Genetic Differentiation and Relationship among <i>Castanopsis chinensis</i>, <i>C. qiongbeiensis</i>, and <i>C. glabrifolia</i> (Fagaceae) as Revealed by Nuclear SSR Markers

<i>Castanopsis chinensis</i> (Spreng.) Hance is widespread in the subtropical forests of China. <i>Castanopsis qiongbeiensis</i> G.A. Fu and <i>Castanopsis glabrifolia</i> J. Q. Li & Li Chen are limited to the coastal beaches of Wenchang county in the northeast of Hainan Island, and have similar morphological characteristics to <i>C. chinensis</i>. It is supposed that <i>C. qiongbeiensis</i> and <i>C. glabrifolia</i> are closely related to <i>C. chinensis</i>. In the present study, the genetic differentiation, gene flow, and genetic relationship of <i>C. chinensis</i>, <i>C. qiongbeiensis</i>, and <i>C. glabrifolia</i> were investigated by using 15 nuclear microsatellite markers; a total of 308 individuals from 17 populations were sampled in the three species. The allelic variation of nuclear microsatellites revealed moderate but significant genetic differentiation (F_CT = 0.076) among <i>C. chinensis</i>, <i>C. qiongbeiensis</i>, and <i>C. glabrifolia</i>, and genetic differentiation between <i>C. chinensis and C. glabrifolia</i> was larger than that between <i>C. chinensis</i> and <i>C. qiongbeiensis</i>. Demographic simulations revealed unidirectional gene flow from <i>C. chinensis</i> to <i>C. glabrifolia</i> and <i>C. qiongbeiensis</i>, which highlight dispersal from mainland to island. The isolation effect of Qiongzhou Strait increased the genetic differentiation of species on both sides of the strait; however, the differentiation was diminished by gene flow that occurred during the historical period when Hainan Island was connected to mainland China. Our results supported the argument that <i>C. glabrifolia</i> should be considered an independent species and argued that <i>C. qiongbeiensis</i> should be regarded as an incipient species and independent conservation unit.