TEMPORAL CHANGES IN THE GENETIC-STRUCTURE OF THE DAPHNIA SPECIES COMPLEX IN TJEUKEMEER, WITH EVIDENCE FOR BACKCROSSING

Temporal genetic variation was studied within the cyclic parthenogenet ic Daphnia galeata-cucullata species complex in Lake Tjeukemeer (The N etherlands). During three successive years, three allozyme loci were s tudied in order to compare: the level of genetic variation in D. galea ta (G), D. cucullata (C) and their interspecific hybrid (C x G). This data set was used to investigate: whether Cx G hybrids are regularly p roduced by the parental species and to what extent backcrossing takes place in this species complex. G and C x G were the most abundant taxa , whereas C was only abundant in late summer and autumn. Genetic diver sity was highest in C, lowest in G, and intermediate in C x G. Pgm and Pgi allele frequencies of C x G were not intermediate to those of the parental species but were more similar to G than to C. In addition, a lmost no significant deviations from Hardy-Weinberg were observed. An UPGMA analysis of Nei's genetic distances showed that C x G hybrids ar e more closely related to G (difference <0.04), than to C (difference 0.30). My data suggest, in accordance with other studies, that C x G h ybrids are probably still formed by the parental species, though not n ecessarily every year. Furthermore, my data suggest that backcrossing occurred frequently, and led to introgression of C alleles into G.