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

Evidence for extensive interspecific hybridization among speciesof the genus Daphnia has been accumulating on a global scale.Although there is evidence for limited gene flow between taxavia hybridization, many species still maintain discrete morphologicaland molecular characteristics. We studied temporal and spatialpatterns of sexual reproduction within the Daphnia galeata–hyalina–cucullatahybrid species complex in a lake (Plußsee), located innorthern Germany. Allozyme electrophoresis allowed us to trackseasonal changes in taxon composition as well as the quantificationof back-crosses. Sexually-reproducing animals (ephippial femalesand males) were mainly found in autumn. The simultaneous presenceof sexual morphs of D. galeata and D. galeata x hyalina withthe dominant D. hyalina taxa makes recent hybridization, aswell as back-crossing, plausible. Males and ephippial femalesof D. hyalina were not back-crossed as were the parthenogeneticfemales. The low number of sexual clones of the hybrid D. galeatax hyalina might reflect its reduced fertility, although thesefew clones were detected in high densities. Only hybrid-clonesthat had a back-cross genotype (towards D. hyalina) exhibitedephippial females and males. This indicates that male and ephippialfemale production within the Daphnia taxa is not random, whichmight increase the chance for the parental Daphnia species toremain distinct.