Unexpected complexity of sex chromosome evolution in haploid plan

Sex chromosomes have evolved independently in numerous lineages across the tree of life, including the ancient and under-studied haploid plant liverwort lineage. However, species with separate sexes sometimes evolve unisexuality and the ability to self-fertilise. Here, we studied chromosome changes during recurrent transitions between sexual systems (dioicy, where the haploid gametophyte stage plants are either male or female, and monoicy, where this stage is unisexual) in the Marchantiales group of the ancient and under-studied haploid plant liverwort lineage. We demonstrate that dioicy re-evolved in this group after monoicy had become established, with de novo establishment of a sex chromosome from an autosome and evolution of a new gene set essential for sexual differentiation of the haploid gametophytes. The changes include unusual major chromosomal rearrangements. These findings indicate unexpectedly complex evolution of sex chromosomes in haploid plants.