Abstract: Sex chromosomes have evolved independently in numerous lineages across the Tree of Life, in both diploid-dominant species, including many animals and plants, and the less studied haploid-dominant plants and algae. Strict genetic sex determination ensures that individuals reproduce by outcrossing. However, species with separate sexes (termed dioecy in diploid plants, and dioicy in haploid plants) may sometimes evolve different sex systems, and become monoicous, with the ability to self-fertilize. Here, we studied dioicy–monoicy transitions in the ancient liverwort haploid-dominant plant lineage, using three telomere-to-telomere gapless chromosome-scale reference genome assemblies from the Ricciaceae group of Marchantiales. Ancestral liverworts are believed to have been dioicous, with U and V chromosomes (chromosome 9) determining femaleness and maleness, respectively. We confirm the finding that monoicy in Ricciocarpos natans evolved from a dioicous ancestor, and most ancestrally U chromosomal genes have been retained on autosomes in this species. We also describe evidence suggesting the possible re-evolution of dioicy in the genus Riccia, with probable de novo establishment of a sex chromosome from an autosome (chromosome 5), and further translocations of genes from the new sex chromosome to autosomes. Our results also indicated that micro-chromosomes are consistent genomic features, and may have evolved independently from sex chromosomes in Ricciocarpos and Riccia lineages.

Key words: bryophytes, dioicy–monoicy transitions, microchromosome, sex chromosome, sex‐determining system