Interspecific hybridization in sympatric species of Daphnia inhabiting lakes in northeastern Poland

Populations of three sympatric species (Daphnia cucullata, Daphnia galeata, and Daphnia hyalina (Daphnia longispina complex)) inhabiting three lakes in northeastern Poland (Roś, Mikołajskie, Wigry) were checked for interspecific hybrids. Hybrid Daphnia coexisting with parental individuals were detected in each of the investigated lakes. Several classes of hybrids (F1, F2, backcrosses), including D. cucullata×galeata×hyalina individuals, have been detected. Hybrids constituted a substantial (13%–38%) part of the total abundance of individuals of the D. longispina complex. Deficiency of heterozygotes (hybrids) detected may indicate the existence of barriers for gene flow between the sympatric Daphnia taxa.