Correlated changes in life history traits in response to selection for faster pre-adult development in the fruit fly Drosophila melanogaster

Insects including the fruit fly Drosophila melanogaster are under intense pressure to develop rapidly because they inhabit ephemeral habitats. We have previously shown that when selection for faster development was artificially imposed on D. melanogaster in the laboratory, reduction of pre-adult development time and shortening of the clock period occurs, suggesting a role for circadian clocks in the regulation of life history traits. Circadian clocks in D. melanogaster have also been implicated in the control of metabolic pathways, ageing processes, oxidative stress and defense responses to exogenous stressors. In order to rigorously examine correlations between pre-adult development time and other life history traits, we assayed pre-adult survivorship, starvation and desiccation resistance, body size and body weight, fecundity and adult lifespan in faster developing populations of D. melanogaster. The results revealed that selection for faster pre-adult development significantly reduced several adult fitness traits in the faster developing flies without affecting pre-adult survivorship. Although overall fecundity of faster developing flies was reduced, their egg output per unit body weight was significantly higher than that of controls, indicating that reduction in adult lifespan might be due to disproportionate investment in reproduction. Thus our results suggest that selection for faster pre-adult development in D. melanogaster yields flies with higher reproductive fitness. Because these flies also have shorter clock periods, our results can be taken to suggest that pre-adult development time and circadian clock period are correlated with various adult life history traits in D. melanogaster, implying that circadian clocks may have adaptive significance.