Seedling development in Hanseniella, Hydrobryum and Thawatchaia (Podostemaceae), and implications on body plan evolution in the Hydrobryum clade

In order to understand the evolution of the body plan characteristic of the unique aquatic Podostemaceae, seedlings of crustose-rooted species of the Hydrobryum clade, i.e., Hanseniella heterophylla, Hydrobryum loeicum, Hy. tardhuangense, Hy. vientianense and Thawatchaia trilobata, were studied by sectioning and scanning electron microscopy. In all the species the plumule/primary shoot and the radicle/primary root did not form in the seedling. The adventitious root formed exogenously from the lateral side of the hypocotyl and developed initially into a semicircular root, which became crustose during early seedling development. Tufted-leaves arose endogenously from the root. Within the context of the phylogeny of the Hydrobryum clade, we postulate an evolutionary scenario in which the plumule was lost at the divergence of the Hydrobryum clade from the Cladopus clade, much later than the radicle was lost at the divergence of the clade of Podostemoideae and Weddellinoideae from Tristichoideae. The seedling body plan, with the exogenous crustose root playing the role of a leading organ, common in the clade of Hanseniella, Hydrobryum and Thawatchaia, appeared after the divergence of Hydrodiscus with a branched shoot as a leading organ.