Stepping stones to Hawaii:: a trans-equatorial dispersal pathway for Metrosideros (Myrtaceae) inferred from nrDNA (ITS plus ETS)

Aim The majority of Hawaii's plants are derived from the south and west of the Pacific rim, suggesting that at least some of the founders have crossed the climatic discontinuity at the equator. However, the pathways of these dispersal events are not-well understood. We sought to elucidate such a pathway for the plant genus Metrosideros Banks ex Gaertn., which appears to have colonized Hawaii from New Zealand. Location We have previously analysed the internal transcribed spacer (ITS) sequence of nrDNA for thirty-six taxa of Metrosideros subgenus Metrosideros distributed across Melanesia and Polynesia. Further we collected two taxa from the Austral and Marquesas Islands. Methods Phylogenetic analysis was undertaken using both the ITS and external transcribed spacer (ETS) regions of nrDNA for twenty-three taxa from Polynesia and East Melanesia. Results The increased resolution achieved by combining ETS data with that of ITS allowed discrimination of two subclades in eastern Oceania. One subclade groups an the Hawaiian taxa together with M. collina (J. R. & G. Forst.) A. Gray from the Marquesas Islands. Main conclusions From an origin in New Zealand, the woody angiosperm Metrosideros is likely to have crossed the equator to Hawaii using a staging point in the remote Marquesas Islands. Colonization of Hawaii by the wind-borne seed of these plants has been achieved against the flow of the prevailing Trade winds north of the equator. A possible climatic mechanism for this dispersal is described.