Synopsis top ↑
The hepatic genus Haplomitrium has traditionally been of great phylogenetic interest due to its wide array of presumed ancestral characters, some of which are unique and others of which are reminiscent of Takakia and other bryophyte groups. However, the evolutionary affinities, taxonomy, and underlying morphogenetic patterns of Haplomitrium remain uncertain. Of the 13 described species, only 5 have been studied, in part, for developmental patterns, mostly at the light microsc0pe level. New discoveries of both unique and moss-like ultrastructural characters in Takakia, a postulated close relative of Haplomitrium, and recent suggestions that Haplomitrium is more closely related to the Metzgeriales than to the Jungermanniales makes renewed investigations, including ultrastructural analyses of Haplomitrium appropriate. In this study, the ontogenetic patterns responsible for morphological features of the genus were investigated using experimental culture techniques, light and electron microscopy. Morphogenetic potentials were examined to gain insight into both infrageneric affinities and the phylogenetic relationships with other bryophyte groups, Takakia and the tracheophytes. Of the 13 species studied, 7 are retained and 6 new synonyms are proposed. Likewise, the only other extant member of the Haplomitriales, Steereomitrium, was synonymized with Haplomitrium. The taxa are grouped into two subgenera: subg. Haplomitrium (with two sections: sect. Haplomitrium and sect. Archibryum) and subg. Calobryum. It is concluded that, the genus is a highly plastic, plesiomorphic taxon, probably of Gondwanian origin. Haplomitrium's assemblage of derived traits negate it as the progenitor of either the Metzgeriales or the Jungermanniales. Rather, it appears to have a shared ancestry with the Metzgeriales, and is consequently best classified as a coordinate order in the newly described subclass Metzgeriidae. The shared ancestry of the Haplomitriales and the Metzgeriales suggests that primal hepatics were cylindrical forms with the potential to give rise independently to both dorsiventral and erect, radial gametophytes in response to selective pressures.