Contemporary Asymmetric Genetic Introgression Between Two Pelophylax Species in Shanghai

Human disturbances are considered to break reproduction barriers among species. Significant increases in hybridization events have been reported among a large number of taxonomic groups in anthropogenic environments, providing novel insights into species evolution mechanisms and conservation management in the Anthropocene. The Eastern Golden Frog (Pelophylax plancyi) and BlackSpotted Frog (P. nigromaculatus) are two sympatric anuran species with a long history of mitochondrial genome introgression in highly urbanized continental East Asia. However, there is only limited understanding of the pattern of their contemporary hybridization and factors influencing their interspecific relationship under anthropogenic disturbances. Here, interspecific hybridization between P. plancyi and P. nigromaculatus at the population level was investigated in Shanghai. All except two haplotypes obtained from both species in Shanghai were mixed together, and located in the introgression clade, implying multiple ancient mitochondrial introgression events occurred in the populations of our study area. Asymmetric genetic introgression was detected by microsatellite markers, with 0.7% of P. plancyi and 14.6% of P. nigromaculatus identified as contemporary admixed individuals. Consistent with the trend of population density, higher genetic diversity of neutral microsatellite loci was found in the more abundant P. plancyi; however, variation in mitochondrial (Cyt-b) and nuclear (POMC) genes was higher in relatively rare P. nigromaculatus. The population density of P. plancyi and number of water patches within local habitats were significantly positively correlated with both occurrences and proportions of admixed individuals in the populations of P. plancyi and P. nigromaculatus. Considering the prevalent transformation of habitats in urbanized areas, these results imply that a high population density in isolated artificially altered habitats is likely to increase interspecific hybridization. Thus, population monitoring and improvement of landscape connectivity between habitats would be needed to control the intensity of interspecific hybridization between P. plancyi and P. nigromaculatus in anthropogenic-disturbed environments.