Dragons in the mist: origin, dispersal and diversification of the Australian genus Dracophyllum (Richeeae, Ericaceae)

The genus Dracophyllum (Ericaceae) has a fragmented distribution across Australasia, but reaches the greatest level of species richness and morphological diversity in New Caledonia and New Zealand. About 50 species are currently recognized, varying from cushion plants to trees up to 14 m tall. They are conspicuous in montane rainforests and wet alpine heathland and are commonly known as ‘dragon heath’ because of their distinctive growth form. We investigated evolutionary processes that contribute to this disparity in species richness. We compiled two complementary data sets for the chloroplast-encoded genes matK and rbcL that included representatives of Dracophyllum, its close relatives Richea and Sphenotoma (together constituting Tribe Richeeae), along with more distant relatives in the Ericaceae. The sequences were subjected to Bayesian and maximum likelihood analyses to assess the robustness of our phylogenetic inferences. We chose four calibration points based on the fossil record and geological events, but nonetheless there was considerable uncertainty surrounding our divergence estimates. In our analyses, Tribe Richeeae formed a monophyletic group that diverged during the Eocene (at least 33.3 Ma) with a crown radiation during the early Miocene (at least 16.5 Ma) that resulted in two disjunct lineages. The crown radiation coincoides roughly with the onset of aridification in central Australia and fragmentation of the rainforests in eastern Australia. The Western Australian genus Sphenotoma formed an isolated evolutionary lineage, while Dracophyllum and Richea together formed a second lineage that was widely distributed in Eastern Australia, Lord Howe Island, New Caledonia and New Zealand. The relationships of the Tasmanian endemic, Dracophyllum milliganii, remain an enigma. It was ambiguously placed in our analyses, emerging as sister either to Sphenotoma or to the Dracophyllum/Richea clade. We documented two instances of long-distance dispersal to New Caledonia and New Zealand in the late Miocene to early Pliocene. However the recent discovery of Oligocene–early Miocene macrofossils that resemble Dracophyllum predates these molecular estimates by several million years. The low level of sequence divergence suggests a rapid and recent species radiation in these two island archipelagos largely within the last 3– 6 million years. This radiation may have been spurred by the Pliocene uplift of the Southern Alps in New Zealand and by episodes of glaciation during the Pleistocene