Temporal changes in the genetic structure of the Daphnia species complex in Tjeukemeer, with evidence for backcrossing

Abstract

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 × G). This data set was used to investigate: whether C × G hybrids are regularly produced by the parental species and to what extent backcrossing takes place in this species complex. G and C × 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 × G. pgm and pgi allele frequencies of C × 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 × G hybrids are more closely related to G (difference <0.04), than to C (difference 0.30). My data suggest, in accordance with other studies, that C × G hybrids are probably still formed by the parental species, though not necessarily every year. Furthermore, my data suggest that backcrossing occurred frequently, and led to introgression of C alleles into G.