Spatial and temporal patterns of sexual reproduction in a hybrid Daphnia species complex

Evidence for extensive interspecific hybridization among species of the genus Daphnia has been accumulating on a global scale. Although there is evidence for limited gene flow between taxa via hybridization, many species still maintain discrete morphological and molecular characteristics. We studied temporal and spatial patterns of sexual reproduction within the Daphnia galeata-hyalina-cucullata hybrid species complex in a lake (Plusssee), located in northern Germany. Allozyme electrophoresis allowed us to track seasonal changes in taxon composition as well as the quantification of back-crosses. Sexually-reproducing animals (ephippial females and males) were mainly found in autumn. The simultaneous presence of sexual morphs of D. galeata and D. galeata x hyalina with the dominant D. hyalina taxa makes recent hybridization, as well as back-crossing, plausible. Males and ephippial females of D. hyalina were not back-crossed as were the parthenogenetic females. The low number of sexual clones of the hybrid D. galeata x hyalina might reflect its reduced fertility, although these few clones were detected in high densities. Only hybrid-clones that had a back-cross genotype (towards D. hyalina) exhibited ephippial females and males. This indicates that male and ephippial female production within the Daphnia taxa is not random, which might increase the chance for the parental Daphnia species to remain distinct