No evidence of parthenogenesis‐inducing bacteria involved in Thripoctenus javae thelytoky: an unusual finding in Chalcidoidea

All Hymenoptera have a haplodiploid mode of sex determination. Although most species reproduce by arrhenotokous parthenogenesis, there are many thelytokous species, in which unfertilized eggs develop into diploid females. Thelytoky can be genetic or due to microbial infection. In the large Chalcidoidea superfamily, thelytokous parthenogenesis is almost always associated with infection of endosymbionts of the genera Wolbachia, Cardinium, and Rickettsia. Thripoctenus javae (Girault) (Hymenoptera: Eulophidae) is a larval parasitoid of the greenhouse thrips Heliothrips haemorrhoidalis (Bouché) (Thysanoptera: Thripidae), an important worldwide pest. Both the host and its parasitoid reproduce by thelytokous parthenogenesis. The main goal of this study was to test whether endosymbiotic bacteria, either those known to induce thelytokous parthenogenesis or other sex‐manipulators, are responsible for thelytoky of two geographically distinct populations of T. javae. We used sequencing of ribosomal ITS2 and 28S‐D2 and mitochondrial COI genes to molecularly characterize the two populations, antibiotic and heat treatments, and FISH of ovaries, for thelytoky studies. It was impossible to revert thelytokous individuals back to sexual reproduction and no evidence of bacterial infection was found in parthenogenetic T. javae females. This makes T. javae the second chalcidoid in which thelytokous reproduction appears not to be associated with the presence of bacterial endosymbionts.     

Epidemiology of asexuality induced by the endosymbiotic Wolbachia across phytophagous wasp species: host plant specialization matters

Among eukaryotes, sexual reproduction is by far the most predominant mode of reproduction. However, some systems maintaining sexuality appear particularly labile and raise intriguing questions on the evolutionary routes to asexuality. Thelytokous parthenogenesis is a form of spontaneous loss of sexuality leading to strong distortion of sex ratio towards females and resulting from mutation, hybridization or infection by bacterial endosymbionts. We investigated whether ecological specialization is a likely mechanism of spread of thelytoky within insect communities. Focusing on the highly specialized genus Megastigmus (Hymenoptera: Torymidae), we first performed a large literature survey to examine the distribution of thelytoky in these wasps across their respective obligate host plant families. Second, we tested for thelytoky caused by endosymbionts by screening in 15 arrhenotokous and 10 thelytokous species for Wolbachia, Cardinium, Arsenophonus and Rickettsia endosymbionts and by performing antibiotic treatments. Finally, we performed phylogenetic reconstructions using multilocus sequence typing (MLST) to examine the evolution of endosymbiont‐mediated thelytoky in Megastigmus and its possible connections to host plant specialization. We demonstrate that thelytoky evolved from ancestral arrhenotoky through the horizontal transmission and the fixation of the parthenogenesis‐inducing Wolbachia. We find that ecological specialization in Wolbachia's hosts was probably a critical driving force for Wolbachia infection and spread of thelytoky, but also a constraint. Our work further reinforces the hypothesis that community structure of insects is a major driver of the epidemiology of endosymbionts and that competitive interactions among closely related species may facilitate their horizontal transmission.     

Evolutionary implications of developmental instability in parthenogenetic drosophila mercatorum. I. Comparison of several strains with different genotypes

Natural populations of sexually reproducing Drosophila mercatorum are capable of a very low rate of parthenogenesis, but this mode of reproduction has apparently never characterized an entirely asexual population in this species. The high abortion rate observed in laboratory parthenogenetic lines suggests that developmental constraints may cause the failure of this trait to spread in nature. To investigate the basis of this developmental instability and how it may affect the evolution of parthenogenesis in natural populations, early embryonic development was compared between one sexual and four parthenogenetic laboratory strains of D. mercatorum. There is a large amount of variation within a given parthenogenetic strain, suggesting that parthenogenesis is associated with a general breakdown of developmental stability. There is relatively little variation among different parthenogenetic strains, suggesting that most abortions are due to a feature inherent to parthenogenetic reproduction rather than a feature of a particular genome. Likewise, there is little variation between parthenogenetic and sexual strains in the causes of abortions, suggesting that the developmental problems encountered by parthenogenetic lineages are not unique to parthenogens. Thus, the failure of parthenogenesis to spread within D. mercatorum can be attributed to no particular developmental constraint per se operating after the initiation of embryogenesis. However, the overall increase in all developmental problems that occurs with the transition from sexual to parthenogenetic development suggests that the high degree of developmental instability associated with parthenogenesis may be considered a developmental constraint in its own right.     

Origin and Genetic Diversity of Diploid Parthenogenetic Artemia in Eurasia

There is wide interest in understanding how genetic diversity is generated and maintained in parthenogenetic lineages, as it will help clarify the debate of the evolution and maintenance of sexual reproduction. There are three mechanisms that can be responsible for the generation of genetic diversity of parthenogenetic lineages: contagious parthenogenesis, repeated hybridization and microorganism infections (e.g. Wolbachia). Brine shrimps of the genus Artemia (Crustacea, Branchiopoda, Anostraca) are a good model system to investigate evolutionary transitions between reproductive systems as they include sexual species and lineages of obligate parthenogenetic populations of different ploidy level, which often co-occur. Diploid parthenogenetic lineages produce occasional fully functional rare males, interspecific hybridization is known to occur, but the mechanisms of origin of asexual lineages are not completely understood. Here we sequenced and analysed fragments of one mitochondrial and two nuclear genes from an extensive set of populations of diploid parthenogenetic Artemia and sexual species from Central and East Asia to investigate the evolutionary origin of diploid parthenogenetic Artemia, and geographic origin of the parental taxa. Our results indicate that there are at least two, possibly three independent and recent maternal origins of parthenogenetic lineages, related to A. urmiana and Artemia sp. from Kazakhstan, but that the nuclear genes are very closely related in all the sexual species and parthenogegetic lineages except for A. sinica, who presumable took no part on the origin of diploid parthenogenetic strains. Our data cannot rule out either hybridization between any of the very closely related Asiatic sexual species or rare events of contagious parthenogenesis via rare males as the contributing mechanisms to the generation of genetic diversity in diploid parthenogenetic Artemia lineages.     

Life-history changes that accompany the transition from sexual to parthenogenetic reproduction in Drosophila mercatorum

In spite of the predicted genetic and ecological costs of sex, most natural populations maintain sexual reproduction, even those capable of facultative parthenogenesis. Unfertilized eggs from natural populations of Drosophila mercatorum occasionally develop into viable adults, but obligately parthenogenetic populations are unknown in this species. To evaluate the microevolutionary forces that both favor and constrain the evolution of parthenogenesis in D. mercatorum, we have measured parthenogenetic rates across a natural, sexually reproducing population and characterized the life-history changes that accompany the transition from sexual to parthenogenetic reproduction in laboratory strains. A highly significant difference in parthenogenetic rate was found between two populations in close geographic proximity, with increased rate found with lower population density. Laboratory strains of parthenogenetic females suffered increased mortality and reduced egg viability relative to their virgin counterparts from a sexual strain. Lifetime egg production was similar across all strains, but a shift in peak egg production to an earlier age also occurred. The combination of these life-history traits resulted in a higher net reproductive value for sexual females, but because they also had a longer generation time, intrinsic rate of increase was not as dramatically different from parthenogenetic females. In environments with high early mortality, there may be no fitness disadvantage to parthenogenesis, but the predicted ecological advantage of a twofold increase in intrinsic rate of increase was not realized. These results support the theory of Stalker (1956) that parthenogenesis is favored in environments in which sexual reproduction is difficult or impossible.     

Thelytokous parthenogenesis and the heterogeneous decay of mating behaviours in a bushcricket (Orthopterida)

Parthenogenesis in bushcrickets has an incidence of less than 1%. In the diploid bushcricket Poecilimon intermedius, rearing indicates obligate, thelytokous parthenogenesis. Antibiotic treatment of P. intermedius was not effective in restoring male production, and negative results from PCR screening excluded feminizing endosymbionts, such as Wolbachia, as a reason for the lack of males. The geographical range of P. intermedius follows the general pattern of geographical parthenogenesis, being more northerly and much larger than in the sexual relatives. This is a rare example of geographical parthenogenesis that is not attributable to endosymbiont infection or polyploidy. Females of the parthenogenetic species show differential decay of mating‐related behaviour. While interspecific matings were readily achieved in captivity, with spermatophores being transferred and sperm successfully entering the females, the parthenogenetic females exhibit no phonotaxis towards singing males.     

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 .     

Facultative symbiont infections affect aphid reproduction

Some bacterial symbionts alter their hosts reproduction through various mechanisms that enhance their transmission in the host population. In addition to its obligatory symbiont Buchnera aphidicola, the pea aphid Acyrthosiphon pisum harbors several facultative symbionts influencing several aspects of host ecology. Aphids reproduce by cyclical parthenogenesis whereby clonal and sexual reproduction alternate within the annual life cycle. Many species, including the pea aphid, also show variation in their reproductive mode at the population level, with some lineages reproducing by cyclical parthenogenesis and others by permanent parthenogenesis. While the role of facultative symbionts has been well studied during the parthenogenetic phase of their aphid hosts, very little is known on their possible influence during the sexual phase. Here we investigated whether facultative symbionts modulate the capacity to produce sexual forms in various genetic backgrounds of the pea aphid with controlled symbiont composition and also in different aphid genotypes from natural populations with previously characterized infection status and reproductive mode. We found that most facultative symbionts exhibited detrimental effects on their hosts fitness under sex-inducing conditions in comparison with the reference lines. We also showed that the loss of sexual phase in permanently parthenogenetic lineages of A. pisum was not explained by facultative symbionts. Finally, we demonstrated that Spiroplasma infection annihilated the production of males in the host progeny by inducing a male-killing phenotype, an unexpected result for organisms such as aphids that reproduce primarily through clonal reproduction.     

Elimination of a specialised facultative symbiont does not affect the reproductive mode of its aphid host

Abstract.  1. Microorganisms that manipulate the reproduction of their hosts through diverse mechanisms including the induction of parthenogenesis are widespread among arthropods.
2. The pea aphid, Acyrthosiphon pisum , shows a variation in its reproductive mode, with lineages reproducing by cyclical parthenogenesis (obligate alternation of parthenogenetic and sexual generations each year) and others by obligate parthenogenesis (continuous asexual reproduction all year round). In addition, the pea aphid harbours, along with Buchnera the primary aphid endosymbiont, several facultative symbionts whose prevalence differs among host populations.
3. The possible influence of a Rickettsia facultative symbiont on the reproductive mode of its host was tested on two pea aphid clones by comparing the response of infected and uninfected individuals with the same genetic background to conditions that typically induce the production of sexual morphs.
4. No significant effect of the Rickettsia infection was found on the type of reproductive morphs produced (sexual vs. asexual) or on their quantities for the two clones.
5. However, the Rickettsia had a detrimental effect on the fitness of its aphid host, in apparent contradiction to the high prevalence of this symbiont in some host populations. It is suggested that this negative impact may disappear under specific environmental conditions, transforming a parasitic association into a mutualistic one.     

Parthenogenetic populations of the freshwater snail Campeloma limum occupy habitats with fewer environmental stressors than their sexual counterparts

1. Sexual organisms should have half the reproductive rate of their parthenogenetic counterparts (i.e. twofold cost of sex), so the plethora of sexual species relative to parthenogenetic species remains an evolutionary paradox. The rarity of parthenogenesis may in part be due to the accumulation of deleterious mutations. Indeed, parthenogenetic populations of the freshwater snail Campeloma limum have a greater mutation load relative to sexual populations of C. limum, although this does not directly affect their reproductive fitness. We hypothesise that although parthenogenesis has no direct effect on fitness in C. limum, mutation accumulation and environmental stress act synergistically to limit the distribution of parthenogenetic populations. 2. We evaluated this hypothesis, predicting that parthenogenetic populations of C. limum would inhabit sites with fewer environmental stressors than their sexual counterparts. We collected water quality, population density and individual size data at multiple time points from eight parthenogenetic and five sexual populations in the south‐eastern United States (Georgia and South Carolina). 3. Consistent with our hypothesis, sexual populations of C. limum inhabited poorer‐quality areas (sites with significantly lower dissolved oxygen and significantly more faecal coliform bacteria) than parthenogenetic populations. Despite these stressors, sexual populations still exhibited significantly higher population density than parthenogenetic populations. 4. Our findings support the hypothesis that mutation‐laden parthenogenetic C. limum populations occupy habitats with fewer environmental stressors relative to their sexual counterparts. Moreover, sexual C. limum populations inhabit lower‐quality habitats where they can presumably evade the twofold cost of sex in the absence of competition from their parthenogenetic counterparts.     

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.     

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.     

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.     

EVOLUTION OF ASEXUALITY VIA DIFFERENT MECHANISMS IN GRASS THRIPS (THYSANOPTERA: Aptinothrips)

Asexual lineages can derive from sexual ancestors via different mechanisms and at variable rates, which affects the diversity of the asexual population and thereby its ecological success. We investigated the variation and evolution of reproductive systems in Aptinothrips, a genus of grass thrips comprising four species. Extensive population surveys and breeding experiments indicated sexual reproduction in A. elegans, asexuality in A. stylifer and A. karnyi, and both sexual and asexual lineages in A. rufus. Asexuality in A. stylifer and A. rufus coincides with a worldwide distribution, with sexual A. rufus lineages confined to a limited area. Inference of molecular phylogenies and antibiotic treatment revealed different causes of asexuality in different species. Asexuality in A. stylifer and A. karnyi has most likely genetic causes, while it is induced by endosymbionts in A. rufus. Endosymbiont‐community characterization revealed presence of Wolbachia, and lack of other bacteria known to manipulate host reproduction. However, only 69% asexual A. rufus females are Wolbachia‐infected, indicating that either an undescribed endosymbiont causes asexuality in this species or that Wolbachia was lost in several lineages that remained asexual. These results open new perspectives for studies on the maintenance of mixed sexual and asexual reproduction in natural populations.     

The impact of endosymbionts on the evolution of host sex-determination mechanisms

The past years have revealed that inherited bacterial endosymbionts are important sources of evolutionary novelty for their eukaryotic hosts. In this review we discuss a fundamental biological process of eukaryotes influenced by bacterial endosymbionts: the mechanisms of sex determination. Because they are maternally inherited, several endosymbionts of arthropods, known as reproductive parasites, have developed strategies to convert non-transmitting male hosts into transmitting females through feminization of genetic males and parthenogenesis induction. Recent investigations have also highlighted that endosymbionts can impact upon host sex determination more subtly through genetic conflicts, resulting in selection of host nuclear genes resisting endosymbiont effects. Paradoxically, it is because of their selfish nature that reproductive parasites are such powerful agents of evolutionary change in their host sex-determination mechanisms. They might therefore represent excellent models for studying transitions between sex-determining systems and, more generally, the evolution of sex-determination mechanisms in eukaryotes.     

Reproduction and dispersal in an ant-associated root aphid community

Clonal organisms with occasional sex are important for our general understanding of the costs and benefits that maintain sexual reproduction. Cyclically parthenogenetic aphids are highly variable in their frequency of sexual reproduction. However, studies have mostly focused on free‐living aphids above ground, whereas dispersal constraints and dependence on ant‐tending may differentially affect the costs and benefits of sex in subterranean aphids. Here, we studied reproductive mode and dispersal in a community of root aphids that are obligately associated with the ant Lasius flavus. We assessed the genetic population structure of four species (Geoica utricularia, Tetraneura ulmi, Forda marginata and Forda formicaria) in a Dutch population and found that all species reproduce predominantly if not exclusively asexually, so that populations consist of multiple clonal lineages. We show that population viscosity is high and winged aphids rare, consistent with infrequent horizontal transmission between ant host colonies. The absence of the primary host shrub (Pistacia) may explain the absence of sex in three of the studied species, but elm trees (Ulmus) that are primary hosts of the fourth species (T. ulmi) occurred within a few km of the study population. We discuss the extent to which obligate ant‐tending and absence of primary hosts may have affected selection for permanent parthenogenesis, and we highlight the need for further study of these aphids in Southern Europe where primary hosts may occur close to L. flavus populations, so that all four root aphid species would have realistic opportunities for completing their sexual life cycle.     

First record of second‐generation facultative parthenogenesis in a vertebrate species,the whitespotted bambooshark Chiloscyllium plagiosum

In this study, two parthenogenetic events within a family of the whitespotted bambooshark Chiloscyllium plagiosum are reported. A captive female produced multiple parthenogens. Unexpectedly, a single specimen of a total of nine parthenogens displayed external claspers characterizing the male sex in chondrichthyans. Upon dissection, internal sexual organs of this specimen were malformed or absent; however, the presence of claspers in this study challenges the as yet assumed sex determination system in this shark species. Even more remarkable was that one of the female parthenogens reproduced asexually again producing viable offspring. As far as is known, this is the first genetically confirmed evidence for second‐generation facultative parthenogenesis in vertebrates. These results support the evolutionary significance of parthenogenesis as an alternative to sexual reproduction.     

Extensive variation in chromosome number and genome size in sexual and parthenogenetic species of the jumping‐bristletail genus Machilis (Archaeognatha)

Parthenogenesis in animals is often associated with polyploidy and restriction to extreme habitats or recently deglaciated areas. It has been hypothesized that benefits conferred by asexual reproduction and polyploidy are essential for colonizing these habitats. However, while evolutionary routes to parthenogenesis are manifold, study systems including polyploids are scarce in arthropods. The jumping‐bristletail genus Machilis (Insecta: Archaeognatha) includes both sexual and parthenogenetic species, and recently, the occurrence of polyploidy has been postulated. Here, we applied flow cytometry, karyotyping, and mitochondrial DNA sequencing to three sexual and five putatively parthenogenetic Eastern‐Alpine Machilis species to investigate whether (1) parthenogenesis originated once or multiply and (2) whether parthenogenesis is strictly associated with polyploidy. The mitochondrial phylogeny revealed that parthenogenesis evolved at least five times independently among Eastern‐Alpine representatives of this genus. One parthenogenetic species was exclusively triploid, while a second consisted of both diploid and triploid populations. The three other parthenogenetic species and all sexual species were diploid. Our results thus indicate that polyploidy can co‐occur with parthenogenesis, but that it was not mandatory for the emergence of parthenogenesis in Machilis. Overall, we found a weak negative correlation of monoploid genome size (Cx) and chromosome base number (x), and this connection is stronger among parthenogenetic species alone. Likewise, monoploid genome size decreased with elevation, and we therefore hypothesize that genome downsizing could have been crucial for the persistence of alpine Machilis species. Finally, we discuss the evolutionary consequences of intraspecific chromosomal rearrangements and the presence of B chromosomes. In doing so, we highlight the potential of Alpine Machilis species for research on chromosomal and genome‐size alterations during speciation.     

PHYLOGENY AND EVOLUTION OF PARTHENOGENETIC WEEVILS OF THE ARAMIGUS TESSELLATUS SPECIES COMPLEX (COLEOPTERA: CURCULIONIDAE: NAUPACTINI): EVIDENCE FROM MITOCHONDRIAL DNA SEQUENCES

Molecular-phylogenetic studies of parthenogenetic animals have been a valuable recent addition to the literature on the evolutionary biology of sex. By illuminating the origins and ages of parthenogenetic lineages, such studies can help to define the temporal scale at which selection acts against parthenogenetic lineages, as well as provide an essential framework for further study. Although parthenogenetic weevils have played an important role in cytogenetic and protein-electrophoretic studies of parthenogenesis, they have not previously been subjects of DNA-based molecular-phylogenetic study. A mitochondrial DNA study of Aramigus tessellatus, a species complex of weevils native to South America, indentified 12 distinct (1–9% divergent) maternal lineages, of which 2 represent sexual populations, while at least 9 represent parthenogenetic lineages. These lineages partially correspond to lineages previously recognized by morphological differences. Phylogenetic analysis found 14 most parsimonious trees, according to which parthenogenesis appears to have arisen 3–7 times. There is a monophyletic group of lineages (the “brown clade”), having up to 4.5% sequence divergence within it, which may be primitively parthenogenetic and over 2 million years old.     

The role of endosymbionts in the evolution of haploid-male genetic systems in scale insects (Coccoidea)

There is an extraordinary diversity in genetic systems across species, but this variation remains poorly understood. In part, this is because the mechanisms responsible for transitions between systems are often unknown. A recent hypothesis has suggested that conflict between hosts and endosymbiotic microorganisms over transmission could drive the transition from diplodiploidy to systems with male haploidy (haplodiploidy, including arrhenotoky and paternal genome elimination [PGE]). Here, we present the first formal test of this idea with a comparative analysis across scale insects (Hemiptera: Coccoidea). Scale insects are renowned for their large variation in genetic systems, and multiple transitions between diplodiploidy and haplodiploidy have taken place within this group. Additionally, most species rely on endosymbiotic microorganisms to provide them with essential nutrients lacking in their diet. We show that species harboring endosymbionts are indeed more likely to have a genetic system with male haploidy, which supports the hypothesis that endosymbionts might have played a role in the transition to haplodiploidy. We also extend our analysis to consider the relationship between endosymbiont presence and transitions to parthenogenesis. Although in scale insects there is no such overall association, species harboring eukaryote endosymbionts were more likely to be parthenogenetic than those with bacterial symbionts. These results support the idea that intergenomic conflict can drive the evolution of novel genetic systems and affect host reproduction.