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.     

Genetic architecture of sexual and asexual populations of the aphid Rhopalosiphum padi based on allozyme and microsatellite markers

Cyclical parthenogens, including aphids, are attractive models for comparing the genetic outcomes of sexual and asexual reproduction, which determine their respective evolutionary advantages. In this study, we examined how reproductive mode shapes genetic structure of sexual (cyclically parthenogenetic) and asexual (obligately parthenogenetic) populations of the aphid Rhopalosiphum padi by comparing microsatellite and allozyme data sets. Allozymes showed little polymorphism, confirming earlier studies with these markers. In contrast, microsatellite loci were highly polymorphic and showed patterns very discordant from allozyme loci. In particular, microsatellites revealed strong heterozygote excess in asexual populations, whereas allozymes showed heterozygote deficits. Various hypotheses are explored that could account for the conflicting results of these two types of genetic markers. A strong differentiation between reproductive modes was found with both types of markers. Microsatellites indicated that sexual populations have high allelic polymorphism and heterozygote deficits (possibly because of population subdivision, inbreeding or selection). Little geographical differentiation was found among sexual populations confirming the large dispersal ability of this aphid. In contrast, asexual populations showed less allelic polymorphism but high heterozygosity at most loci. Two alternative hypotheses are proposed to explain this heterozygosity excess: allele sequence divergence during long-term asexuality or hybrid origin of asexual lineages. Clonal diversity of asexual lineages of R. padi was substantial suggesting that they could have frozen genetic diversity from the pool of sexual lineages. Several widespread asexual genotypes were found to persist through time, as already seen in other aphid species, a feature seemingly consistent with the general-purpose genotype hypothesis.     

Molecular markers linked to breeding system differences in segregating and natural populations of the cereal aphid Rhopalosiphum padi L.

The aphid Rhopalosiphum padi shows coexistence of sexual and asexual populations, providing an opportunity to study the evolution of breeding system variation in the context of theories on the origin and maintenance of sex. However, assessments of the distribution of sexual and asexual lineages of this aphid are complicated by the difficulties in rapidly characterizing their breeding system. To facilitate this task and to gain insight into the genetic relatedness between sexual and asexual genotypes, molecular markers linked to breeding system differences were recently developed. In this study, we have successfully converted a random amplified polymorphic DNA (RAPD) marker associated with life-cycle variation in R. padi into a codominant sequences-characterized amplified region (SCAR). Segregating and natural populations of known breeding systems were examined to evaluate the life cycle-SCAR marker association. Complete linkage was found in segregating populations while the association averaged 94% in field populations. Detailed analysis of allelic distribution and sequence divergence of the SCAR locus among sexual and asexual populations provides further evidence for a unique and apparently ancient loss of sexuality in this aphid. It also suggests that occasional gene flow occurs between populations differing in their breeding system, mediated by males produced by 'asexual' lineages. This system provides the possibility for the recurrent emergence of new asexual lineages, ensuring the longer persistence of asexuality, and would have important implications for the assessment of costs and benefits of sex in aphids.     

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.     

Obligate asex in a rotifer and the role of sexual signals

Transitions to asexuality have occurred in many animals and plants, yet the biological mechanisms causing such transitions have often remained unclear. Cyclical parthenogens, such as cladocerans, rotifers or aphids often give rise to obligate asexual lineages. In many rotifers, chemical signals that accumulate during population crowding trigger the induction of sexual stages. In this study, I tested two hypotheses on the origin of obligate parthenogenesis in the rotifer Brachionus calyciflorus: (i) that obligate parthenogens have lost the responsiveness to the sexual signal; and (ii) that obligate parthenogens have lost the ability to produce the sexual signal. Pairwise cross-induction assays among three obligate parthenogenetic strains and two cyclically parthenogenetic (sexual) strains were used to test these hypotheses. I found that obligate parthenogens can induce sexual reproduction in sexual strains, but not vice versa. This demonstrates that obligate parthenogens do still produce the sexual signal, but have lost responsiveness to that signal.     

Climate and agricultural context shape reproductive mode variation in an aphid crop pest

In aphids, reproductive mode is generally assumed to be selected for by winter climate. Sexual lineages produce frost-resistant eggs, conferring an advantage in regions with cold winters, while asexual lineages predominate in regions with mild winters. However, habitat and resource heterogeneities are known to exert a strong influence on sex maintenance and might modulate the effect of climate on aphid reproductive strategies. We carried out a hierarchical sampling in northern France to investigate whether reproductive mode variation of the aphid Rhopalosiphum padi is driven by winter climate conditions, by habitat and resource heterogeneities represented by a range of host plants or by both factors. We confirmed the coexistence in R. padi populations of two genetic clusters associated with distinct reproductive strategies. Asexual lineages predominated, whatever the surveyed year and location. However, we detected a between-year variation in the local contribution of both clusters, presumably associated with preceding winter severity. No evidence for host-driven niche differentiation was found in the field on six Poaceae among sexual and asexual lineages. Two dominant multilocus genotypes (∼70% of the sample), having persisted over a 10-year period, were equally abundant on different plant species and locations, indicating their large ecological tolerance. Our results fit theoretical predictions of the influence of winter climate on the balance between sexual and asexual lineages. They also highlight the importance of current agricultural practices which seem to favour a small number of asexual generalist genotypes and their migration across large areas of monotonous environments.     

The cost of sex and competition between cyclical and obligate parthenogenetic rotifers

The ubiquity of sexual reproduction is an evolutionary puzzle because asexuality should have major reproductive advantages. Theoretically, transitions to asexuality should confer substantial benefits in population growth and lead to rapid displacement of all sexual ancestors. So far, there have been few rigorous tests of one of the most basic assumptions of the paradox of sex: that asexuals are competitively superior to sexuals immediately after their origin. Here I examine the fitness consequences of very recent transitions to obligate parthenogenesis in the cyclical parthenogenetic rotifer Brachionus calyciflorus. This experimental system differs from previous animal models, since obligate parthenogens were derived from the same maternal genotype as cyclical parthenogens. Obligate parthenogens had similar fitness compared with cyclical parthenogens in terms of the intrinsic rate of increase (calculated from life tables). However, population growth of cyclical parthenogens was predicted to be much lower: sexual female offspring do not contribute to immediate population growth in Brachionus, since they produce either males or diapausing eggs. Hence, if cyclical parthenogens constantly produce a high proportion of sexual offspring, there is a cost of sex, and obligate parthenogens can invade. This prediction was confirmed in laboratory competition experiments.     

Temporal differentiation and spatial coexistence of sexual and facultative asexual lineages of an aphid species at mating sites

Cases of coexisting sexual and asexual relatives are puzzling, as evolutionary theory predicts that competition for the same ecological niches should lead to the exclusion of one or the other population. In the cyclically parthenogenetic aphid, Rhopalosiphum padi, sexual and facultative asexual lineages are admixed in space at the time of sexual reproduction. We investigated how the interaction of reproductive mode and environment can lead to temporal niche differentiation. We demonstrated theoretically that differential sensitivity of sexual and facultatively asexual aphids to an environmental parameter (mating host suitability) shapes the two strategies: whereas the sexual lineages switch earlier to the production of sexual forms, the facultative asexual lineages delay and spread out their investment in sexual reproduction. This predicted pattern of niche specialization is in agreement with the temporal structure revealed in natura by demographic and genetic data. We propose that partial loss of sex by one pool of aphids and subsequent reduction in gene flow between lineages may favour temporal specialization through disruptive selection.     

Multiple direct transitions from sexual reproduction to apomictic parthenogenesis in Timema stick insects

Transitions from sexual reproduction to parthenogenesis may occur along multiple evolutionary pathways and involve various cytological mechanisms to produce diploid eggs. Here, we investigate routes to parthenogenesis in Timema stick insects, a genus comprising five obligate parthenogens. By combining information from microsatellites and karyotypes with a previously published mitochondrial phylogeny, we show that all five parthenogens likely evolved spontaneously from sexually reproducing species, and that the sexual ancestor of one of the five parthenogens was probably of hybrid origin. The complete maintenance of heterozygosity between generations in the five parthenogens strongly suggests that eggs are produced by apomixis. Virgin females of the sexual species were also able to produce parthenogenetic offspring, but these females produced eggs by automixis. High heterozygosity levels stemming from conserved ancestral alleles in the parthenogens suggest, however, that automixis has not generated the current parthenogenetic Timema lineages but that apomixis appeared abruptly in several sexual species. A direct transition from sexual reproduction to (at least functional) apomixis results in a relatively high level of allelic diversity and high efficiency for parthenogenesis. Because both of these traits should positively affect the demographic success of asexual lineages, spontaneous apomixis may have contributed to the origin and maintenance of asexuality in Timema .     

Admixed sexual and facultatively asexual aphid lineages at mating sites

Cyclically parthenogenetic organisms may have facultative asexual counterparts. Such organisms, including aphids, are therefore interesting models for the study of ecological and genetic interactions between lineages differing in reproductive mode. Earlier studies on aphids have revealed major differences in the genetic outcomes of populations that are possibly resulting mostly either from sexual or from asexual reproduction. Besides, notable gene flow between sexual and asexual derivatives has been suspected, which could lead to the emergence of new asexual lineages. The present study examines the interplay between these lineages and is based on analyses of population structure of individuals that may contribute to the pool of sexual reproductive forms in the host alternating aphid Rhopalosiphum padi. Using a Bayesian assignment method, we first show that the sexual forms of R. padi on mating sites encompass two genetically distinct clusters of individuals in the western part of France. The first cluster included unique genotypes of sexual lineages, while the second cluster included facultatively asexual lineages in numerous copies, the reproductive mode of the two clusters being confirmed by reference clones. Sexual reproductive forms produced by sexual and facultatively asexual lineages are thus admixed at mating sites which gives a large opportunity for the two clusters to mate with each other. Nevertheless, this study also highlights, as previously demonstrated, that the two clusters retained high genetic differentiation. Possible explanations for the inferred limited genetic exchanges are advanced in the discussion, but further dedicated investigations are required to solve this paradox.     

Short-term population differences in the genetic architecture of life history traits related to sexuality in an aphid species

One of the most important factors that determine the evolutionary trajectory of a suite of traits in a population is the structure of the genetic variance-covariance matrix (G). We studied the cyclically parthenogenetic aphid Rhopalosiphum padi, whose populations exhibit two types of reproductive lineages respectively specialized in sexuality (that is, cyclically parthenogenetic lineages) and in asexuality (that is, obligate parthenogenetic lineages). We compared the quantitative genetics of life histories in these two lineage types. Our results suggest that both, the elements and the whole structure of the resulting G matrices differ in the very short term, between lineage types. This would involve the evolution toward different evolutionary optima in the same population, depending on whether sexual or asexual lineages predominate. Since sexual and asexual lineages vary seasonally in their abundance, a fluctuating selective regime has been proposed for this species, which would contribute to the maintenance of the reproductive polymorphism that these populations exhibit.     

Environmentally related patterns of reproductive modes in the aphid Myzus persicae and the predominance of two 'superclones' in Victoria, Australia

Asexual organisms that naturally coexist with sexual relatives may hold the key to understanding the maintenance of sex and recombination, a long-standing problem in evolutionary biology. This situation applies to the peach-potato aphid, Myzus persicae, in southeastern Australia where cyclical parthenogens form mixed populations with obligate parthenogens. We collected M. persicae from several areas across Victoria, genotyped them at seven microsatellite loci and experimentally determined their reproductive mode. The geographic distribution of reproductive modes was correlated with two environmental variables that differentially affect obligate and cyclical parthenogens; obligate parthenogens were less frequent in areas with cold winters because they cannot produce frost-resistant eggs while cyclical parthenogens were limited by the availability of their primary host, peach, on which sexual reproduction takes place. Clonal diversity increased with the proportion of cyclical parthenogens in a sample because they tended to have unique microsatellite genotypes, whereas many obligate parthenogens were copies of the same genotype. Two obligately asexual genotypes stood out as being very abundant and widespread, one constituting 24% and the other 17.4% of the entire collection. Both of these highly successful genotypes were present in the majority of all collection sites. Genetic population structure was weak, albeit significant, with a multilocus FST of only 0.021 when samples were reduced to only one representative of each genotype. Interestingly, obligate parthenogens were, on average, more heterozygous and exhibited larger allele size differences between the two alleles at individual loci than cyclical parthenogens. This striking pattern could result from hybridization, for which we have no evidence, or may reflect the previously proposed model of biased mutational divergence of microsatellite alleles within asexual aphid lineages.     

Geographic ranges, population structure, and ages of sexual and parthenogenetic snail lineages

Abstract A sexual reproduction is thought to doom organisms to extinction due to mutation accumulation and parasite exploitation. Theoretical models suggest that parthenogens may escape the negative effects of conspecific and biological enemiecs through escape in space. Through intensive sequencing of a mitochondrial DNA (mtDNA) and a nuclear intron locus in sexual and pathenogenetic freshwater snails (Campelom), I examine three questionss: (1) Are sexual mtDNA lineage more restricted geographically than parthenogenetic mtDNA lineages? (2) Are independent pathenogenetic lineages shorter lived than sexual lineages? (3) Do pathenogens have higher intraindividual nuclear sequence diversity and form well‐differentiated monophyletic groups as expected under the Meselson effect? Geographic ranges of parthenogenetic lineages are significantly larger than geographic ranges of sexual lineages. Based on coalescence times under different deographic assumptions, asexual lineages are short lived, but there is variation in clonal ages. Although alternative explanations exit, these results suggest that asexual lineages may persist in the short term through dispersal, and that various constraints may cause geographic restriction of sexual lineagess. Both allotriploid and diploid Campleloma parthenogens have significantly higher allelic divergence within individuals, but show limited nuclear sequence divergence from sexual ancestors. In contrast to previous allozyme evidence for nonhybrid origins of diploid Campeloma parthenogens, cryptic hybridization may account for elevated heterozygosity.     

Strong biases in the transmission of sex chromosomes in the aphid Rhopalosiphum padi

The typical life cycle of aphids involves several parthenogenetic generations followed by a single sexual one in autumn, i.e. cyclical parthenogenesis. Sexual females are genetically identical to their parthenogenetic mothers and carry two sex chromosomes (XX). Male production involves the elimination of one sex chromosome (to produce X0) that could give rise to genetic conflicts between X-chromosomes. In addition, deleterious recessive mutations could accumulate on sex chromosomes during the parthenogenetic phase and affect males differentially depending on the X-chromosome they inherit. Genetic conflicts and deleterious mutations thus may induce transmission bias that could be exaggerated in males. Here, the transmission of X-chromosomes has been studied in the laboratory in two cyclically parthenogenetic lineages of the bird cherry-oat aphid Rhopalosiphum padi . X-chromosome transmission was followed, using X-linked microsatellite loci, at male production in the two lineages and in their hybrids deriving from reciprocal crosses. Genetic analyses revealed non-Mendelian inheritance of X-chromosomes in both parental and hybrid lineages at different steps of male function. Putative mechanisms and evolutionary consequences of non-Mendelian transmission of X-chromosomes to males are discussed.     

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.     

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.     

The genetic outcomes of sex and recombination in long-term functionally parthenogenetic lineages of Australian Sitobion aphids

The typical life cycle of an aphid is cyclical parthenogenesis which involves the alternation of sexual and asexual reproduction. However, aphid life cycles, even within a species, can encompass everything on a continuum from obligate sexuality, through facultative sexuality to obligate asexuality. Loss of the sexual cycle in aphids is frequently associated with the introduction of a new pest and can occur for a number of environmental and genetic reasons. Here we investigate loss of sexual function in Sitobion aphids in Australia. Specifically, we aimed to determine whether an absence of sexual reproduction in Australian Sitobion results from genetic loss of sexual function or environmental constraints in the introduced range. We addressed our aims by performing a series of breeding experiments. We found that some lineages have genetically lost sexual function while others retain sexual function and appear environmentally constrained to asexuality. Further, in our crosses, using autosomal and X-linked microsatellite markers, we identified processes deviating from normal Mendelian segregation. We observed strong deviations in X chromosome transmission through the sexual cycle. Additionally, when progeny genotypes were examined across multiple loci simultaneously we found that some multilocus genotypes are significantly over-represented in the sample and that levels of heterozygosity were much higher than expected at almost all loci. This study demonstrates that strong biases in the transmission of X chromosomes through the sexual cycle are likely to be widespread in aphids. The mechanisms underlying these patterns are not clear. We discuss several possible alternatives, including mutation accumulation during periods of functional asexuality and genetic imprinting.     

The effects of reproductive specialization on energy costs and fitness genetic variances in cyclical and obligate parthenogenetic aphids

Organisms with coexisting sexual and asexual populations are ideal models for studying the consequences of either reproductive mode on the quantitative genetic architecture of life-history traits. In the aphid Rhopalosiphum padi, lineages differing in their sex investment coexist but all share a common parthenogenetic phase. Here, we studied multiple genotypes of R. padi specialized either for sexual and asexual reproduction and compared their genetic variation in fitness during the parthenogenetic phase. Specifically, we estimated maintenance costs as standard metabolic rate (SMR), together with fitness (measured as the intrinsic rate of increase and the net reproductive rate). We found that genetic variation (in terms of broad-sense heritability) in fitness was higher in asexual genotypes compared with sexual genotypes. Also, we found that asexual genotypes exhibited several positive genetic correlations indicating that body mass, whole-animal SMR, and apterous individuals production are contributing to fitness. Hence, it appears that in asexual genotypes, energy is fully allocated to maximize the production of parthenogenetic individuals, the simplest possible form of aphid repertoire of life-histories strategies.     

Evolutionary and functional insights into reproductive strategies of aphids

Aphids are among the few organisms capable of reproducing either sexually or asexually. This plasticity in reproductive mode is viewed as an adaptive response to cope with seasonal changes. Clonal reproduction occurs during the growing season allowing rapid population increase, while sexual reproduction occurs during late summer and leads to frost-resistant eggs that can survive winter conditions. This shift between these two extreme reproductive modes is achieved by using the same genotype, i.e. within the same genetic clone, and is triggered by photoperiodic changes perceived by the aphid brain or visual system. Advances have been made recently to depict genetic programs that relate to the regulation of reproductive modes in aphids. These studies have benefited from the rapid development of genomic and post-genomic resources obtained through the International Aphid Genomics Consortium. Here, we underline the importance of several candidate genes in the switch from clonal to sexual reproduction in aphids and whose roles await full validation. Besides reproductive mode variation expressed at the genotypic level, aphid species also frequently encompass lineages which have lost the sexual phase and hence the alternating clonal and sexual reproductive phases of the life cycle. This coexistence of sex and asexual reproduction within the same species raises questions on its evolutionary and ecological significance. We summarize the knowledge accumulated to date on the maintenance of sex as well as on the origin and evolution of asexuality in aphids. By combining functional genomics, genetic and ecological approaches on reproductive plasticity and polymorphism, we hope to obtain an integrative view of the evolutionary forces shaping aphid reproductive strategies, from gene to population and species levels.     

Gene flow between sexual and facultatively asexual lineages of an aphid species and the maintenance of reproductive mode variation

Many organisms considered as strictly clonal may in fact experience some rare events of sexual reproduction with their sexual relatives. However, the rate of sexual–asexual gene flow has rarely been assessed mainly because its evaluation is difficult to achieve in the field. In the cyclically parthenogenetic aphid Rhopalosiphum padi , two main sets of lineages, differing in their investment in sexual reproduction and in their genetic attributes, co-exist even at a very fine scale: the 'sexual' lineages which have a full commitment to the sexual reproduction, and the 'facultatively asexual' lineages, which allocate investment in the sexual and parthenogenetic reproduction. This system offers a unique opportunity to tackle the genetic interactions between two contrasting reproductive modes. Here, we provide evidence that gene flow occurred between sexual and facultatively asexual lineages of R. padi. We carefully examined the shuffling in phenotypic and genotypic variation following a sexual reproduction event that took place in the field. Combining genotypic data and phenotypic measurements showed that this gene mixing led to the production of a wide array of reproductive modes, including strictly asexual lineages. Finally, we discuss the central role played by facultatively asexual lineages on the maintenance of reproductive mode variation.