Diversification of Chionochloa (Poaceae) and biogeography of the New Zealand Southern Alps

Aim We test hypotheses regarding the origin of diversity and patterns of species richness in and around the New Zealand Southern Alps with 25 species of Chionochloa (Poaceae, Danthonioideae). Location New Zealand. Methods We inferred a well-resolved and mostly robustly supported chloroplast phylogeny based on multiple DNA sequence markers (trnT-L-F, rpl16, trnD-psbM, atpB-rbcL, matK and ndhF), sampling 92% of the recognized species and 82% of the subspecific taxa. Nuclear ribosomal internal transcribed spacer sequences were also sampled, but proved uninformative. Biogeographic reconstruction and character optimization were done using both parsimony and likelihood approaches, and molecular dating used relaxed clock approaches. Results Most of the species diversity in Chionochloa stemmed from a common ancestor in the southern South Island with subsequent dispersal between areas. One clade of apparently cryptic taxa diversified within the central South Island 'endemism gap', persisting there throughout at least the latter half of the Pleistocene. Exclusively alpine and other habitat specialist species originated independently, the former relatively recently (between 7.6 Ma and the present). Main conclusions The phylogeny of Chionochloa and other published phylogenies of New Zealand plant groups demonstrate that the higher degree of endemism in the north and south of the New Zealand South Island relative to a central endemism gap cannot be explained by Alpine Fault displacement. Furthermore, our results suggest that if extinctions resulting from glaciations played a role in the origin of the central endemism gap, their impact was less than might be presumed on the basis of the distribution of taxa as they are currently defined. The diversification of Chionochloa and a number of New Zealand plant groups, such as Ranunculus, was contemporaneous with the initiation of the uplift of the Southern Alps. In contrast to patterns of diversifications within the alpine regions typical of the hyperdiverse Andes, exclusively alpine species in New Zealand arose independently from ancestors distributed in more lowland areas. Similarly, habitat specialists in Chionochloa arose independently from more generalist ancestors. Thus, although diversification in these groups may have been stimulated by mountain building and Pleistocene climatic oscillations, cladogenesis did not occur within the high alpine habitat itself.