Late-Cenozoic origin and diversification of Chatham Islands endemic plant species revealed by analyses of DNA sequence data

Abstract

The relationships of 35 taxa endemic to the Chatham Islands and their close relatives were investigated using DNA sequence data. This study implies that the closest relatives of many of the endemic species are widespread and common species that occur on either of the main New Zealand islands. Hypotheses that the majority of the Chatham Island endemic plants have their closest relatives in northern North Island or southern South Island are not supported. The closest relative for the monotypic Myosotidium hortensium occurs in the Mediterranean, indicating successful long-distance transoceanic dispersal. Patterns of dispersal and estimates of divergence times were determined, and we relate these data to hypotheses of the islands' late-Cenozoic geological history. The Chatham Island endemic taxa differ from their closest relatives by between 0.0 and 6.4% sequence divergence (mostly < 1.5%). Divergence time estimates determined for 19 taxa using molecular clock analyses identified a large group of taxa whose divergence times were < 2.4 Myr, and four species with divergence times > 3.54 Myr. Divergence time estimates for Chatham Island endemic plants are congruent with two late-Cenozoic geological events. First, the emergence of land 6.0 Myr following eruption of the Mangere Volcano, and second, uplift of the submerged Southern Volcanics, Mangere Volcano and Rangitihi Volcanics between 3.0 and 2.0 Myr.