Spaak, P. (1996) Temporal changes in the genetic structure of the Daphnia species complex in Tjeukemeer, with evidence for backcrossing. Heredity, 76, 539-548. ISSN 0018-067X.
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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 (difference <0.04), than to C (difference 0.30). My data suggest, in accordance with other studies, that C x 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. [KEYWORDS: allozymes; Cladocera; interspecific hybridization; introgression; parthenogenesis Life-history variation; interspecific hybridization; lake constance; hybrid zones; populations; diversity; hyalina;galeata; variability; cucullata]
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