Evolutionary history of sex and accessory chromosomes in hornworts

Hornworts represent one of the six major lineages of land plants already present in the Devonian (Villarreal et al., 2010; Frangedakis et al., 2021, 2023). As one of the three lineages of bryophytes, the hornwort life cycle is gametophyte-dominant, with the sporophyte dependent on the maternal gametophyte. As with the other lineages of bryophytes, the mosses and liverworts, dioicy (separate sexes) is prevalent, with c. 40% of hornwort species being dioicous, with the other 60% monoicous (Villarreal & Renner, 2013). In cases that have been examined, sex is often correlated with the presence of sex chromosomes, with females harboring a U chromosome and males a V chromosome, and the sporophyte having a UV karyotype.