Evolutionary transitions from unisexuality to hermaphroditism in mosses are associated with polyploidy. Jesson et al. investigate this association in populations of Atrichum undulatum s.l. Unisexual individuals are haploid, diploid or triploid, while hermaphrodites are diploid or triploid. However, many diploid populations are strictly separate-sexed, suggesting that hermaphroditism is not a necessary result of genome doubling.