Identification of a sex-determining region potentially involved in resolving genetic conflicts over sex ratio.

Sex-determining genes remain largely uncharacterized outside classical models in vertebrates and insects, leaving a gap in our understanding of their evolutionary emergence. The terrestrial isopod Armadillidium vulgare provides an excellent model for investigating this question, as it presents multiple genetic sex determinants. Some lineages possess a masculinizing dominant allele at a locus called the 'M gene', which is analogous to an XY system. This allele is hypothesized to have been selected due to the deficit of males caused by a non-Mendelian feminizing factor previously described. The existence of the M gene was inferred from crosses carried out in the 1990s, but its molecular nature remains unresolved. Here, we conducted a genome-wide single-nucleotide polymorphisms analysis combining pooled sequencing of male and female progenies with sequencing of individual parents across two families. Bayesian estimation of haplotype frequencies in progenies pinpointed a candidate genomic region of approximately two megabases. Notably, this region contains the gene encoding the androgenic gland hormone, a protein involved in male sexual differentiation. Our findings lay the groundwork for functional investigations of the M gene, offering novel insights into the dynamics of sex determination in terrestrial isopods and into the turnover of sex chromosomes in response to sex-ratio distortion.