Population admixture and high larval viability among urban toads

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.

Pleistocene glacial cycles drove lineage diversification and fusion in the Yosemite toad (Anaxyrus canorus)

“…There are many examples of intraspecific admixture increasing the viability of translocated larvae and adults (e.g., Japanese common toads: Hase et al. ; European toads: Zeisset and Beebee ). Given the extremely short and variable hydroperiods Yosemite toads experience during larval development, it seems likely that tadpole physiology would be under strong selection.…”

“…Under conditions where ponds are shallow and tend to desiccate, admixture in pond-breeding anurans is more widespread (Kingsolver et al 2002;Pfennig 2007), and natural selection can more efficiently replace old traits with new, successful ones because they are already at high frequencies (Hedrick 2013). There are many examples of intraspecific admixture increasing the viability of translocated larvae and adults (e.g., Japanese common toads: Hase et al 2013; European toads: Zeisset and Beebee 2013). Given the extremely short and variable hydroperiods Yosemite toads experience during larval development, it seems likely that tadpole physiology would be under strong selection.…”

Female polyandry and size-assortative mating in isolated local populations of the Japanese common toadBufo japonicus

“…All tissue samples were digested overnight at 55 °C in a solution (0.3% sodium dodecyl sulphate, 400 m m NaCl, 5 m m EDTA, 20 m m Tris‐HCl, pH 8.0) that contained 200 μg mL −1 proteinase K. To estimate the frequency of polyandry, we genotyped all tissue samples on the basis of seven microsatellite loci: Bbufu 11, Bbufu 13, Bbufu 23, Bbufu 49, Bbufu 39, Bbufu 62 (Brede et al ., ) and Bjap 14 (Hase, Nikoh & Shimada, ) by PCR using 10–50 ng DNA. Following the method described by Hase et al .…”

“…Following the method described by Hase et al . (), the loci were amplified with two or three multiplex reactions using a Multiplex PCR Kit (Qiagen), according to the manufacturer's protocol. The PCR amplification cycles comprised an initial denaturation period at 95 °C for 15 min, followed by 25–35 cycles of 30 s at 94 °C for denaturation, primer‐specific touchdown annealing steps for 90 s (described by Hase et al ., ) and an extension step at 72 °C for 1 min.…”

“…All tissue samples were digested overnight at 55°C in a solution (0.3% sodium dodecyl sulphate, 400 mM NaCl, 5 mM EDTA, 20 mM Tris-HCl, pH 8.0) that contained 200 μg mL −1 proteinase K. To estimate the frequency of polyandry, we genotyped all tissue samples on the basis of seven microsatellite loci: Bbufu11, Bbufu13, Bbufu23, Bbufu49, Bbufu39, Bbufu62 (Brede et al, 2001) and Bjap14 (Hase, Nikoh & Shimada, 2013) by PCR using 10-50 ng DNA. Following the method described by Hase et al (2013), the loci were amplified with two or three multiplex reactions using a Multiplex PCR Kit (Qiagen), according to the manufacturer's protocol. The PCR amplification cycles comprised an initial denaturation period at 95°C for 15 min, followed by 25-35 cycles of 30 s at 94°C for denaturation, primer-specific touchdown annealing steps for 90 s (described by Hase et al, 2013) and an extension step at 72°C for 1 min.…”