MOLECULAR EVOLUTION OF THE COTTOID FISH ENDEMIC TO LAKE BAIKAL DEDUCED FROM NUCLEAR-DNA EVIDENCE

Lake Baikal in Eastern Siberia contains a remarkable flock of 29 speci es of teleost fishes of the suborder Cottoidei (sculpins, bullheads) t hat are endemic to the lake and its associated rivers and occupy all d epth habitats down to over 1500 m. The species are divided into three families, the Cottidae with 7 species? the Abyssocottidae with 20 spec ies, and the Comephoridae with 2 species. Nucleotide sequences of the rod opsin gene from 12 of these species, plus a non-Baikal marine spec ies, have been used to examine the evolutionary relations and the dive rgence time of the flock. Phylogenetic trees, generated by neighbor-jo ining and maximum parsimony, indicate that the unique Comephoridae fam ily with its viviparity and unusual appearance is closely related to t he Cottidae and Abyssocottidae, whereas the genus Cottocomephorus, at present placed in the Cottidae, was the first to diverge from the ance stral species and forms a separate lineage. The major adaptation to de ep water would appear to be of relatively recent origin, and there is evidence that the ancestral species occupied a shallow-water marine or brackish habitat. Estimates of antiquity obtained from synonymous sub stitutions place the origin of the species flock at around 4.9 million years ago. (C) 1997 Academic Press.