Abstract
In terms of evolutionary biology, a population admixture of more than two distinct lineages may lead to strengthened genetic variation through hybridization. However, a population admixture arising from artificial secondary contact poses significant problems in conservation biology. In urban Tokyo, a population admixture has emerged from two lineages of Japanese common toad: native Bufo japonicus formosus and nonnative B. japonicus japonicus, of which the latter was introduced in the early 20th century. To evaluate the degree of genetic disturbance in the admixed population of these two subspecies, we analyzed genotypes of toads distributed within and outside Tokyo by assessing mtDNA and seven microsatellite loci. We found that the introduced B. japonicus japonicus genotype dominates six local populations in the Tokyo admixture zone and was clearly derived from past introgressive hybridization between the two subspecies. These observations were supported by morphological assessments. Furthermore, the average larval survival rate in Tokyo was significantly higher than that outside Tokyo, suggesting that the temporary contribution of introduced toads occurred through introgression. The fitness of toads in urban Tokyo may thus be increasing with the assistance of nonnative individuals.
Highlights
Population admixture is an important topic in evolutionary biology because it offers the advantage of local adaptation by increasing genetic variation through hybridization within and between species (Song et al 2011)
Our previous phylogenetic analysis revealed that Japanese common toads in the Tokyo area have two major mitochondrial lineage types: western Japan (Western) and eastern Japan (Eastern), which correspond to B. japonicus japonicus and B. japonicus formosus, respectively (Hase et al 2012)
Unlike previous reports of amphibians in urban areas (Hitchings and Beebee 1997, 1998), we found no decrease in genetic diversity or larval viability
Summary
Population admixture is an important topic in evolutionary biology because it offers the advantage of local adaptation by increasing genetic variation through hybridization within and between species (Song et al 2011). Some introduced species successfully adapt to an artificial environment and spread throughout the habitat overcoming the native species (Tait et al 2005; Wania et al 2006; Niinemets and Penuelas 2008). This often involves hybridization with natives and poses an unpredictable threat to local ecosystems (Trusty et al 2007; Ryan et al 2009)
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