Inferring the late Quaternary phylogeography of Pseudopanax crassifolius using microsatellite analysis
Geologic processes have shaped the New Zealand archipelago throughout its existence. The last major geologic event was the Pleistocene glaciations beginning around 2.5 million years ago. This cold period left its mark in the phylogeography (the geographic distribution of genetic variation) of New Zealand’s globally significant biota. Studies into the phylogeography of New Zealand have largely focused on species with limited distributions through rarity or ecological preferences. This study focuses on the ubiquitous species Pseudopanax crassifolius (Sol. Ex A. Cunn) K. Koch, also known commonly as Horoeka or Lancewood. This species is widespread and almost continuously distributed throughout New Zealand giving a broad scale look at the patterns and processes that have influenced the formation of New Zealand’s natural history. Seven microsatellite loci and two rps4 chloroplast haplotypes were utilised to study 247 Pseudopanax crassifolius and nine P. chathamicus individuals sampled from populations from around New Zealand. Pseudopanax crassifolius was found to have levels of genetic diversity and overall differentiation consistent with common widespread trees. The genetic structuring suggests P. crassifolius is not a single homogenous population across a southern cluster. The geographic structuring of genetic variation within these clusters is poor. The genetic patterns and the spatial distribution of these patterns may reflect the response of Pseudopanax crassifolius to changing environmental conditions during the late Quaternary following the maximum extent of the last glacial maximum (LGM) period. During the maximally cold periods of the LGM, P. crassifolius is likely to have been eliminated or at least greatly reduced in the south and west coast of the South Island. In the remainder of the South Island and throughout the North Island it remained widespread. The heterogeneous pattern of genetic variation with little geographic correlation in the northern cluster may reflect either the extent of the historic distribution of the species or the effect of gene flow between populations acting to inhibit population structuring from establishing. The reduction in genetic diversity and the homogeneity of structure in the south indicate a pattern of leading edge re-colonisation into southern areas as conditions became more favourable following the LGM. The leading edge mode is supported by asymmetric introgression of rps4 haplotype seen between P. crassifolius and P. ferox along the east coast of the South Island. This study also investigated levels of differentiation between Pseudopanax crassifolius and P. chathamicus. There is limited evidence of differentiation based on microsatellite markers. There is therefore no strong genetic evidence for either the support or rejection of the current species delimitation of the crassifolius group of Pseudopanax species. The two species are morphologically different and geographically isolated. This, alongside evidence from previous studies suggest that P. chathamicus is possibly an example of a group undergoing incipient allopatric speciation. A recent founder event is proposed with enough potential diversity carried in two individual fruit to account for the diversity seen in P. chathamicus.