males of their own lineage. These queens fail to produce worker offspring; they produce only purebred daughter queens by sexual reproduction. We discuss how the production of reproductive queens through sexual, intralineage crosses may favour the stability of social hybridogenesis in this species.     

A single locus determines thelytokous parthenogenesis of laying honeybee workers (Apis mellifera capensis)

The evolution and maintenance of parthenogenetic species are a puzzling issue in evolutionary biology. Although the genetic mechanisms that act to restore diploidy are well studied, the underlying genes that cause the switch from sexual reproduction to parthenogenesis have not been analysed. There are several species that are polymorphic for sexual and parthenogenetic reproduction, which may have a genetic basis. We use the South African honeybee subspecies Apis mellifera capensis to analyse the genetic control of thelytoky (asexual production of female workers). Due to the caste system of honeybees, it is possible to establish classical backcrosses using sexually reproducing queens and drones of both arrhenotokous and thelytokous subspecies, and to score the frequency of parthenogenesis in the resulting workers. We found Mendelian segregation for thelytoky of egg-laying workers, which appears to be controlled by a single major gene (th). The segregation pattern indicates a recessive allele causing thelytoky. We found no evidence for maternal transmission of bacterial endosymbionts controlling parthenogenesis. Thelytokous parthenogenesis of honeybee workers appears to be a classical qualitative trait, because we did not observe mixed parthenogenesis (amphitoky), which might be expected in the case of multi-locus inheritance.     

Effects of clonality on the genetic variability of rare,insular species: the case of Ruta microcarpa from the Canary Islands

Many plant species combine sexual and clonal reproduction. Clonal propagation has ecological costs mainly related to inbreeding depression and pollen discounting; at the same time, species able to reproduce clonally have ecological and evolutionary advantages being able to persist when conditions are not favorable for sexual reproduction. The presence of clonality has profound consequences on the genetic structure of populations, especially when it represents the predominant reproductive strategy in a population. Theoretical studies suggest that high rate of clonal propagation should increase the effective number of alleles and heterozygosity in a population, while an opposite effect is expected on genetic differentiation among populations and on genotypic diversity. In this study, we ask how clonal propagation affects the genetic diversity of rare insular species, which are often characterized by low levels of genetic diversity, hence at risk of extinction. We used eight polymorphic microsatellite markers to study the genetic structure of the critically endangered insular endemic Ruta microcarpa. We found that clonality appears to positively affect the genetic diversity of R. microcarpa by increasing allelic diversity, polymorphism, and heterozygosity. Moreover, clonal propagation seems to be a more successful reproductive strategy in small, isolated population subjected to environmental stress. Our results suggest that clonal propagation may benefit rare species. However, the advantage of clonal growth may be only short‐lived for prolonged clonal growth could ultimately lead to monoclonal populations. Some degree of sexual reproduction may be needed in a predominantly clonal species to ensure long‐term viability.     

The approach to equilibrium of multilocus genotype diversity under clonal selection and cyclical parthenogenesis

Sexual generations in cyclical parthenogens are typically separated by multiple generations of clonal reproduction. In contrast to sexual reproduction, during parthenogenesis the genome of the parent is passed on to the offspring as a unit. The absence of recombination during parthenogenesis leads to differences in the action of natural selection in the two reproductive phases. In addition, since recombination is a sampling process, random genetic drift is potentially more important in sexual reproduction than in parthenogenesis. A recent development in the study of rotifer population genetics is the use of microsatellites to characterize natural populations. Microsatellites are selectively neutral, show patterns of Mendelian inheritance and tend to be much more variable than allozymes. An advantage over allozymes is that microsatellite DNA can be cloned with PCR and thus multiple loci can be assayed from a single individual. We use a new computer model in this paper to investigate the response of selectively active and selectively neutral genes to evolutionary forces during cyclical parthenogenesis. Selectively active alleles may respond differently to selection in the parthenogenetic and sexual phases of cyclical parthenogenesis. Even when strong clonal selection is acting on loci associated with adaptation, the view that emerges with microsatellites may be one of Hardy-Weinberg and linkage equilibrium. Thus studies using selectively neutral loci may fail to detect clonal selection even when it is an important feature of the rotifer population's adaptive structure.     

Sexual cooperation: mating increases longevity in ant queens

Divergent reproductive interests of males and females often cause sexual conflict . Males of many species manipulate females by transferring seminal fluids that boost female short-term fecundity while decreasing their life expectancy and future reproductivity . The life history of ants, however, is expected to reduce sexual conflict; whereas most insect females show repeated phases of mating and reproduction, ant queens mate only during a short period early in life and undergo a lifelong commitment to their mates by storing sperm . Furthermore, sexual offspring can only be reared after a sterile worker force has been built up . Therefore, the males should also profit from a long female lifespan. In the ant Cardiocondyla obscurior, mating indeed has a positive effect on the lifetime reproductive success of queens. Queens that mated to either one fertile or one sterilized male lived considerably longer and started laying eggs earlier than virgin queens. Only queens that received viable sperm from fertile males showed increased fecundity. The lack of a trade-off between fecundity and longevity is unexpected, given evolutionary theories of aging . Our data instead reveal the existence of sexual cooperation in ants.     

Androgenesis is a maternal trait in the invasive ant Wasmannia auropunctata

Androgenesis is the production of an offspring containing exclusively the nuclear genome of the fathering male via the maternal eggs. This unusual mating system is generally considered a male trait, giving to androgenetic males a substantial fitness advantage over their sexually reproducing relatives. We here provide the first empirical study of the evolutionary outcomes of androgenesis in a haplo-diploid organism: the invasive ant Wasmannia auropunctata. Some of the populations of this species have a classical haplo-diploid sexual mating system. In other populations, females and males are produced through parthenogenesis and androgenesis, respectively, whereas workers are produced sexually. We conducted laboratory reciprocal-cross experiments with reproductive individuals from both types of populations and analysed their progenies with genetic markers, to determine the respective contribution of males and females to the production of androgenetic males. We found that androgenesis was a parthenogenetic female trait. A population genetic study conducted in natura confirmed the parthenogenetic female origin of androgenesis, with the identification of introgression events of sexual male genotypes into androgenetic/parthenogenetic lineages. We argue that by producing males via androgenesis, parthenogenetic queen lineages may increase and/or maintain their adaptive potential, while maintaining the integrity of their own genome, by occasionally acquiring new male genetic material and avoiding inbreeding depression within the sexually produced worker cast.     

Sex allocation in clonal plants: might clonal expansion enhance fitness gains through male function?

The returns on investment in sexual reproduction are described by fitness gain curves and the shapes of these curves affect, among other things, the evolutionary stability of reproductive systems. The available evidence indicates that gain curves for male function decelerate, corresponding to diminishing fitness returns on investment in pollen. In contrast, the gain curve for female function is thought to decelerate less strongly than it does for male function (e.g., if seed fertility is limited by more by resources than by mating opportunities). Here we suggest that when the shapes of the female and male gain curves differ, clonality alters the rates of return on investment via the two sex functions. In particular, we propose that clonal expansion might increase fitness gains through male function because the subdivision of reproductive effort among ramets allows each ramet to take advantage of the steepest parts of the male gain curve. We examined the interaction between clonal expansion and fitness gains using numerical analysis of a model of sex allocation in which we assumed that there is no mating interference among ramets. We found that clonal expansion led to substantial increases in fitness through male function, but to decreases in fitness through female function. Under intermediate investment in clonal growth, marginal fertility gains through the two sex functions did not intersect over a broad range of sex allocation patterns, suggesting that clonality could favor the evolution of separate sexes. Finally, we suggest an alternative explanation for the common observation of male-biased sex ratios in clonal dioecious plants. If male function fitness is maximized under higher rates of clonal expansion than for female function, greater frequencies of male ramets might reflect the outcome of fertility selection, rather than constraints on clonal expansion imposed by greater costs of reproduction for females.     

Genetic relationships between native and introduced populations of the little fire ant Wasmannia auropunctata

Native to much of Central and South America, the little fire ant Wasmannia auropunctata has been rapidly spreading throughout the world. In its introduced range, W. auropunctata is frequently linked with drastic reductions of ant diversity; anecdotal reports of damaging attacks on vertebrates are also common. As it poses an ever-increasing threat to biodiversity, W. auropunctata has emerged as a model system for the study of ecological differences between native and invasive ant populations. These studies have been hampered by a lack of information on the genetic relatedness between native and introduced populations. By investigating the genetic structure of W. auropunctata populations, we provide a framework for conducting phylogenetically independent tests of differences between these ants in their native and invasive ranges. Phylogenetic analyses, based on the cox1 / cox2 region of mtDNA, revealed at least three separate source populations of W. auropunctata distributed across two large clades. Much of the Caribbean region, presumably part of the native range, is inhabited by a clade of ants sharing very similar or identical mtDNA haplotypes, suggesting the possibility of multiple introductions or high levels of gene flow across that area. Most invasive populations in the Pacific were closely related to these ants. The invasive populations in Gabon and New Caledonia arise from another, relatively distantly related clade. Phylogenetically independent contrasts confirm McGlynn's (1999) observation that invasive W. auropunctata populations are smaller than native populations. Given the complex phylogeographical structure of W. auropunctata populations, future comparative work should correct for phylogenetic effects.     

Mating system and population structure in the desert ant Cataglyphis livida

We investigated population genetic structure, mating system, worker reproduction and thelytokous parthenogenesis in the desert ant Cataglyphis livida. Pedigree analyses at polymorphic microsatellite loci show that colonies are headed by a single queen, and that queens are mated with two to eight males. No inbreeding was found in the population sampled. Colonies are genetically differentiated and exhibit no isolation-by-distance pattern, consistent with independent foundation of new colonies. Workers do reproduce and lay haploid (arrhenotokous) eggs in queenless colonies; conversely, we found no evidence of worker reproduction in queenright nests. In contrast with C. cursor, where new queens are produced by thelytokous parthenogenesis, female sexuals and workers of C. livida arise from classical sexual reproduction. We discuss the parallels and contrasts between the mating system and population structure in C. livida and the other Cataglyphis species studied so far.     

Asexuality alone does not explain the success of clonal forms in insects with geographical parthenogenesis

Asexual forms of invertebrates are relatively common. They are often more successful than their sexual progenitors. Especially in insects, the pattern called geographical parthenogenesis shows that asexuality is important in speciation and ecological adaptation. In geographical parthenogenesis the clones have a wider distribution than the sexual forms they originate from. This indicates that they have a broader niche they may utilize successfully. The cause of this apparent success is, however, hard to come by as the term asexuality covers separate phenomena that are hard to disentangle from the mode of reproduction itself. Asexual insects are often polyploid, of hybrid origin, or both and these phenomena have been argued to explain the distribution patterns better than clonality. In this study we survey the literature on arthropods with geographical parthenogenesis in an attempt to clarify what evidence there is for the different phenomena explaining the success of the clonal forms. We focus on the few species where knowledge of distribution of different ploidy levels allows for a distinction of contributions from different phenomena to be made. Our survey support that asexuality is not the only factor underlying the success of all asexuals. Evidence about the importance of a hybrid origin of the clones is found to be meagre as the origin of clones is unknown in the majority of cases. Asexuality, hybridity and polyploidy are intertwined phenomena that each and all may contribute to the success of clonal taxa. Polyploidy, however, emerges as the most parsimonious factor explaining the success of these asexual invertebrate taxa.