SEXUAL REPRODUCTION IN DAPHNIA - INTERSPECIFIC DIFFERENCES IN A HYBRID SPECIES COMPLEX

To investigate whether reproductive variation in the Daphnia galeata-c ucullata-hyalina hybrid species complex can explain the occurrence of hybrids and backcrosses, a total of 43 clones from three north German lakes were tested for allocation to sexual reproduction under equal st ress conditions. Six replicates per clone were followed until the seve nth adult instar. I used the following cues to promote sexual reproduc tion: short photoperiod, water from a crowded Daphnia culture, fish-co nditioned water and low food concentration. For each animal, clutch si ze and clutch sex were recorded. Ephippia, which were empty since the animals were cultured individually, were only produced by D. cucullata and D. cucullata x hyalina (26% and 6% of the broods, respectively), whereas almost all taxa produced males (the range was 2-15%). Intraspe cific variation for male and ephippial production was also found. The fact that the taxa tested show different responses to the stimuli offe red may indicate that there is reproductive isolation between them. Th is is supported by field data from the Tjeukemeer (The Netherlands) wh ich shows that D. galeata mostly produces ephippia in spring and D. cu cullata in autumn. The presence of hybrids with ephippia in both seaso ns, however, shows that backcrossing is also possible.