Abstract Background Sex chromosomes evolve from autosomes and are expected to be initially homomorphic, though they may become heteromorphic over time. In diploid systems, one sex chromosome often degenerates, whereas homomorphy should persist longer in organisms with haploid UV sex chromosomes. Intriguingly, both homomorphic and heteromorphic UV sex chromosomes are prevalent in liverworts, yet the genomic structure and evolutionary origins of homomorphic sex chromosomes remain unexplored. Results We investigate the structure and gene content of the homomorphic UV sex chromosomes of the liverwort Blasia pusilla . We uncover a previously undescribed organization comprising a large recombining pseudoautosomal region (PAR) and a terminal, non-recombining sex-determining region (SDR) on both U and V chromosomes. Notably, the SDR of Blasia pusilla is homologous to the compact SDR of the heteromorphic UV chromosomes in Marchantia polymorpha , a liverwort species lacking well-defined PARs. Conclusions Our results show that the UV sex chromosomes of Blasia pusilla have a well-defined SDR and a PAR. We hypothesize that this organization is more prevalent in the homomorphic UV sex chromosomes of liverworts than previously expected and could potentially represent an ancestral feature of the Marchantiidae subclass. However, we cannot exclude the possibility of an origin via fusion between an autosome and a micro-sex chromosome, similar to the UV chromosomes of Marchantia polymorpha . Our study has significant implications for understanding sex chromosome evolution in plants possessing a UV sex-determining system.
Yue et al. (Mon,) studied this question.