Reticulate evolution in terrestrial snails of Euhadra peliomphala species complex

Recent studies have shown the significant role of introgression in evolution, yet the confident evidence of introgression has come from biased taxa. Although the Mollusca were representative taxa to show introgression-driven evolution, conclusive examination has been rare. Here, we investigated the genetic structure and evolutionary history of terrestrial snails in the Euhadra peliomphala species complex (Heterobranchia: Camaenidae). We collected 197 individuals from 196 sites in Japan, covering the entire distribution, and obtained genome-wide single nucleotide polymorphisms (SNPs) using double digest restriction site-associated DNA sequencing. The population genetic analysis showed that the study individuals are clustered into 12 lineages, which are distributed parapatrically. Our hybrid test detected hybrid zones between several pairs of the lineages. Additionally, we reconstructed a phylogenetic network of these lineages and detected three reticulate events, including introgression from an extinct lineage. The ages of the introgression events suggests that geological activity and climate change were involved. We also found human-induced introgression. Overall, the present research showed a highly reticulated evolutionary history of the E. peliomphala species complex, and provided confident evidence of introgression in terrestrial snails. This study should play a major role as a basis for further research on introgression-driven evolution within the E. peliomphala species complex.