Kim H G, Keeley S C, Vroom P S, Jansen R K
Department of Botany and Institute of Cellular and Molecular Biology, University of Texas, Austin, TX 78713, USA.
Proc Natl Acad Sci U S A. 1998 Dec 22;95(26):15440-5. doi: 10.1073/pnas.95.26.15440.
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.
海洋岛屿特有植物的祖先识别往往因岛屿和大陆类群之间极端的形态差异而变得复杂。对于距离任何大陆源3900公里的夏威夷群岛来说尤其如此。我们研究了Hesperomannia属的起源，该属有三个物种，是夏威夷特有的，一直被归入向日葵科的Mutisieae族。利用叶绿体基因ndhF（其中ndhF是叶绿体NADH脱氢酶的ND5蛋白）对该科所有族的代表进行系统发育分析表明，Hesperomannia属于Vernonieae族。利用ndhF序列和核糖体DNA的内部转录间隔区对Vernonieae族进行系统发育比较，发现Hesperomannia是非洲Vernonia物种的姐妹群。从非洲向东北方向远距离扩散到东南亚并跨越许多太平洋岛屿链是这种不寻常生物地理联系最有可能的解释。根据DNA序列估计，非洲Vernonia和Hesperomannia之间的分歧时间为1700万至2600万年，早于现有的八个夏威夷岛屿的形成时间。这些估计与一个假说一致，即Hesperomannia的祖先在渐新世晚期至中新世中期之间到达夏威夷-帝王链的一个低岛，当时这些岛屿高于海平面。在其到达之后，东南太平洋岛屿链成为向现有的夏威夷群岛扩散的垫脚石。
