Introgressive hybridization between two nonnative apple snails in China: widespread hybridization and homogenization in egg morphology.

BACKGROUND Apple snails from the genusPomaceahave spread widely in paddy fields and other wetlands of southern China since their introduction in the 1980s.Pomaceaspp. are commonly identified using mitochondrial COI sequences. However, sequencing the nuclear elongation factor 1-alpha (EF1 alpha) gene revealed genetic introgression between field populations ofP. canaliculataandP. maculata, which produce surviving hybrids in laboratory crossbreeding experiments. RESULTS In this study, we sequenced 1054 EF1 alpha clones to design specific primers and established a fast and accurate multiplex polymerase chain reaction (PCR) method for genotyping EF1 alpha. Combined with genotypingP. canaliculataandP. maculatabased on mitochondrial COI and nuclear EF1 alpha, we revealed the genetic introgression patterns of 30 apple snail populations in China. Purebred and hybrid individuals ofP. canaliculatawere widely distributed, while pure maculata-EF1 alpha type was detected only in a few individuals identified asP. canaliculatabased on COI sequences. Each egg clutch had one to three genetic patterns, indicating multiple paternity or segregation in the progeny of hybrids. The higher percentages of hybrids in both wild populations and progeny than the homozygotes indicated a potential heterosis in the apple snail populations. Additionally, egg size and clutch size of the apple snails became homogeneous among the non-native populations exhibiting introgression hybridization. CONCLUSION Our findings emphasize the value of apple snails as a model to study the mechanisms and impacts of introgressive hybridization on fitness traits.