Many plant and animal species are known to form interspecific hybrids (i.e. crossing of different species). Studies of hybrid formation allow investigation into the processes of natural selection and speciation, and have become a prominent tool for theoretical and empirical studies in evolutionary biology. Waterfleas of the genus Daphnia form interspecific hybrids in many lakes across Europe. Since Daphnia hybrids and species reproduce cyclic parthenogenetically, hybrids can circumvent potential selective disadvantages (such as infertility) and can in theory persist indefinitely as clonal lineages. Morphological and genetic methods were developed and applied to reveal distributional pattern and to overcome taxonomic uncertainties among several Daphnia populations across The Netherlands and other European countries. Alcohol preserved material of several Cladoceran species (Anomopoda, Ctenopoda, Haplopoda and Onychopoda) was subjected to a simple and effective DNA preparation and amplification technique. In contrast to the majority of animal hybrids which occur in narrow corridors bounded by parental species, Daphnia species and their interspecific hybrids were found to co-occur in syntopy in many lakes across large geographic areas.
Although Daphnia species of the D. longispina group are genetically distinct, hybridization is found among many different species, and abundant hybrids may be temporarily more fit than parental species. Although the species D. cucullata and D. galeata preferentially reproduce sexually during different periods of the year, the overlap is sufficient to form F1-hybrids and backcross-hybrids. Hybrid swarms were found to consist of F1-hybrids and to a much lower proportion, backcross-hybrids. Most interspecific hybrids were produced via a cross between D. cucullata females and D. galeata males. The joint application of morphological, nuclear and mitochondrial genetic markers revealed non-hierarchical phylogenetic relationships and different levels of genetic and phenotypic divergence among Daphnia species. Interspecific hybridization appears to enable sufficient gene flow to permit introgression and reticulate evolution, but selection against backcross genotypes appears to restrict gene flow and gene pool merging.
Multivariate statistical analyses revealed an association between environmental factors and the relative abundance of Daphnia taxa. These results indicate that species and hybrids differ in habitat selection and that hybrid maintenance in Daphnia is dependent on variation in environmental factors. The observed patterns of interspecific hybridization suggest that Daphnia species, F1-hybrids and backcrosses vary in their relative fitness. Tests of hybrid models therefore require the assessment of fitness and response to selection regimes among genotypic hybrid classes.