Datensatz

Zusammenfassung

The juvenile hormone, methyl farnesoate (MF), and its analog insecticides have been used successfully to
induce the production of males in cladocerans under long-day conditions in the laboratory. However,
without hormone addition Daphnia do not usually produce male offspring under long photoperiods,
while short photoperiods are a stimulus for the induction of males. We used 21 clones of Daphnia pulex
differing in their propensity to produce males under short-day conditions to test whether the treatment
with MF would result in an additive effect of shifting the sex ratio towards males. Contrary to our expectations,
clones with a high tendency of male production showed a reduced sex ratio in response to MF
treatment under short-day conditions, but clones that produced normally few males or did not produce
males were stimulated by 700 nM MF to produce up to 40% males. We suggest that the endocrine disruptive
effect of MF or juvenile hormone analogs in the field may depend on the clonal composition of the
cladoceran population and on the natural photoperiod. This may affect the seasonal occurrence of sexual
reproduction and eventually cause a mismatch between the presence of males and ephippial females.