Contrasting phylogeographical patterns of two closely related species, Machilus thunbergii and Machilus kusanoi (Lauraceae), in Taiwan

Aim: The purpose of this paper was to study the patterns of genetic variation, demographic history, haplotype relationships and potential location of diversity centres of two closely related species, Machilus thunbergii and Machilus kusanoi.
Location: The phylogeography of M. thunbergii and M. kusanoi was examined by sampling 110 and 106 individuals from 25 and 16 sampling sited, respectively, across their distributional range in Taiwan. Machilus thunbergii is distributed on the Asian mainland, South Korea, southern Japan, the Ryukyus, Taiwan and the Philippines, whereas M. kusanoi is endemic to Taiwan. These two species are closely related, and both are widely distributed in Taiwan but occupy different altitudinal zones and habitates.
Results:Extremely low levels of genetic differentiation were found for both species. This result suggested that these two species probably survived in multiple relict refugia with different population sizes throughout the island during low temperature periods of the Pleistocene. In addition, nested clade analysis (NCA) of cpDNA haplotypes indicated that restricted gene flow with isolation-by-distance characterized the recolonization after the Pleistocene by Tashueshan and Shiouhluan populations of M. thunbergii in the north-central area west of the Central Mountain Range (CMR). In contrast, NCA analysis indicated that a major diversity centre on the southern tip of the island (Kending population) and contiguous range expansion characterized the recolonization by M. kusanoi of northern areas along the east side of the CMR. The major diversity centres found for the two species examined were further supported by the results of the mean Fst for individual populations in the comparison with other populations, and of the contribution of the divergence component to the total diversity.
Main conclusions: This research supports the multiple relict refugia hypothesis for both species investigated. Populations of M. thunbergii at Shiouhluan and Tashueshan in the north-central area west of the CMR represent a diversity centre currently expanding its size. A diversity centre at the southern-edge population of M. kusanoi, and a contiguous range expansion from Kending, were found. These results indicate that the M. thunbergii populations at Tashueshan and Shiouhluan and the M. kusanoi population at Kending, and even Soukar, are evolutionarily significant units for conservation programmes.

研究目标：本研究旨在探究两个近缘物种——红楠（Machilus thunbergii）与台湾润楠（Machilus kusanoi）的遗传变异模式、种群历史、单倍型关系，以及多样性中心的潜在分布区域。

研究区域与采样：本研究以两个物种在台湾岛及周边的分布范围为采样区域，分别从25个采样点采集110份红楠样本、16个采样点采集106份台湾润楠样本，以解析二者的系统地理学格局。红楠的分布范围涵盖亚洲大陆、韩国南部、日本南部、琉球群岛、中国台湾及菲律宾；而台湾润楠为中国台湾特有种。两个物种亲缘关系密切，在台湾岛均有广泛分布，但各自占据不同的海拔梯度与生境类型。

研究结果：两个物种均呈现极低水平的遗传分化。这一结果表明，在更新世低温期，两个物种可能分别以不同种群规模，在台湾岛的多个孑遗避难所中存活至今。此外，对叶绿体DNA（chloroplast DNA）单倍型的嵌套分支分析（nested clade analysis, NCA）显示，红楠分布于中央山脉（Central Mountain Range, CMR）西侧中北部区域的塔舒山（Tashueshan）与秀峦（Shiouhluan）种群，其更新世后的重新拓殖过程呈现有限基因流与距离隔离模式。与之相对，嵌套分支分析显示，台湾润楠以位于岛南端的垦丁（Kending）种群为主要多样性中心，沿中央山脉东侧向北侧区域进行的连续范围拓殖构成了其更新世后的重新拓殖模式。通过对各群体与其他群体的平均固定指数（fixation index, Fst）以及分化组分对总多样性的贡献值进行分析，进一步验证了本研究中两个物种的主要多样性中心。

主要结论：本研究结果支持两个被研究物种均存在多个孑遗避难所的假说。红楠分布于中央山脉西侧中北部区域的秀峦与塔舒山种群，构成了当前种群规模正在扩张的多样性中心。台湾润楠的南部边缘种群（以垦丁种群为核心）同样为一个多样性中心，其种群通过从垦丁出发的连续范围拓殖实现扩散。本研究结果表明，红楠的塔舒山、秀峦种群，以及台湾润楠的垦丁甚至苏卡（Soukar）种群，均为保护规划中的进化显著单元（evolutionarily significant unit, ESU）。