Diapause is widely believed to be an adaptation allowing for the temporal avoidance of periods of harsh environmental conditions. Resting forms are typically resistant to many unfavourable factors, hence when deposited in the environment they may assure protection against various selective forces. Unlike in the case of abiotic factors, surprisingly little evidence has pointed to a selective role for biotic forces (other than that of food depletion) in the evolution of the mechanism of diapause. In my studies, I have sought to check whether resting-egg formation could be selected for by fish predation as a mechanism of genome protection in planktonic DAPHNIA MAGNA.
Experimental animals came from a fish-inhabited lake, the Binnensee, in which the typical winter diapause of D. MAGNA had been shown to be complemented by occasional periods of resting eggs formation in summer, in the season of peak fish predation. Due to its relatively large size D. MAGNA is highly vulnerable to visually-feeding fish predators, with the result that the two co-occur only occasionally.
In a set of laboratory experiments I tested for a diapause response of cloned D. MAGNA females to chemical compounds associated with fish predation. Experimental animals were exposed to water formerly inhabited by planktivorous fish fed with conspecific prey. The chemical cues of predation are widely used by planktonic organisms to trigger various defence responses.
The first finding was that a high proportion of experimental females produced ephippial (resting) eggs in initial broods, when exposed to the chemical cues from the early ontogenetic stages onwards.
Second, this response was found to be triggered by at least two signals of different origin i.e. fish kairomones and “alarm cues” deriving from injured conspecific prey. A similar response was observed irrespective of whether the hypothetical alarm substance associated with predator odour came from DAPHNIA specimens actually eaten by fish or from crushed conspecific individuals. There was no similar response to either single cues or chemicals originating from crushed chironomid larvae combined with fish kairomones. The combined signals may be used by DAPHNIA to assess the real danger of fish predation more reliably. While alarm substances may better reflect the significance of the current predation regime on conspecifics, kairomones might inform prey about which predator is active at the time.
Third, I found the diapause response of experimental females to cues of fish predation to be affected by food concentration. While at a low food level almost all tested females produced resting eggs in the early broods, a greater food concentrations were associated with steadily lower fractions of ephippial females, as well as with ever greater postponement of the onset of resting-egg production till later broods. In the absence of cues of predation neither high nor low food concentrations induced diapause in DAPHNIA. This flexible response of endangered prey is considered adaptive. It seems that food conditions may affect prey’s chances of survival and successful reproduction, and hence influence the “decision” of endangered prey to produce resting forms.
All these findings strongly support a hypothesis regarding the selective role of fish predation in the evolution of the diapause response in D.MAGNA.