Out of Asia and into India:: on the molecular phylogeny and biogeography of the endemic freshwater gastropod Paracrostoma Cossmann, 1900 (Caenogastropoda: Pachychilidae)

Pachychilidae are distributed in the tropical regions of the southern continents implying a Gondwanan history. In the present study, we investigate the phylogenetic relationships of the freshwater pachychilid gastropod Paracrostoma endemic to Southern India using molecular genetic and morphological data, including the first examination of soft body material of the type species, Paracrostoma huegelii. In addition, two new species, Paracrostoma tigrina sp. nov. and Paracrostoma martini sp. nov., are described. Our systematic revision shows that former taxonomic concepts of Paracrostoma were misleading. We demonstrate that the monophyletic Paracrostoma is restricted to Southern India and nested within a clade of South-east Asian taxa composed of Brotia and Adamietta. The mitochondrial phylogeny is corroborated by the presence of a subhaemocoelic brood pouch that represents a synapomorphy shared by members of only this group of taxa from the Asian mainland and India. Thus, in contrast to several other zoogeographical model cases, our study suggests that pachychilid freshwater gastropods colonized India out of South-east Asia, probably after the collision of both landmasses during the Eocene. By contrast, a simple vicariance scenario involving the Mesozoic raft of originally Gondwanan elements on the drifting Madagascar - India plate and later colonization of Asia from India fails to explain this distributional pattern. Therefore, Pachychilidae do not follow the predictions of the vicariant biotic ferry hypothesis, which has been suggested for a number of other organisms. We conclude that the origins of the Indian biota are more complex and diverse than assumed under the standard Mesozoic vicariance model. (C) 2007 The Linnean Society of London.