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23Sex chromosomes are highly variable in some taxonomic groups, but the evolutionary 24 mechanisms underlying this diversity are not well understood. In terrestrial isopod crustaceans, 25 interactions with Wolbachia, a vertically transmitted endosymbiont causing male-to-female sex 26 reversal, are thought to drive rapid evolutionary turnovers in sex chromosomes. Here, we use 27 surgical manipulations and genetic crosses, plus genome sequencing, to examine sex 28 chromosomes in the terrestrial isopod Trachelipus rathkei. Although an earlier cytogenetics 29 study suggested a ZZ/ZW sex chromosome system in this species, we surprisingly find that in 30 our study population, sex is determined by an XX/XY system. Consistent with a recent 31 evolutionary origin for this XX/XY system, the putative male-specific region of the genome is 32 small. The genome shows evidence of sequences horizontally acquired from past Wolbachia 33 infections, as well as evidence of Y-linked duplications of the androgenic gland hormone gene, 34 thought to be a possible target for sex reversal by Wolbachia. Overall, these results are 35 consistent with the hypothesis that reproductive endosymbionts such as Wolbachia can promote 36 quick turnover of sex determination mechanisms in their hosts. 37 38 39 Orzack et al. 1980). 87 88Unfortunately, we still have a limited understanding of why evolutionary turnovers of sex 89 chromosomes are rare in some groups but frequent in others. A variety of models have been 90 proposed to explain why these turnovers occur, including sexual antagonism, deleterious