Towards a molecular systematics of the Lake Baikal/Lake Tuva sponges.

Lake Baikal is famous for its extensive biodiversity that is equaled only by few other lakes. Fascinatingly, about 80% of all the animals the lake hosts are endemic. Sponges (Porifera) that live in symbiosis with photosynthetic algae are the most abundant animal taxon found in the littoral zone of Lake Baikal and have been grouped to the family Lubomirskiidae. In recent years, several attempts to determine the phylogenetic relationship between Lubomirskiidae and cosmopolitan freshwater sponges have been undertaken. Yet the results obtained remain inconclusive. Here, we strive to determine the phylogeny of freshwater sponges with the focus on endemic Lake Baikal species, also taking into account two poriferan species that were collected during an expedition in 2006 in two other isolated Siberian lakes, Lake Chagytai and Lake Tore-Khol. Since its discovery at the beginning of the twentieth century, the Lake Chagytai species was grouped to the Lubomirskiidae and called Baikalospongia dzhegatajensis. However, analyses of molecular sequence data [internal transcribed spacer 2 (ITS2), ribosomal DNA (rDNA)] and morphological markers (spicules, habitus) inferred a close relationship to the cosmopolitan genus Ephydatia and also to the Lake Tore-Khol species that had not so far been described. Thus, both species were tentatively termed Ephydatia tuva (Lake Chagytai) and E. altaiensis (Lake Tore-Khol). We hypothesize that these new species might have evolved from Ephydatia-like ancestors through adaptation to the unique environmental conditions of both lakes. To test the ITS data, an unlinked genetic locus was chosen for further phylogenetic analyses, the protein-coding gene silicatein. These analyses provided not only a more robust resolution between the Lubomirskiidae, but also corroborated the grouping of the Lake Chagytai and Lake Tore-Khol species to the genus Ephydatia. In addition, the phylogenetic analyses suggest a Spongilla-like founder generation of poriferan species in Lake Chagytai and Lake Tore-Khol. In conclusion, we propose that the process of speciation in Lake Baikal and Lake Chagytai/Lake Tore-Khol, from a cosmopolitan Spongilla-like ancestor to more than ten endemic species follows allopatric speciation patterns and is of the peripatric type.