DEVELOPMENTAL MORPHOLOGY OF SEEDLING AND SHOOT AND PHYLOGENETIC RELATIONSHIP OF DIPLOBRYUM KOYAMAE (PODOSTEMACEAE)

We demonstrated that saltational evolution was recurrent in the body plans of seedlings of the aquatic angiosperm Podostemaceae, in contrast to other angiosperms with seedlings having almost common body plans. Diplobryum koyamae, transferred to the genus Hydrodiscus described in this paper, has long-floating shoots with an anchoring disk-like base and is rootless. Such a body plan is distinct from other members of Asian Podostemoideae comprising reduced or moderate shoots borne on the root. Here, our molecular phylogenetic analysis revealed that H. koyamae is sister to a crustose-rooted group of Hanseniella, Hydrobryum and Thawatchaia within Asian Podostemoideae. The germinating embryo was devoid of plumule and radicle, and comprised a single cotyledon and a short hypocotyl, which produced an adventitious shoot endogenously. The leaves are formed in the absence of the shoot apical meristem, accompanying the separation of lightly stained cells. Comparison with other species of Asian Podostemoideae having the plumule and the adventitious root in the seedling, along with their phylogenetic relationship, suggests that saltational evolution occurred in the seedling body plan of H. koyamae leading to the extraordinary adult body plan, as in the separate clade of Dalzellia, Indodalzellia, and Indotristicha of the subfamily Tristichoideae.