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 parthenogenetic Daphnia galeata-cucullata species complex in Lake Tjeukemeer (The Netherlands). During three successive years, three allozyme loci were studied in order to compare: the level of genetic variation in D. galeata (G), D. cucullata (C) and their interspecific hybrid (C x G). This data set was used to investigate: whether Cx G hybrids are regularly produced 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 diversity 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, almost no significant deviations from Hardy-Weinberg were observed. An UPGMA analysis of Nei's genetic distances showed that C x G hybrids are more closely related to G (differenc