VARIATION OF ALLOZYME LOCI IN ENDEMIC GAMMARIDS OF LAKE BAIKAL

The outstandingly rich and highly endemic fauna of Lake Baikal include s more than 200 species of gammarids (Crustacea, Amphipoda), most of w hich occur only in Baikal. Allelic variation at 23 enzyme loci has bee n studied for the first time in four species of Baikalian gammarids. T he levels of polymorphism are similar to those observed in other inver tebrates, including gammarids: c. 20-25% of the loci are polymorphic w ith 2.53 to 3.3 alleles and mean heterozygosity of 0.048 to 0.105 per polymorphic locus. The genotype frequencies in local populations in mo st cases markedly deviate from Hardy-Weinberg equilibrium with a defic iency of heterozygotes. Populations were shown to be strongly subdivid ed even over a range of less than 100 m. On the larger scale genetic d ifferences among the populations do not show any strong correlation wi th geographical separation. Genetic distances among species were found to be rather low: from 0.159 between closely related, but morphologic ally clearly distinct species of the genus Spinacanthus to 0.546 betwe en the representatives of different genera. We discuss these patterns of population structure with respect to the problem of the origin of e ndemic faunas in ancient lakes. Our results suggest that speciation oc curred rapidly and relatively recently. Genetic distances within and a mong species support models of speciation based on the founder effect or differentiation of a peripheral population rather than on divergenc e of large subpopulations. The rare of morphological evolution in Baik alian gammarids is very high compared with their biochemical evolution .