Novel colour polymorphisms in a hybrid zone of Mandarina (Gastropoda: Pulmonata)

Abstract

A hybrid zone of the land snails Mandmina mandarina and Mandmina chich@nana in the oceanic Bonin Islands was studied morphologically and genetically to show the potential of hybridization as a source of morphological novelties. These species are clearly distinguishable on the basis of dozymes, colour polymorphisms, shell form and genital morphology, but exhibit a hybrid zone from the northeast to southwest of Chichijima island. There is a cline in the frequency of the colour patterns characteristic of each of the species, and specimens with intermediate colour pattern on the shells appear in the hybrid zone. Not only specimens with colour patterns that are overlapping of the patterns of the two species, but also specimens with unique colour patterns appear in the hybrid populations. These unique colour patterns are not found in the pure populations of both species or other Mandorina species in Chichijima Islands, and it is suggested that these are produced by the hybridization. Because of the appearance of many types of unique colour patterns, variability of the colour polymorphism in the hybrid populations are remarkably higher than that in the pure populations. This result suggests that the novel morphology is produced by the hybridization between species with distinctive morphology. This reveals the importance of hybridization as a source of morphological variation, diversity and evolutionary novelty.