Molecular phylogenetics and micromorphology of Australasian Stipeae (Poaceae), and the interrelation of whole-genome duplication and evolutionary radiations in this grass tribe

Abstract

The mainly Australian grass genus Austrostipa with ca. 64 species represents a remarkable example of an evolutionary radiation. To investigate aspects of diversification, macro- and micromorphological variation in this genus we conducted a molecular phylogenetic and scanning electron microscopy (SEM) analysis including representatives from all of its accepted subgenera. Plastid DNA variation within Austrostipa was low and only few lineages were resolved. Nuclear ITS and Acc1 yielded comparable groupings of taxa and resolved subgenera Arbuscula, Petaurista, Bambusina in a common clade and as monophyletic. In summary, we suggest recognizing nine subgenera in Austrostipa. Because of its taxonomic significance in Stipeae, we studied the lemma epidermal structure in 34 representatives of Austrostipa. In most species, the lemma epidermal pattern (LEP) was relatively uniform (maize-like LEP), but in six species it was more similar to that of Stipa s.str., Neotrinia, Ptilagrostis and Orthoraphium. The species representing subgenera Lobatae, Petaurista, Bambusina and A. muelleri from subg. Tuberculatae were well-separated from all the other species included in the analysis. Two different sequence copies of Acc1 were found in polyploid Austrostipa and Anemanthele. Each of the copy types formed a single clade. This was also true of the sampled species of Stipa s.str., but their clades were strongly separated from those of Austrostipa and Anemanthele. This underlines the statement of Tzvelev (1977) that most if not all contemporary Stipeae are of hybrid origin and demonstrates it for the first time unambiguously on the molecular level. Chromosome number variation is surveyed and reviewed for the whole tribe Stipeae and interpreted in a molecular phylogenetic framework. The rather coherent picture of chromosome number variation underlines the phylogenetic and evolutionary significance of this character. The closest extant relatives of Austrostipa and Anemanthele are in the clade of Achnatherum s.str., Celtica, Oloptum and Stipellula. These genera are most abundant in Central and Eastern Asia, which makes a colonization of Australian and New Zealand from this region more likely, perhaps via long-distance dispersal, than colonization of Australia from southern South America via Antarctica as previously invoked. Supporting Information may be found online in the Supporting Information section at the end of the article.