First dual-sex hornwort genomes probe sex chromosome evolution in a haploid system

Hornworts (Anthocerophyta) are a group of seed-free, nonvascular plants known for their horn-shaped sporophyte generation. Shared only with some other seed-free plants and fungi, their unusual haploid-dominant life cycle sees the haploid gametophyte phase as an independent organism and the diploid sporophyte phase as an outgrowth dependent on the gametophyte. Dioicy, or separate sexes, has evolved independently in about 40% of extant species, every time with a UV sex chromosome system. Theoretical models predict that hornworts’ haploid-dominant lifecycle minimizes genetic “shielding” that typically occurs in diploid heterogametic (XY/ZW) systems, thereby improving selection’s ability to purge deleterious mutations on these UV chromosomes and perhaps accelerating sex chromosome differentiation toward sex-specific functions. Therefore, we ask: do hornwort sex chromosomes exhibit either sex-specific differentiation or degradation when compared to heterogametic systems? Here, we assemble the first full male and female genomes of two hornwort species and employ annotation summaries to demonstrate that, though these sex chromosomes are degraded relative to autosomes, degradation is roughly equivalent between sexes. Further, synteny analysis suggests considerable divergence between independently evolved sex chromosome systems.

Coming from a school with a very small plant biology curriculum, I was awed by the breadth and depth of plant science at BTI and Cornell. The opportunity to meet professionals who have devoted their lives to the taxa that fascinate me has galvanized my interest in graduate school. For example, the commitment of my mentor, Peter Schafran, to hornworts and quillwortd—obscure clades of seed-free plants—inspires me to discover my own fervor for natural history because I find few things more meaningful. I’m grateful for the wet lab, computational, and field work I was privileged to partake in, for these helped me discover the obstacles between where I am now and where I wish to be in my career.