Characterization of a sex-determining region and its genomic context via statistical estimates of haplotype frequencies in daughters and sons sequenced in pools

Abstract

Sex chromosomes are generally derived from a pair of autosomes that have acquired a locus controlling sex. Sex chromosomes usually evolve reduced recombination around this locus and undergo a long process of molecular divergence. Although sex chromosomes have been intensively studied in several model taxa, the actual loci controlling sex are difficult to identify in highly diverged sex chromosomes, hence they are known in relatively few species. Taxa with evolutionarily young sex chromosomes can help fill this gap in knowledge. Here we aimed at pinpointing the sex-determining region (SDR) of Armadillidium vulgare, a terrestrial isopod with female heterogamety (ZW females and ZZ males) and which presumably presents evolutionarily young sex chromosomes. To locate the SDR, we assessed SNP allele frequencies in F1 daughters and sons sequenced in pools (pool-seq) in several families. We developed a Bayesian method that uses the SNP genotypes of individually sequenced parents and poolseq data from F1 siblings to estimate the genetic distance between a given genomic region (contig) and the SDR. This allowed us to assign more than 43 Megabases of contigs to sex chromosomes. By taking advantage of the several F1 families, we delineated a very short genomic region (~65 kilobases) that did not show evidence for recombination with the SDR. In this region, the comparison of sequencing depths between sexes outlined female-specific genes that may be involved in sex determination. Overall, our results provide strong evidence for an extremely low divergence of sex chromosomes in A. vulgare.