Molecular phylogenetics of the genus Costularia (Schoeneae, Cyperaceae) reveals multiple distinct evolutionary lineages

Abstract

<p>We investigated the monophyly of <i>Costularia</i> (25 species), a genus of tribe Schoeneae (Cyperaceae) that illustrates a remarkable distribution pattern from southeastern Africa, over Madagascar, the Mascarenes and Seychelles, to Malesia and New Caledonia. A further species, <i>Tetraria borneensis</i>, has been suggested to belong to <i>Costularia</i>. Relationships and divergence times were inferred using an existing four marker phylogeny of Cyperaceae tribe Schoeneae expanded with newly generated sequence data mainly for <i>Costularia s.l.</i> species. Phylogenetic reconstruction was executed using Bayesian inference and maximum likelihood approaches. Divergence times were estimated using a relaxed molecular clock model, calibrated with fossil data. Based on our results, <i>Tetraria borneensis</i> is not related to the species of <i>Costularia. Costularia s.l.</i> is composed of four distinct evolutionary lineages. Two lineages, one including the type species, are part of the <i>Oreobolus</i> clade, i.e. a much reduced genus <i>Costularia</i> restricted to southeastern Africa, Madagascar, the Mascarenes and Seychelles, and a small endemic genus from New Caledonia for which a new genus <i>Chamaedendron</i> is erected based on <i>Costularia</i> subgenus <i>Chamaedendron</i>. The other two lineages are part of the <i>Tricostularia</i> clade, i.e. a separate single-species lineage from the Seychelles for which a new genus (<i>Xyroschoenus</i>) is described, and <i>Costularia</i> subgenus <i>Lophoschoenus</i>. For the latter, more research is needed to test whether they are congeneric with the species placed in the reticulate-sheathed <i>Tetraria</i> clade.</p>