The sex chromosomes of bryophytes: Recent insights, open questions, and reinvestigations of Frullania dilatata and Plagiochila asplenioides

The three bryophyte lineages have long-lived gametophytes that are either bisexual, producing both male- and female gametes, or sexually specialized and then producing only one type of gamete. Phylogenies suggest repeated evolutionary switches between these systems, implying that bryophyte sex chromosomes may have been gained and lost repeatedly. How this occurred is poorly understood, even though plant sex chromosomes were first discovered in liverworts. We explain how the sex chromosomes of haploid-dominant organisms are distinct from the better-studied X-Y and Z-W systems in the tree of life, summarise what is known about their distribution and genetic composition, and present new cytogenetic data for Frullania dilatata and Plagiochila asplenioides, the former with two U chromosomes and one V chromosome, the latter with one U chromosome and two V chromosomes; male and female C-values in F. dilatata are correspondingly asymmetric (the C-value of P. asplenioides is only known for female nuclei). So far, there is a lack of high-throughput sequencing, quantification, and in situ study of the repetitive DNA, organellar DNA, and transposable elements, and it is therefore not known what causes the size difference of U and V chromosomes from the autosomes or each other. Heterochromatin was also first discovered in bryophytes, but its function in their sex regulation has not been addressed. Studies of bryophyte sex chromosomes with combined cytogenetic and genomic approaches are fundamental for a fuller understanding of sex chromosome evolution across the tree of life.