Abstract
The neurotransmitter dopamine has been shown to play an important role
in modulating behavioural, morphological and life-history responses to
food abundance. However, costs of expressing high dopamine levels remain
poorly studied and are essential for understanding the evolution of the
dopamine system. Negative maternal effects on offspring size from
enhanced maternal dopamine levels have previously been documented in
Daphnia. Here, we tested whether this translates into fitness costs in
terms of lower starvation resistance in offspring. We exposed Daphnia
magna mothers to aqueous dopamine (2.3 mg/L or 0 mg/L for the control)
at two food levels (ad libitum versus 30% ad libitum) and recorded a
range of maternal life history traits. The longevity of their offspring
was then quantified in the absence of food. In both control and dopamine
treatments, mothers that experienced restricted food ration had lower
somatic growth rates and higher age at maturation. Maternal food
restriction also resulted in production of larger offspring that had a
superior starvation resistance, compared to ad libitum groups. However,
although dopamine exposed mothers produced smaller offspring than
controls at restricted food ration, these smaller offspring survived
longer under starvation. Hence, maternal dopamine exposure provided an
improved offspring starvation resistance. We discuss the relative
importance of proximate and ultimate causes for why D. magna may not
evolve towards higher endogenous dopamine levels despite the fitness
benefits this appears to have.