The evolution of extreme shell shape variation in the land snail Ainohelix editha: A phylogeny and hybrid zone analysis

Ainohelix editha from Hokkaido, Japan, exhibit great geographical variation in their shell morphology. In particular, A. editha in two quite separate locations, Shimamaki and Samani, are striking because they are extremely flat and have a sharp keel, whereas at adjacent sites the shells are globular or depressed-globular. We used mitochondrial 16S rRNA and nuclear ITS-2 sequences to infer a phylogeny among 47 snails from 29 locations. Snails from the two keeled-flat populations clustered separately in the phylogeny, suggesting that this unusual shell form could have evolved independently. A morphological analysis of shells collected along a transect between keeled-flat and globular snail sites showed a cline for shell shape and the angle of the keel. Two different mtDNA lineages were found across the transect, with a cline for an ITS-2 single nucleotide polymorphism. Together, the results may suggest a lack of reproductive isolation between keeled-flat and globular snails, with possible introgression by hybridization.