Heterogametic Females Reveal a <scp>ZW</scp> Sex Determination System and a Putative Sex Chromosome for the Chilean Jack Mackerel, <i>Trachurus murphyi</i>

Sex determination is the establishment of an organism's sex, usually by the inheritance at the fertilisation of certain genes present in sex chromosomes. However, this process is not universal, and indeed, sex might be determined through different factors, including genetic, environmental, behavioural, physiological or the interplay among them. Identification of the sex determination system, sex chromosomes and sex-linked markers is essential for understanding the genetics of sex determination in non-model organisms, which in turn can be used for several applications such as conservation and management. In fish, sex determination is a very flexible process and varies considerably among genera and families; even within individuals, it is subjected to modification by external factors. Here we report the discovery of the sex determination system, sex-linked loci and a putative sex chromosome for the Chilean jack mackerel Trachurus murphyi. Using a genome-wide approach, we identified 20 high-confidence sex-linked loci and found that females are heterogametic while males are homogametic, thus supporting a ZZ/ZW sex-determination system. All high-confidence sex-linked loci appear gametologous loci and are mapped in chromosome 13 of T. trachurus, a closely related species of T. murphyi. The female-to-male depth ratio analysis showed that most loci with a ratio close to 0.5 are located on this chromosome. Additionally, we generated a small GTseq panel that includes 13 loci supporting sex identification in individuals. The sexually identified chromosome has a strong effect on the population genetics analyses revealed by principal component analyses and F_ST statistics. Our results indicate that T. murphyi shows a ZW sex-determination system and that chromosome 13 might be a sex chromosome, likely a Z chromosome. Altogether, our results provide new insights into sex determination systems in T. murphyi and T. trachurus and also constitute a new genomic resource for future applications in the conservation and management of these two economically relevant jack mackerel species.