Systematic position of the enigmatic liverwort Mizutania (Mizutaniaceae, Marchantiophyta) inferred from molecular phylogenetic analyses

Phylogenetic analyses of liverworts using chloroplast rbcL and rps4 sequences were performed with special reference to the enigmatic thalloid liverwort, Mizutania riccardioides. The results showed that Mizutania is nested within leafy liverwort family Calypogeiaceae (subclass Jungermanniidae), thereby refuting the traditional interpretation that Mizutania is a member of the simple thalloid liverworts (subclass Metzgeriidae) related to Aneuraceae. Male plants with three ranked bracts, which are newly discovered in this study, also show a close morphological affinity to those of leafy liverworts. Parallel evolution of the flattened gametophyte seems to have occurred sporadically in the Jungermanniidae in association with adaptation to prostrate growth on the ground or on living leaves. The unistratose thalloid features of Mizutania are interpreted as an extremely reduced or highly specialized form of a creeping leafy liverwort. The rudimentary sexual branches which develop on the margins of the thallus are similar in many characteristics to those of Calypogeiaceae. We reduce the monotypic family Mizutaniaceae to a synonym of Calypogeiaceae.