The mysterious sex chromosomes of haploid plants

The sex chromosomes of haploid plants, including mosses and liverworts (members of the bryophytes) have been much less studied than those of diploid plants. However, it has long been known that having separate sexed individuals is much commoner in bryophytes (Perley and Jesson 2015), than in diploid plants (in which the proportion of dioecious species has consistently been estimated at around 5% (see Charlesworth 1985; Renner 2014; Yampolsky and Yampolsky 1922). Some plants of both types may not have genetic sex-determination (Pannell 1997; Tanurdzic and Banks 2004; Zimmerman 1991), but early cytogenetic studies in bryophytes observed heteromorphic sex chromosomes (Allen 1917, 1919), with the karyotypes of male and female gametophytes differing clearly. Unbiased estimates are not yet available, and species numbers studied are small, but the data (reviewed by Allen 1945 and Renner, Heinrichs et al. 2017) suggest that around half of separate sexed bryophyte species have visibly different sex chromosomes, at least as many as in dioecious diploid angiosperms (Ming, Bendahmane et al. 2011; Westergaard 1958). Overall, therefore, many more bryophyte than angiosperm species are available for studying sex chromosomes.