LET US NOT BE UNFAIR TO ASEXUALS: THEIR EPHEMERALITY MAY BE EXPLAINED BY NEUTRAL MODELS WITHOUT INVOKING ANY EVOLUTIONARY CONSTRAINTS OF ASEXUALITY

Phylogenetic studies typically demonstrate lower evolutionary ages of clones, relative to their sexual ancestors. This has often been attributed to heightened extinction risk of asexual organisms. We previously criticized such interpretations and demonstrated that the life span of clones is ultimately limited by neutral drift depending on the rate at which new clones are spawned into an asexual community of a finite size. Therefore, it is important to investigate whether the natural rates of such influxes are sufficiently high to account for the relative ephemerality of clones without assuming their increased extinction rate. I applied the neutral clonal turnover model to phylogenies of polyploid asexual ferns and simulated the coalescent trees over a wide range of demographic structures and sampling schemes. On parameterizing the model with biologically relevant estimates of population sizes and plant polyploidization rates, simulated clonal assemblages appeared younger than their sexual counterparts even in the absence of selection against clones. Therefore, differences observed between the ages of sexual and clonal lineages may be explained by the neutral clonal turnover. Researchers should consider the possibility that natural clones may get lost by neutral drift before their fate could eventually be affected by any long‐term constraints of asexuality.     

Dynamic Formation of Asexual Diploid and Polyploid Lineages: Multilocus Analysis of Cobitis Reveals the Mechanisms Maintaining the Diversity of Clones

Given the hybrid genomic constitutions and increased ploidy of many asexual animals, the identification of processes governing the origin and maintenance of clonal diversity provides useful information about the evolutionary consequences of interspecific hybridization, asexuality and polyploidy. In order to understand the processes driving observed diversity of biotypes and clones in the Cobitis taenia hybrid complex, we performed fine-scale genetic analysis of Central European hybrid zone between two sexual species using microsatellite genotyping and mtDNA sequencing. We found that the hybrid zone is populated by an assemblage of clonally (gynogenetically) reproducing di-, tri- and tetraploid hybrid lineages and that successful clones, which are able of spatial expansion, recruit from two ploidy levels, i.e. diploid and triploid. We further compared the distribution of observed estimates of clonal ages to theoretical distributions simulated under various assumptions and showed that new clones are most likely continuously recruited from ancestral populations. This suggests that the clonal diversity is maintained by dynamic equilibrium between origination and extinction of clonal lineages. On the other hand, an interclonal selection is implied by nonrandom spatial distribution of individual clones with respect to the coexisting sexual species. Importantly, there was no evidence for sexually reproducing hybrids or clonally reproducing non-hybrid forms. Together with previous successful laboratory synthesis of clonal Cobitis hybrids, our data thus provide the most compelling evidence that 1) the origin of asexuality is causally linked to interspecific hybridization; 2) successful establishment of clones is not restricted to one specific ploidy level and 3) the initiation of clonality and polyploidy may be dynamic and continuous in asexual complexes.     

Twigs on the tree of life? Neutral and selective models for integrating macroevolutionary patterns with microevolutionary processes in the analysis of asexuality

Neutral models characterize evolutionary or ecological patterns expected in the absence of specific causal processes, such as natural selection or ecological interactions. In this study, we describe and evaluate three neutral models that can, in principle, help to explain the apparent 'twigginess' of asexual lineages on phylogenetic trees without involving the negative consequences predicted for the absence of recombination and genetic exchange between individuals. Previously, such phylogenetic twiggyness of asexual lineages has been uncritically interpreted as evidence that asexuality is associated with elevated extinction rates and thus represents an evolutionary dead end. Our first model uses simple phylogenetic simulations to illustrate that, with sexual reproduction as the ancestral state, low transition rates to stable asexuality, or low rates of ascertained 'speciation' in asexuals, can generate twiggy distributions of asexuality, in the absence of high extinction rates for asexual lineages. The second model, developed by Janko et   al . (2008 ), shows that a dynamic equilibrium between origins and neutral losses of asexuals can, under some conditions, generate a relatively low mean age of asexual lineages. The third model posits that the risk of extinction for asexual lineages may be higher than that of sexuals simply because asexuals inhabit higher latitudes or altitudes, and not due to effects of their reproductive systems. Such neutral models are useful in that they allow quantitative evaluation of whether empirical data, such as phylogenetic and phylogeographic patterns of sex and asexuality, indeed support the idea that asexually reproducing lineages persist over shorter evolutionary periods than sexual lineages, due to such processes as mutation accumulation, slower rates of adaptive evolution, or relatively lower levels of genetic variability.     

On the fate of sexual traits under asexuality

Environmental shifts and life‐history changes may result in formerly adaptive traits becoming non‐functional or maladaptive. In the absence of pleiotropy and other constraints, such traits may decay as a consequence of neutral mutation accumulation or selective processes, highlighting the importance of natural selection for adaptations. A suite of traits are expected to lose their adaptive function in asexual organisms derived from sexual ancestors, and the many independent transitions to asexuality allow for comparative studies of parallel trait maintenance versus decay. In addition, because certain traits, notably male‐specific traits, are usually not exposed to selection under asexuality, their decay would have to occur as a consequence of drift. Selective processes could drive the decay of traits associated with costs, which may be the case for the majority of sexual traits expressed in females. We review the fate of male and female sexual traits in 93 animal lineages characterized by asexual reproduction, covering a broad taxon range including molluscs, arachnids, diplopods, crustaceans and eleven different hexapod orders. Many asexual lineages are still able occasionally to produce males. These asexually produced males are often largely or even fully functional, revealing that major developmental pathways can remain quiescent and functional over extended time periods. By contrast, for asexual females, there is a parallel and rapid decay of sexual traits, especially of traits related to mate attraction and location, as expected given the considerable costs often associated with the expression of these traits. The level of decay of female sexual traits, in addition to asexual females being unable to fertilize their eggs, would severely impede reversals to sexual reproduction, even in recently derived asexual lineages. More generally, the parallel maintenance versus decay of different trait types across diverse asexual lineages suggests that neutral traits display little or no decay even after extended periods under relaxed selection, while extensive decay for selected traits occurs extremely quickly. These patterns also highlight that adaptations can fix rapidly in natural populations of asexual organisms, in spite of their mode of reproduction.     

EVALUATION OF ELEVATED PLOIDY AND ASEXUAL REPRODUCTION AS ALTERNATIVE EXPLANATIONS FOR GEOGRAPHIC PARTHENOGENESIS IN EUCYPRIS VIRENS OSTRACODS

Transitions from sexual to asexual reproduction are often coupled with elevations in ploidy. As a consequence, the importance of ploidy per se for the maintenance and spread of asexual populations is unclear. To examine the effects of ploidy and asexual reproduction as independent determinants of the success of asexual lineages, we sampled diploid sexual, diploid asexual, and triploid asexual Eucypris virens ostracods across a European wide range. Applying nuclear and mitochondrial markers, we found that E. virens consists of genetically highly differentiated diploid sexual populations, to the extent that these sexual clades could be considered as cryptic species. All sexual populations were found in southern Europe and North Africa and we found that both diploid asexual and triploid asexual lineages have originated multiple times from several sexual lineages. Therefore, the asexual lineages show a wide variety of genetic backgrounds and very strong population genetic structure across the wide geographic range. Finally, we found that triploid, but not diploid, asexual clones dominate habitats in northern Europe. The limited distribution of diploid asexual lineages, despite their shared ancestry with triploid asexual lineages, strongly suggests that the wider geographic distribution of triploids is due to elevated ploidy rather than to asexuality.     

Evolutionary history of contagious asexuality in Daphnia pulex

Asexual taxa are short-lived, suggesting that transitions to asexuality represent evolutionary dead-ends. However, with high rates of clonal origin and coexistence of asexuals and sexuals via selective asymmetries, asexuality may persist in the long term as a result of a dynamic equilibrium between clonal origin and extinction. Few such systems have been studied in detail. Here, we investigate the evolutionary history of asexual lineages of Daphnia pulex, which are derived from sexual relatives via the inheritance of a dominant female-limited meiosis-suppressing locus and inhabit ponds throughout northeastern North America (NA). Our extensive sampling and subsequent phylogenetic analysis using mitochondrial sequence data reveals a young and genetically diverse asexual assemblage, reflecting high rates of clonal origin due to the contagious nature of asexuality. Yet, asexuality is restricted to two phylogroups (B and C) with historical and/or present associations with northeastern NA and is absent from a northwestern phylogroup (A), supporting a recent northeastern origin of asexuality in this species. Furthermore, macrogeographic patterns of genetic variability indicate that phylogroups B and C recolonized northeastern NA from opposite directions, yet their presently overlapping geographic distributions are similarly divided into an eastern asexual and a western sexual region. We attribute these patterns to a recent contagious spread of asexuality from a northeastern source. If environment-mediated selective asymmetries play no significant role in determining the outcome of competitive interactions between sexuals and asexuals, regions of contact may be setting the stage for continued asexual conquests.     

MOLECULAR MARKERS INDICATE RARE SEX IN A PREDOMINANTLY ASEXUAL PARASITOID WASP

The parasitoid wasp genus Lysiphlebus (Hymenoptera: Braconidae: Aphidiinae) contains a taxonomically poorly resolved group of both sexual (arrhenotokous) species and asexual (thelytokous) clones. Maximum-parsimony and maximum-likelihood analyses of mitochondrial DNA sequence data from specimens collected across Western Europe showed that asexuality, which does not appear to be caused by the bacterium Wolbachia, is concentrated in two geographically widespread lineages, the older of which diverged from the closest extant sexual taxa approximately 0.5 million years ago. However, the DNA sequences of a nuclear intron (elongation factor—1α) showed no congruence with this pattern, and a much higher frequency of heterozygotes with very high allelic diversity was observed among the asexual females compared to that among females from the sexual species. This pattern is consistent with maternally inherited asexuality coupled with a history of rare sex with members of several closely related sexual populations or species. Our observations reinforce recent arguments that rare sex may be more important for the persistence of otherwise asexual lineages than hitherto appreciated.     

Loss of sexual recombination and segregation is associated with increased diversification in evening primroses

The loss of sexual recombination and segregation in asexual organisms has been portrayed as an irreversible process that commits asexually reproducing lineages to reduced diversification. We test this hypothesis by estimating rates of speciation, extinction, and transition between sexuality and functional asexuality in the evening primroses. Specifically, we estimate these rates using the recently developed BiSSE (Binary State Speciation and Extinction) phylogenetic comparative method, which employs maximum likelihood and Bayesian techniques. We infer that net diversification rates (speciation minus extinction) in functionally asexual evening primrose lineages are roughly eight times faster than diversification rates in sexual lineages, largely due to higher speciation rates in asexual lineages. We further reject the hypothesis that a loss of recombination and segregation is irreversible because the transition rate from functional asexuality to sexuality is significantly greater than zero and in fact exceeded the reverse rate. These results provide the first empirical evidence in support of the alternative theoretical prediction that asexual populations should instead diversify more rapidly than sexual populations because they are free from the homogenizing effects of sexual recombination and segregation. Although asexual reproduction may often constrain adaptive evolution, our results show that the loss of recombination and segregation need not be an evolutionary dead end in terms of diversification of lineages.     

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.     

Multiple routes to asexuality in an aphid species.

Cyclical parthenogens, including aphids, are important models for studying the evolution of sex. However, little is known about transitions to asexuality in aphids, although the mode of origin of asexual lineages has important consequences for their level of genetic diversity, ecological adaptability and the outcome of competition with their sexual relatives. Thus, we surveyed nuclear, mitochondrial and biological data obtained on cyclical and obligate parthenogens of the bird cherry-oat aphid, Rhopalosiphum padi (L), to investigate the frequency of transitions from sexuality to permanent asexuality. Many instances of asexual lineages retaining the ability to produce males are known in aphids, so particular attention was paid to the existence of occasional matings between females from sexual lineages and males produced by asexual lineages, which have the potential to produce new asexual lineages. Phylogenetic inference based on microsatellite and mitochondrial data indicates at least three independent origins of asexuality in R. padi, yielding the strongest evidence to date for multiple origins of asexuality in an aphid. Moreover, several lines of evidence demonstrate that transitions to asexuality result from two mechanisms: a complete spontaneous loss of sex and repeated gene flow from essentially asexual lineages into sexual ones.     

DO ASEXUAL POLYPLOID LINEAGES LEAD SHORT EVOLUTIONARY LIVES? A CASE STUDY FROM THE FERN GENUS ASTROLEPIS

A life-history transition to asexuality is typically viewed as leading to a heightened extinction risk, and a number of studies have evaluated this claim by examining the relative ages of asexual versus closely related sexual lineages. Surprisingly, a rigorous assessment of the age of an asexual plant lineage has never been published, although asexuality is extraordinarily common among plants. Here, we estimate the ages of sexual diploids and asexual polyploids in the fern genus Astrolepis using a well-supported plastid phylogeny and a relaxed-clock dating approach. The 50 asexual polyploid samples we included were conservatively estimated to comprise 19 distinct lineages, including a variety of auto- and allopolyploid genomic combinations. All were either the same age or younger than the crown group comprising their maternal sexual-diploid parents based simply on their phylogenetic position. Node ages estimated with the relaxed-clock approach indicated that the average maximum age of asexual lineages was 0.4 My, and individual lineages were on average 7 to 47 times younger than the crown- and total-ages of their sexual parents. Although the confounding association between asexuality and polyploidy precludes definite conclusions regarding the effect of asexuality, our results suggest that asexuality limits evolutionary potential in Astrolepis.     

Phylogenetic evidence for hybrid origins of asexual lineages in an aphid species

Understanding the mode of origin of asexuality is central to ongoing debates concerning the evolution and maintenance of sexual reproduction in eukaryotes. This is because it has profound consequences for patterns of genetic diversity and ecological adaptability of asexual lineages, hence on the outcome of competition with sexual relatives both in short and longer terms. Among the possible routes to asexuality, hybridization is a very common mechanism in animals and plants. Aphids present frequent transitions from their ancestral reproductive mode (cyclical parthenogenesis) to permanent asexuality, but the mode of origin of asexual lineages is generally not known because it has never been thoroughly investigated with appropriate molecular tools. Rhopalosiphum padi is an aphid species with coexisting sexual (cyclically parthenogenetic) and asexual (obligately parthenogenetic) lineages that are genetically distinct. Previous studies have shown that asexual lineages of R. padi are heterozygous at most nuclear loci, suggesting either that they have undergone long-term asexuality (under which heterozygosity tends to increase) or that they have hybrid origins. To discriminate between these alternatives, we conducted an extensive molecular survey combining the sequence analysis of alleles of two nuclear DNA markers and mitochondrial DNA haplotypes in sexual and asexual lineages of R. padi. Both nuclear and cytoplasmic markers clearly showed that many asexual lineages have hybrid origins, the first such demonstration in aphids. Our results also indicated that asexuals result from multiple events of hybridization between R. padi and an unknown sibling species, and are of recent origin (contradicting previous estimates that asexual R. padi lineages were of moderate longevity). This study constitutes another example that putatively ancient asexual lineages are actually of much more recent origin than previously thought. It also presents a robust approach for testing whether hybrid origin of asexuality is also a common phenomenon in aphids.     

Habitat heterogeneity favors asexual reproduction in natural populations of grassthrips

Explaining the overwhelming success of sex among eukaryotes is difficult given the obvious costs of sex relative to asexuality. Different studies have shown that sex can provide benefits in spatially heterogeneous environments under specific conditions, but whether spatial heterogeneity commonly contributes to the maintenance of sex in natural populations remains unknown. We experimentally manipulated habitat heterogeneity for sexual and asexual thrips lineages in natural populations and under seminatural mesocosm conditions by varying the number of hostplants available to these herbivorous insects. Asexual lineages rapidly replaced the sexual ones, independently of the level of habitat heterogeneity in mesocosms. In natural populations, the success of sexual thrips decreased with increasing habitat heterogeneity, with sexual thrips apparently only persisting in certain types of hostplant communities. Our results illustrate how genetic diversity‐based mechanisms can favor asexuality instead of sex when sexual lineages co‐occur with genetically variable asexual lineages.     

A population of sexual Daphnia pulex resists invasion by asexual clones

Asexual reproduction avoids the costs associated with sex, predicting that invading asexual clones can quickly replace sexual populations. Daphnia pulex populations in the Great Lakes area are predominately asexual, but the elimination of sexual populations by invading clones is poorly understood. Asexual clones were detected at low frequency in one rare sexual population in 1995, with some increase in frequency during 2003 and 2004. However, these clones remained at low frequency during further yearly sampling (2005–2013) with no evidence that the resident sexual population was in danger of elimination. There was evidence for hybridization between rare males produced by asexual clones and sexual females with the potential to produce new asexual genotypes and spread the genetic factors for asexuality. In a short-term laboratory competition experiment, the two most common asexual clones did not increase in frequency relative to a genetically diverse sexual population due in part to a greater investment in diapausing eggs that trades-off current population growth for increased contribution to the egg bank. Our results suggest that a successful invasion can be prolonged, requiring a combination of clonal genotypes with high fitness, persistence of clones in the egg bank and negative factors affecting the sexual population such as inbreeding depression resulting from population bottlenecks.     

Genetic Control of Contagious Asexuality in the Pea Aphid

Although evolutionary transitions from sexual to asexual reproduction are frequent in eukaryotes, the genetic bases of such shifts toward asexuality remain largely unknown. We addressed this issue in an aphid species where both sexual and obligate asexual lineages coexist in natural populations. These sexual and asexual lineages may occasionally interbreed because some asexual lineages maintain a residual production of males potentially able to mate with the females produced by sexual lineages. Hence, this species is an ideal model to study the genetic basis of the loss of sexual reproduction with quantitative genetic and population genomic approaches. Our analysis of the co-segregation of ∼300 molecular markers and reproductive phenotype in experimental crosses pinpointed an X-linked region controlling obligate asexuality, this state of character being recessive. A population genetic analysis (>400-marker genome scan) on wild sexual and asexual genotypes from geographically distant populations under divergent selection for reproductive strategies detected a strong signature of divergent selection in the genomic region identified by the experimental crosses. These population genetic data confirm the implication of the candidate region in the control of reproductive mode in wild populations originating from 700 km apart. Patterns of genetic differentiation along chromosomes suggest bidirectional gene flow between populations with distinct reproductive modes, supporting contagious asexuality as a prevailing route to permanent parthenogenesis in pea aphids. This genetic system provides new insights into the mechanisms of coexistence of sexual and asexual aphid lineages.     

Is polyploidy a persevering accident or an adaptive evolutionary pattern? The case of the brine shrimp Artemia

Asexual organisms are confronted with substantial drawbacks, both immediate and delayed, threatening their evolutionary persistence. Yet, genetic associations with asexuality may refresh the gene pool promoting adaptation of clonal lineages; polyploidy is one of them. Parthenogenesis itself and/or polyploidy are responsible for the maintenance and spread of clones in Artemia, a sexual-asexual genus of halophilic anostracans. We applied flow cytometry, microsatellite genotyping, and mtDNA sequencing to 23 asexual populations. Artemia parthenogens have evolved multiple times either through hybridization or spontaneously. Nine out of 23 populations contained clones of mixed ploidy (2n, 3n, 4n). Most clones were diploid (20/31) while two and nine clones were triploid and tetraploid, respectively. Apomictic triploids and tetraploids formed two distinct groups of low genetic diversity compared with the more divergent automictic diploids. Polyploidy is also polyphyletic in Artemia, with triploids and tetraploids having independent origins from different sexual ancestors. We discern a pattern of geographical parthenogenesis with all clonal groups being more widespread than their closest sexuals. In favour of a specialist model, asexual diploids are restricted to single locations and are strikingly segregated from generalist triploids and tetraploids occupying a variety of sites. This is a rare pattern of mixed life-history strategies within an asexual complex.     

LARGE POPULATION SIZE PREDICTS THE DISTRIBUTION OF ASEXUALITY IN SCALE INSECTS

Understanding why some organisms reproduce by sexual reproduction while others can reproduce asexually remains an important unsolved problem in evolutionary biology. Simple demography suggests that asexuals should outcompete sexually reproducing organisms, because of their higher intrinsic rate of increase. However, the majority of multicellular organisms have sexual reproduction. The widely accepted explanation for this apparent contradiction is that asexual lineages have a higher extinction rate. A number of models have indicated that population size might play a crucial role in the evolution of asexuality. The strength of processes that lead to extinction of asexual species is reduced when population sizes get very large, so that the long‐term advantage of sexual over asexual reproduction may become negligible. Here, we use a comparative approach using scale insects (Coccoidea, Hemiptera) to show that asexuality is indeed more common in species with larger population density and geographic distribution and we also show that asexual species tend to be more polyphagous. We discuss the implication of our findings for previously observed patterns of asexuality in agricultural pests.     

The maintenance of sexual reproduction in a structured population

We present the results of a computer simulation model in which a sexual population produces an asexual mutant. We estimate the probability that the new asexual lineage will go extinct. We find that whenever the asexual lineage does not go extinct the sexual population is out-competed, and only asexual individuals remain after a sufficiently long period of time has elapsed. We call this type of outcome an asexual takeover. Our results suggest that, given repeated mutations to asexuality, asexual takeover is likely in an unstructured environment. However, if the environment is subdivided into demes that are connected by migration, then asexual takeover becomes less likely. The probability of asexual takeover declines towards zero as the number of demes increases and as the rate of migration decreases. The reason for this is that asexuality leads to a greater loss of fitness due to mutation and genetic drift, in comparison to what occurs under sexual reproduction. Population subdivision slows the spread of asexual lineages, which allows more time for the genetic degeneration caused by asexuality to take place.     

Adaptive Significance and Long-Term Survival of Asexual Lineages

Important questions remain about the long-term survival and adaptive significance of eukaryotic asexual lineages. Numerous papers dealing with sex advantages still continued to compare parthenogenetic populations versus sexual populations arguing that sex demonstrates a better fitness. Because asexual lineages do not possess any recombination mechanisms favoring rapid changes in the face of severe environmental conditions, they should be considered as an evolutionary dead-end. Nevertheless, reviewing literature dealing with asexual reproduction, it is possible to draw three stimulating conclusions. (1) Asexual reproduction in eukaryotes considerably differs from prokaryotes which experience recombination but neither meiosis nor syngamy. Recombination and meiosis would be a driving force for sexual reproduction. Eukaryotes should therefore be considered as a continuum of sexual organisms that are more or less capable (and sometimes incapable) of sexual reproduction. (2) Rather than revealing ancestral eukaryotic forms, most known lineages of asexual eukaryotes have lost sex due to a genomic conflict affecting their sexual capacity. Thus, it could be argued that hybridization is a major cause of their asexuality. Asexuality may have evolved as a reproductive mechanism reducing conflict within organisms. (3) It could be proposed that, rather than being generalists, parthenogenetic hybrid lineages could be favored when exploiting peculiar restricted ecological niches, following the “frozen niche variation” model. Although hybrid events may result in sex loss, probably caused by genomic conflict, asexual hybrids could display new original adaptive traits, and the rapid colonization of environments through clonal reproduction could favor their long-term survival, leading to evolutionary changes and hybrid speciation. Examination of the evolutionary history of asexual lineages reveals that evolutionary processes act through transitional stages in which even very small temporary benefits may be enough to counter the expected selective disadvantages.     

No sex in fungus-farming ants or their crops

Asexual reproduction imposes evolutionary handicaps on asexual species, rendering them prone to extinction, because asexual reproduction generates novel genotypes and purges deleterious mutations at lower rates than sexual reproduction. Here, we report the first case of complete asexuality in ants, the fungus-growing ant Mycocepurus smithii, where queens reproduce asexually but workers are sterile, which is doubly enigmatic because the clonal colonies of M. smithii also depend on clonal fungi for food. Degenerate female mating anatomy, extensive field and laboratory surveys, and DNA fingerprinting implicate complete asexuality in this widespread ant species. Maternally inherited bacteria (e.g. Wolbachia, Cardinium) and the fungal cultivars can be ruled out as agents inducing asexuality. M. smithii societies of clonal females provide a unique system to test theories of parent–offspring conflict and reproductive policing in social insects. Asexuality of both ant farmer and fungal crop challenges traditional views proposing that sexual farmer ants outpace coevolving sexual crop pathogens, and thus compensate for vulnerabilities of their asexual crops. Either the double asexuality of both farmer and crop may permit the host to fully exploit advantages of asexuality for unknown reasons or frequent switching between crops (symbiont reassociation) generates novel ant–fungus combinations, which may compensate for any evolutionary handicaps of asexuality in M. smithii.