ORIGIN AND RECENT ENDEMIC DIVERGENCE OF A CASPIAN MYSIS SPECIES FLOCK WITH AFFINITIES TO THE "GLACIAL RELICT" CRUSTACEANS IN BOREAL LAKES.

Abstract

Aspects of the evolution of intralacustrine species flocks and of the origin of the Arctic or "glacial-relict" zoogeographical element in Eurasian inland waters were elucidated in an allozyme study of the crustacean genus Mysis. This element, of supposedly northern marine ancestry, is represented by vicarious taxa in the deeper parts of the Caspian Sea (an enclosed ancient basin) and in young boreal lakes. The three endemic Caspian Mysis species studied are very close genetically (Nei's D = 0.06), which suggests a recent intrabasin radiation and rapid morphological divergence. This is in contrast to the pattern in postglacial Holarctic boreal lakes, where the Mysis relicta group is represented by a set of morphologically uniform but probably much older sibling species (D = 0.3-0.6). The results provide a parallel to those on the recent diversification of some fish species flocks in ancient freshwater lakes. The situation is, however, unusual in that the Caspian sympatric Mysis flock is pelagic, and conditions promoting speciation through allopatric isolation or spatial segregation by trophic substrate specialization seem implausible. The monophyletic Caspian Mysis clade shows a relatively strong divergence from both the northern lacustrine and the Arctic marine congeners (D = 0.6-1.0); the phylogenetic branching order of these three zoogeographical groups is not conclusively resolved. The results contradict the prevailing hypothesis of a recent Pleistocene origin of the Caspian Arctic element by invasion from Eastern European continental proglacial lakes that drained south to the Caspian basin during the glacial maxima and served as refugia for the boreal lacustrine taxa.