Whole-genome resequencing reveals chromosomal fusion-driven early stages of XY chromosomes evolution in the darkbarbel catfish (Tachysurus vachellii).

This study investigates the early stages of putative sex chromosome evolution in the darkbarbel catfish (Tachysurus vachellii) through whole-genome resequencing of 31 individuals. We identified the sex-determining region (SDR) on chromosome 3 (chr3), with an FST value of 0.5, indicating the presence of relatively putative young XY sex chromosomes. Our analyses revealed significant genetic differentiation between male and female groups, suggesting that these young sex chromosomes contribute to observed genetic divergence. Notably, among SDR, pseudoautosomal region (PAR), and autosomes, PAR exhibits the highest synonymous substitutions (dS) values. And the significantly reduced ratio of nonsynonymous-to-synonymous substitutions (dN/dS) in SDR demonstrates intense purifying selection acting on this region, and the progressive dN/dS gradient (SDR < PAR < autosomes) reveals an incipient differentiation continuum from PAR to SDR. Our findings provide valuable insights into the dynamics of sex chromosome evolution in fish and underscore the need for further research to explore these mechanisms.