Biogeographic patterns in alpine New Zealand: using nuclear and chloroplast loci to investigate divergence and secondary contact in Notothlaspi (Brassicaceae)

Alpine plants must contend with harsh climatic conditions and the challenges imposed by fragmented, patchy habitats, which often restrict gene flow and increase genetic structure. This study presents the first population genetic investigation of a New Zealand alpine plant group, Notothlaspi Hook. f. (Brassicaceae). The primary aims were to explore the spatial structure of genetic variation and gene flow across and within the three recognised Notothlaspi species. Microsatellite and chloroplast markers revealed genetic divergence between the Notothlaspi australe and ­Notothlaspi rosulatum lineages and a close relationship between N. australe and Notothlaspi viretum. There is also evidence of likely secondary contact between N. australe and N. rosulatum, and potentially N. australe and N. viretum in the Richmond Ranges where there is geographic overlap. Notothlaspi australe exhibited hierarchical nuclear genetic structure that does not appear consistent with a model of isolation by distance, but is consistent with the spatially fragmented habitats that is occupies. In contrast, N. rosulatum exhibited higher chloroplast haplotype diversity than N. australe, but lower nuclear differentiation among populations, consistent with the more continuous available habitat across its range. The sampled N. viretum individuals were most genetically similar to sympatric N. australe in analyses of both nuclear and chloroplast data. However, consistent production of up to four alleles at microsatellite loci suggests a duplication event that should be investigated more fully in future studies. Overall, these findings are consistent with historical periods of contact and isolation likely linked to periods of climatic fluctuations and mountain uplift.