Identification of a sex-determining locus potentially involved in a conflict 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 and sex chromosome formation. Terrestrial isopods, particularly the common pill bug Armadillidium vulgare, provide an excellent model for investigating these processes due to the rapid turnover in sex-determination mechanisms they undergo. In A. vulgare, multiple genetic determinants coexist. Notably, a feminizing factor is transmitted at a rate exceeding Mendelian expectations, resulting in female-biased populations. Some lineages possess a masculinizing allele at a locus referred to as the "M gene", which is functionally analogous to an XY system. Its masculinizing dominant allele is hypothesized to have been selected due to the deficit of males caused by the feminizing factor. 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 SNP analysis combining pooled sequencing of male and female progenies with sequencing of individual parents across two families. Bayesian estimation of haplotype frequencies in progenies enabled us to delimit a candidate genomic region of approximately two megabases containing 34 annotated genes. Most notably, one of these genes encodes the androgenic gland hormone, a protein involved in male sexual differentiation. Our findings lay the groundwork for detailed genetic and 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.