Repeated range expansion and niche shift in a volcanic hotspot archipelago: Radiation of C4 Hawaiian Euphorbia subgenus Chamaesyce (Euphorbiaceae)

Woody perennial plants on islands have repeatedly evolved from herbaceousmainland ancestors. Although the majority of species in Euphorbia subgenusChamaesyce section Anisophyllum (Euphorbiaceae) are small and herbaceous, aclade of 16 woody species diversified on the Hawaiian Islands. They are found in abroad range of habitats, including the only known C4 plants adapted to wet forestunderstories. We investigate the history of island colonization and habitat shift inthis group. We sampled 153 individuals in 15 of the 16 native species of HawaiianEuphorbia on six major Hawaiian Islands, plus 11 New World close relatives, toelucidate the biogeographic movement of this lineage within the Hawaiian islandchain. We used a concatenated chloroplast DNA data set of more than eight kilobases in aligned length and applied maximum likelihood and Bayesian inference forphylogenetic reconstruction. Age and phylogeographic patterns were co-estimatedusing BEAST. In addition, we used nuclear ribosomal ITS and the low-copy genesLEAFY and G3pdhC to investigate the reticulate relationships within this radiation.Hawaiian Euphorbia first arrived on Kauài or Nìihau ca. 5 million years ago andsubsequently diverged into 16 named species with extensive reticulation. Duringthis process Hawaiian Euphorbia dispersed from older to younger islands throughopen vegetation that is disturbance-prone. Species that occur under closed vegetation evolved in situ from open vegetation of the same island and are only foundon the two oldest islands of Kauài and Òahu. The biogeographic history ofHawaiian Euphorbia supports a progression rule with within-island shifts from opento closed vegetation.