Living apart if you can – how genetically and developmentally controlled sex has shaped the evolution of liverworts

Sexual differentiation in bryophytes occurs in the dominant gametophytic generation. Over half of bryophytes are dioicous, and this pattern in liverworts is even more profound as over 70% of species are dioicous. However, the evolutionary mechanisms leading to the prevalence of dioicy and the shifts of sexual systems between dioicy and monoicy have remained poorly known. These essential factors in reproductive biology are explored here in light of phylogenetics combined with evidence of genomic characterization of sex chromosomes and sex-determination, as well as cytology. Our analyses and discussions on liverworts are focused on: (1) ancestry and shifts in sexuality, (2) evolution of sex chromosomes and maintenance of haploid dioicy, and (3) environmental impact on the evolution of monoicism. We show that the dioicous condition is ancestral in liverworts, and the evolution of sexual systems is both conserved and stable with an ancient origin, but also highly dynamic in certain more recently divereged lineages. We assume that the haploid dioicy maintained in the course of evolution must be linked to the genetically controlled sex-determination, and transition from genetically to developmentally controlled sex determination, the evolution of monoicism, is driven by ephemeral and unstable environments. Monoicy is less stable in the long-term than dioicy, and thus, ultimately, dioicy is selected in liverworts. It is concluded that sexual dimorphism is maintained through a highly dynamic evolutionary process, sex chromosomes bearing distinct set of evolutionary forces that can have large impacts on genome evolution and may also promote speciation.