Comparative phylogeography of American and Japanese red maples based on chloroplast and nuclear ribosomal DNA

Background/Question/Methods

Red maple (Acer rubrum) and Japanese red maple (A. pycnanthum) are disjunct sister species whose morphological and ecological characteristics are known to be very similar. Whereas red maple is one of the most widely distributed and abundant species in eastern North America, Japanese red maple is rare and has a very narrow geographic distribution in Japan. The objective of our research is to compare phylogeographic characteristics of the two maples using chloroplast and nuclear DNA. Results will be useful to determine their taxonomical and evolutionary relationships.We sequenced the two chloroplast DNA regions (trnL-F, 389bp; atpB-rbcL, 737bp) and one nuclear ribosomal DNA region (ITSi-ITS4, 674 bp). The sample sizes were 100–122 for red maple, and 46 for Japanese red maple. Leaves were collected from natural populations throughout their geographic ranges. As a reference, we also sequenced 7–10 samples of silver maple (A. saccharinum), which is the other member of section Rubra, genus Acer. Single base pair-length mutations in highly repetitive sequence motifs were excluded when we identified haplotypes.
Results/Conclusions
Based on the analyses of chloroplast DNA, we found a total of 8 haplotypes in the three species. Six haplotypes were found in red maple, two in Japanese red maple, and two in silver maple. Japanese red maple was clearly differentiated from the other two species by having unique haplotypes. The two haplotypes of silver maple also were found in red maple. Reading the ITS region, there were 15 haplotypes in the three species; 13 in red maple, one in Japanese red maple, and three in silver maple. Again, the haplotype of Japanese red maple was unique and were not found in the other two species. Extant populations of Japanese red maple appear to have been isolated from the other two species for a very long time. Red maple and silver maple shared two haplotypes in the ITS region. Gene introgression may contribute to occurrence of these haplotypes. In both chloroplast and ITS regions, a high number of haplotypes of red maple was found in the southern region, i.e., Florida, Georgia, South Carolina, and Mississippi. High genetic diversity of the species is likely in this region.
Key words: red maple, phylogeography, chloroplast DNA, ITS, genetic variation