Phylogeography and systematics of Acanthogammarus s. str., giant amphipod crustaceans from Lake Baikal

摘要

To illustrate the dynamics of evolution of the rich endemic crustacean diversity in the ancient Siberian Lake Baikal, we explored the phylogenetic relationships and intralake phylogeographic structure in members of the genus Acanthogammarus Stebbing (Amphipoda, Gammaridea, Acanthogammaridae), which involves some of the most spectacular gigantic freshwater amphipods in the world. Mitochondrial cytochrome oxidase I gene sequence data gave only partial support to either of the recent competing views of systematic diversity. The three species Acanthogammarus victorii, Acanthogammarus maculosus and Acanthogammarus lappaceus along with a fourth, undescribed taxon endemic to the Ushkanii archipelago, made a monophyletic group corresponding to the subgenus Ancyracanthus. A sister relationship of the vicarious 'subspecies'A. victorii and A. maculosus was not supported, rather the southerly A. victorii was basal to the cluster of the three other northerly, overlapping species. The data generally supported full species rank for the previously debated subspecies taxa, apart from the purported distinction between 'Acanthogammarus longispinus' and Acanthogammarus lappaceus, which seems to represent intraspecies morphological variation, partly related to sexual dimorphism. The sister status of Acanthogammarus godlewskii and A. gracilispinus (and thus subgenus Acanthogammarus) had weak support; reassessment of museum collections showed that they both are widespread and sympatric throughout the lake. The observed interspecies sequence differences were 13-22% (23-64% in terms of model-based distance), suggesting divergence times of several million years under conventional calibrations, calling the applicability of external clocks into question. Strong patterns of intraspecific, intralake phylogeographic subdivision were found in A. lappaceus, A. maculosus and A. gracilispinus, each comprising several disjunct lineages with 1-5% divergence. Other species showed only slight differentiation. In all, the patterns of geographical differentiation were unique for each species, varying from moderate allele frequency differentiation to well-established geographically segregated lineages, presumably of Early and Middle Pleistocene age. The patterns of differentiation in Acanthogammarus support an important role of allopatric diversification in the evolution of Lake Baikal fauna.