Molecular evidence for an African origin of the Hawaiian endemic Hesperomannia (Asteraceae)

Abstract

Identification of the progenitors of plants endemic to oceanic islands often is complicated by extreme morphological divergence between island and continental taxa. This is especially true for the Hawaiian Islands, which are 3,900 km from any continental source. We examine the origin of Hesperomannia, a genus of three species endemic to Hawaii that always have been placed in the tribe Mutisieae of the sunflower family. Phylogenetic analyses of representatives from all tribes in this family using the chloroplast gene ndhF (where ndhF is the ND5 protein of chloroplast NADH dehydrogenase) indicate that Hesperomannia belongs to the tribe Vernonieae. Phylogenetic comparisons within the Vernonieae using sequences of both ndhF and the internal transcribed spacer regions of nuclear ribosomal DNA reveal that Hesperomannia is sister to African species of Vernonia. Long-distance dispersal northeastward from Africa to southeast Asia and across the many Pacific Ocean island chains is the most likely explanation for this unusual biogeographic connection. The 17- to 26-million-year divergence time between African Vernonia and Hesperomannia estimated by the DNA sequences predates the age of the eight existing Hawaiian Islands. These estimates are consistent with an hypothesis that the progenitor of Hesperomannia arrived at one of the low islands of the Hawaiian-Emperor chain between the late Oligocene and mid-Miocene when these islands were above sea level. Subsequent to its arrival the southeast Pacific island chains served as steppingstones for dispersal to the existing Hawaiian Islands.

Figure 1

ndhF tree of the Asteraceae showing the tribal placement of Hesperomannia (see arrow). This is one of 180 equally parsimonious trees with 1,587 steps, a consistency index of 0.596, and retention index of 0.564. Numbers above and below the nodes indicate numbers of supporting characters and bootstrap support, respectively. Dashed lines indicate nodes that collapse in the strict consensus tree. Brackets show tribal and subfamilial classification following Jansen and Kim (23). A, Barnadesioideae; B, Cichorioideae; C, Asteroideae.

Figure 2

ndhF tree of the Vernonieae showing the placement of Hesperomannia (bold). This is one of eight equally parsimonious trees with 224 steps, a consistency index of 0.812, and a retention index of 0.812. Numbers above the nodes indicate number of supporting characters. Numbers below the nodes indicate bootstrap values. Dashed lines indicate nodes that collapse in the strict consensus tree.

Figure 3

ITS tree of the Vernonieae showing the placement of Hesperomannia (bold). This is the single most parsimonious tree with 1,262 steps, a consistency index of 0.623, and a retention index of 0.636. Numbers above the nodes indicate number of supporting characters. Numbers below the nodes indicate bootstrap values.