Phylogeny and biogeography of Castanea (Fagaceae) based on chloroplast sequence data

Castanea (Fagaceae), a tertiary disjunct genus which comprises three sections and seven species, is widely distributed in the northern hemisphere. The striking similarity between the floras of eastern Asia and eastern North America and the difference in chestnut blight resistance among species has been of interest to botanists for a century. To infer the biogeographical history of the genus, the phylogeny of Castanea was estimated from separate and combined analyses of five chloroplast DNA sequence data sets (chloroplast trnT-L-F, the rpl16 intron, the partial ndhF gene and 3' flanking region, and the intergenic spacers ycf6-psbM and orf62-trnGM). The length of the total sequenced cpDNA regions comprised over 7.5kb per taxon, which represented 4.8% of the chloroplast genome of 155kb. A total of 13 haplotypes were detected in all species of Castanea. The cpDNA pattern was geographically structured. Higher levels of cpDNA diversity were found in Asia as compared to North America and Europe. It is hypothesized that Castanea originated in Eastern Asia and speciation is assumed to be by vicariance separating the Japanese species, followed by dispersal from eastern Asia to Europe and onto North America and vicariance through separation of land masses.; The genus Castanea is supported as a monophyletic clade, and section Eucastanon is a paraphyletic group. C. crenata, the Japanese chestnut, was shown to represent an early divergence in the genus, estimated at 22.6 mybp, forming a sister group to the remaining members of the genus. Three Chinese species formed a well-supported, sister clade to the North American and European clade. The North America species are supported as a clade with C. pumila var. ozarkensis as the basal lineage, which is sister to the group of C. pumila var. pumila and C. dentata. Morphological evolution of one nut per bur occurred independently in two continents possibly at least 3 times in the genus. Allegheny (var. pumila) and Ozark chinkapins (var. ozarkensis) have been considered as two varieties of one species C. pumila. However, considering all the sequence data, the two varieties of C. pumila should be raised to species level. Divergence of C. pumila var. ozarkensis occurred earlier, and C. pumila var. pumila was a more recent event from C. dentata's ancestor.; Different portions of the chloroplast genome evolved at different rates and therefore can be utilized at different phylogenetic levels. The trnT-L-F region, particularly the trnT-L spacer, provided sufficient signal to produce a well-resolved hypothesis of relationships among the Castanea species and was most informative at the species level. rpl16 and ndhF coding regions indicate deep evolutionary splits between C. crenata and other Castanea species as well as the outgroups, and should be useful in phylogenetic analyses at relatively high taxonomic levels. Although the orf62- trnGM spacer data set did not resolve relationships among the Castanea species, this region is informative at the population level.