Reconsidering invasion history of common land snails in Japan through genome-wide analyses

Understanding the method and history of introduction in non-native species are essential for conservation biology, especially in regions like Japan that have a long history of human activity. Bradybaena similaris is a circumtropical land snail that is considered a non-native species from Southeast Asia or East Asia to Japan, which was believed to be introduced by human activities such as sugarcane or sweet potato establishment several hundred years ago. Bradybaena pellucida, a sister species of B. similaris , is native to western Japan, but has recently spread throughout the eastern part of the country. We investigated genetic diversification pattern of non-native organisms using the two land snail species. We clarified the phylogenetic relationships of B. similaris and B. pellucida based on genome-wide SNPs and mtDNA. We revealed the population demographic history using ABC analysis with SNP data. Bradybaena similaris of mainland Japan could be distinguished from populations on the continent. Introgressive hybridization between the species occurred approximately 29,700 years ago. The present findings provide two new hypotheses: that agriculture began in Japan about 2800 years ago, Japanese populations of B. similaris were introduced from the continent more than 29,000 years ago by human activity other than agriculture; or B. similaris is native to a part of mainland Japan. The history of a non-native species with human activity is complex, urging us to reconsider previous hypotheses.