The terrestrial isopod Armadillidium vulgare is an original model to study the evolution of sex determination and symbiosis in animals. Its sex can be determined by ZW sex chromosomes, or by feminizing Wolbachia bacterial endosymbionts. Here, we report the sequence and analysis of the ZW female genome of A. vulgare. A distinguishing feature of the 1.72 gigabase assembly is the abundance of repeats (68% of the genome). We show that the Z and W sex chromosomes are essentially undifferentiated at the molecular level and the W-specific region is extremely small (at most several hundreds of kilobases). Our results suggest that recombination suppression has not spread very far from the sex-determining locus, if at all. This is consistent with A. vulgare possessing evolutionarily young sex chromosomes. We characterized multiple Wolbachia nuclear inserts in the A. vulgare genome, none of which is associated with the W-specific region. We also identified several candidate genes that may be involved in the sex determination or sexual differentiation pathways. The A. vulgare genome serves as a resource for studying the biology and evolution of crustaceans, one of the most speciose and emblematic metazoan groups.

Sex determination systems are highly variable in many taxa, sometimes even between closely related species. Yet the number and direction of transitions between these systems have seldom been characterized, and the underlying mechanisms are still poorly understood. Here we generated transcriptomes for 19 species of terrestrial isopod crustaceans, many of which are infected byWolbachiabacterial endosymbionts. Using 88 single-copy orthologous genes, we reconstructed a fully resolved and dated phylogeny of terrestrial isopods. An original approach involving crossings of sex-reversed individuals allowed us to characterize the heterogametic systems of five species (one XY/XX and four ZW/ZZ). Mapping of these and previously known heterogametic systems onto the terrestrial isopod phylogeny revealed between 3 and 13 transitions of sex determination systems during the evolution of these taxa, most frequently from female to male heterogamety. Our results support that WW individuals are viable in many species, suggesting sex chromosomes are at an incipient stage of their evolution. Together, these data are consistent with the hypothesis that nucleo-cytoplasmic conflicts generated byWolbachiaendosymbionts triggered recurrent turnovers of sex determination systems in terrestrial isopods. They further establish terrestrial isopods as a model to study evolutionary transitions in sex determination systems and pave the way to molecularly characterize these systems.

Microbial endosymbiosis is widespread in animals, with major ecological and evolutionary implications. Successful symbiosis relies on efficient vertical transmission through host generations. However, when symbionts negatively affect host fitness, hosts are expected to evolve suppression of symbiont effects or transmission. Here, we show that sex chromosomes control vertical transmission of feminizing Wolbachia endosymbionts in the isopod Armadillidium nasatum. Theory predicts that the invasion of an XY/XX species by cytoplasmic sex ratio distorters is unlikely because it leads to fixation of the unusual (and often lethal or infertile) YY genotype. We demonstrate that A. nasatum X and Y sex chromosomes are genetically highly similar and that YY individuals are viable and fertile, thereby enabling Wolbachia spread in this XY-XX species. Nevertheless, we show that Wolbachia cannot drive fixation of YY individuals, because infected YY females do not transmit Wolbachia to their offspring, unlike XX and XY females. The genetic basis fits the model of a Y-linked recessive allele (associated with an X-linked dominant allele), in which the homozygous state suppresses Wolbachia transmission. Moreover, production of all-male progenies by infected YY females restores a balanced sex ratio at the host population level. This suggests that blocking of Wolbachia transmission by YY females may have evolved to suppress feminization, thereby offering a whole new perspective on the evolutionary interplay between microbial symbionts and host sex chromosomes.

The discovery of many fragments of viral genomes integrated in the genome of their eukaryotic host (endogenous viral elements; EVEs) has recently opened new avenues to further our understanding of viral evolution and of host-virus interactions. Here, we report the results of a comprehensive screen for EVEs in crustaceans. Following up on the recent discovery of EVEs in the terrestrial isopod, Armadillidium vulgare, we scanned the genomes of six crustacean species: a terrestrial isopod (Armadillidium nasatum), two water fleas (Daphnia pulex and D. pulicaria), two copepods (the salmon louse, Lepeophtheirus salmonis and Eurytemora affinis), and a freshwater amphipod (Hyalella azteca). In total, we found 210 EVEs representing 14 different lineages belonging to five different viral groups that are present in two to five species: Bunyaviridae (−ssRNA), Circoviridae (ssDNA), Mononegavirales (−ssRNA), Parvoviridae (ssDNA) and Totiviridae (dsRNA). The identification of shared orthologous insertions between A. nasatum and A. vulgare indicates that EVEs have been maintained over several millions of years, although we did not find any evidence supporting exaptation. Overall, the different degrees of EVE degradation (from none to >10 nonsense mutations) suggest that endogenization has been recurrent during the evolution of the various crustacean taxa. Our study is the first to report EVEs in D. pulicaria, E. affinis and H. azteca, many of which are likely to result from recent endogenization of currently circulating viruses. In conclusion, we have unearthed a large diversity of EVEs from crustacean genomes, and shown that four of the five viral groups we uncovered (Bunyaviridae, Circoviridae, Mononegavirales, Parvoviridae) were and may still be present in three to four highly divergent crustacean taxa. In addition, the discovery of recent EVEs offers an interesting opportunity to characterize new exogenous viruses currently circulating in economically or ecologically important copepod species.