Molecular phylogenetics and species delimitation in annual species of Hydrocotyle (Araliaceae) from South Western Australia

Abstract

Hydrocotyle L. is a cosmopolitan genus, with approximately 130 species, of mostly perennial herbs in the family Araliaceae. The genus includes around 95 perennial and 35 annual species, with all annual species endemic to Australia. In this study, I used sequences of a nDNA marker (ETS) and two cpDNA markers (psbA–trnH and trnL–trnF) to reconstruct phylogenetic relationships among the annual species of Hydrocotyle. The final sampling included 21 species of Hydrocotyle and seven outgroup taxa. The phylogenetic analyses of the combined molecular dataset (i.e., ETS, psbA–trnH and trnL–trnF) recovered three major clades within Hydrocotyle, defined by annual or perennial life histories, and the presence/absence of floral bracts. The topology reconstructed here indicates that there is insufficient molecular evidence for the formal recognition of two putative new species, Hydrocotyle sp. Hamelinensis and Hydrocotyle sp. Puberula, previously identified on the basis of morphology. More specifically, H. sp. Hamelinensis is conspecific with H. tetragonocarpa Bunge, while H. sp. Puberula is conspecific with H. scutellifera Benth. Morphological studies of H. tetragonocarpa revealed that this species is andromonoecious and has heterocarpic schizocarps. Fertile plants of H. tetragonocarpa bear fruit that are either ovoid and wingless or broadly obcordate and winged. A detailed re-examination of the schizocarp surfaces of H. scutellifera indicated that these schizocarps are either glabrous or papillate, with neither of these two character states co-varying with any other vegetative or reproductive traits. The phylogeny of the annual Hydrocotyle recovered a paraphyletic Hydrocotyle, with Neosciadium glochidiatum (Benth.) Domin nested within the genus. This study is the first to reconstruct a molecular phylogeny of Hydrocotyle using samples of both annual and perennial taxa. The findings from this study provide a phylogenetic framework for future systematic and taxonomic research within the genus.