High Degree of Mitochondrial Haplotype Diversity in the Japanease Common Toad Bufo japonicus in Urban Tokyo

Abstract

The Japanese common toad Bufo japonicus is widely distributed across mainland Japan and is classified into two subspecies, B. japonicus japonicus and B. japonicus formosus, in the western and eastern regions, respectively. To investigate the genetic diversity of B. japonicus at the breeding pond (local population) level, we sequenced 831 base pairs of mitochondrial DNA (mtDNA) cytochrome b (Cyt b) from 75 individuals collected from nine ponds in urban Tokyo and the surrounding area. Phylogenetic and population genetic analyses revealed high mtDNA haplotype diversity (Hd, 0.716 (mean) ± 0.230 (SD)) within local populations (breeding ponds). Most local populations had multiple haplotypes of the mitochondrial Cyt b gene, and seven of the 18 haplotypes were identified in two or more local populations. These results indicate that mitochondrial gene flow had occurred across different breeding sites. We also identified five haplotypes that belonged to the western clade and correspond to B. japonicus japonicus. Our results provide genetic evidence that B. japonicus japonicus was introduced artificially from Western Japan to Tokyo, where it occupied the natural habitat of B. japonicus formosus. The urban Tokyo area was found to represent an admixed population consisting of both native and non-native B. japonicus subspecies.

Population admixture and high larval viability among urban toads

“…Our previous phylogenetic analysis revealed that Japanese common toads in the Tokyo area have two major mitochondrial lineage types: Western and Eastern, which correspond to B. japonicus japonicus (hereafter, japonicus ) and B. japonicus formosus (ditto, formosus ), respectively (Hase et al 2012). These findings are consistent with the two lineage patterns identified by RFLP genotyping.…”

“…11. Molecular phylogenetic analysis of Cyt b sequences was performed together with previously reported analyses (Igawa et al 2006; Hase et al 2012). The phylogenetic analysis strongly supported the above-mentioned results and the findings are detailed in Figure A1.…”

“…The collection locality (population sites) and the sample attributes are shown in Figure 2 and Table 1. We collected 293 specimens of B. japonicus (including 75 specimens that were used in Hase et al 2012) from three districts: the eastern region of Japan – native B. japonicus formosus (Eastern group: Pops. [local populations] 1–5 and 12–15); the western region of Japan – native B. japonicus japonicus (Western group: Pops.…”

“…Sexes were only determined in a subset of adults.1Individuals partly including the same samples as Hase et al(2012). …”

“…Two divergent lineages of the Japanese common toad are found in urban Tokyo: the native B. japonicus formosus and the nonnative B. japonicus japonicus , which was artificially introduced in the early 20th century. These two subspecies of toads have established admixed local populations (Hase et al 2012). Bufo japonicus japonicus and B. japonicus formosus are naturally distributed in the western and eastern regions of mainland Japan, respectively (Matsui 1984).…”