Abstract
The broadleaved evergreen forests of the East Asian warm temperate zone are characterised by their high biodiversity and endemism, and there is therefore a need to extend our understanding of its genetic diversity and phylogeographic patterns. Castanopsis (Fagaceae) is one of the dominant tree species in the broadleaved evergreen forests of Japan. In this study we investigate the genetic diversity, genetic structure and leaf epidermal morphology of 63 natural populations of C. sieboldii and C. cuspidata, using 32 Expressed Sequence Tag associated microsatellites. The overall genetic differentiation between populations was low (G ST = 0.069 in C. sieboldii and G ST = 0.057 in C. cuspidata). Neighbor-joining tree and Bayesian clustering analyses revealed that the populations of C. sieboldii and C. cuspidata were genetically clearly differentiated, a result which is consistent with the morphology of their epidermal cell layers. This suggests that C. sieboldii and C. cuspidata should be treated as independent species, although intermediate morphologies are often observed, especially at sites where the two species coexist. The higher level of genetic diversity observed in the Kyushu region (for both species) and the Ryukyu Islands (for C. sieboldii) is consistent with the available fossil pollen data for Castanopsis-type broadleaved evergreen trees during the Last Glacial Maximum and suggests the existence of refugia for Castanopsis forests in southern Japan. Within the C. sieboldii populations, Bayesian clustering analyses detected three clusters, in the western and eastern parts of the main islands and in the Ryukyu Islands. The west-east genetic differentiation observed for this species in the main islands, a pattern which is also found in several plant and animal species inhabiting Castanopsis forests in Japan, suggests that they have been isolated from each other in the western and eastern populations for an extended period of time, and may imply the existence of eastern refugia.
Highlights
The Quaternary climate cycles played an important role in shaping the distribution of biodiversity among current populations, even in warm-temperate zones, where the land was not covered by ice sheets [1,2]
We focused on the Castanopsis (Fagaceae)-type broadleaved evergreen forest community in Japan, which characterizes the biodiversity and endemism of the East Asia
Castanopsis sieboldii var. sieboldii is mainly found in coastal regions, whereas C. cuspidata var. cuspidata is restricted to interior upland terrain [27]
Summary
The Quaternary climate cycles played an important role in shaping the distribution of biodiversity among current populations, even in warm-temperate zones, where the land was not covered by ice sheets [1,2]. Avise [3] proposed that comparing genetic data from multiple co-distributed taxa could be useful in elucidating the relative influences of major historical events on current patterns of biodiversity. Such studies have been conducted for several areas of the world, including Europe [1,4], North America [5,6], and Asia (with a particular focus on Japan). We aimed to elucidate the effects of past climatic changes more clearly on the current genetic diversity of the species that inhabit warm-temperate zone, because the effects of climate change are severe for these members of the community
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