Epiphytism and generic endemism in the Hypnodendrales: Cyrtopodendron, Franciella and macro-morphological plasticity

The Hypnodendrales are an order of generally very robust pleurocarpous mosses in which most taxa have one of three distinct growth forms. The group includes four monotypic genera, two of which, Cyrtopodendron and Franciella, are endemic to New Caledonia. We investigate the phylogenetic relationships of these two taxa and of Pterobryella rígida, also a New Caledonian endemic, and demonstrate that Cyrtopodendron and Franciella should be recognised within Pterobryella and Spiridens respectively as P. vieillardii and S. spiridentoides. We further use Bayesian methods to reconstruct ancestral character states for macro-morphological type and substrate preference within the order, to evaluate hypotheses of correlated evolution of these morphological and ecological characters, and to test for directionality in their evolution. The results suggest that macro-morphology evolves in conjunction with shifts between epiphytic and non-epiphytic substrates, with the dendroid architecture strongly associated with non-epiphytic substrates and the morphologies we term "stipitate-frondose" and "spiridentoid" associated with epiphytism. Ancestral character state reconstruction challenges previous hypotheses that the strictly dendroid form is plesiomorphic within the Hypnodendrales and suggests that the stipitate-frondose architecture may have arisen very early within the evolution of the group, although states for the earliest nodes remain ambiguous. The spiridentoid growth form characteristic of Spiridens, Cyrtopus and Bescherellia may be inherited from a shared common ancestor, with the dendroid morphology of Hypnodendron s.str. being derived, while within Pterobryella the balance of evidence favours an epiphytic, stipitate-frondose common ancestor from which the lithophytic species found on New Caledonia and Lord Howe are descended. Transitions between the major growth forms found within the order are highly homoplastic, facilitating adaptive shifts between epiphytic and non-epiphytic substrates and explaining the paraphyly of some previously recognised genera and families.