Genetic structure of the spawning population of Danube shad, Alosa pontica, Eichwaldt 1838 (Clupeiformes, Alosiinae)

The genetic structure of the population of Danube shad, Alosa pontica, has been studied by means of analysis of 3 polymorphic biochemical gene loci. The results of the study provide evidence of its genetic heterogeneity which is expressed by: firstly, an unbalanced ratio of genotype loci manifested by an excess of rare homozygotes and deficite of the correspoding heterozygotes, and secondly, differences in the allele frequencies between shad populations arriving for spawning in March-April and May-June. It is suggested that there could be two causes of heterogeneity: one due to the introgressive hybridization between various forms of A. pontica and another to the differences between early and late spring races of shad which to a certain extent are reproductively autonomous.