The effect of different light regimes on adult life span in Drosophila melanogaster is partly mediated through reproductive output

The effects of different light regimes on the fitness of organisms have typ ically been studied using mean or median adult life span as the sole index of physiological well-being. it is, however, known that life span is invers ely related to reproductive output in many species. Moreover, the effects o f a given environmental treatment on life span can be due to effects on eit her age-independent mortality or the "rate of aging," or a combination of b oth. Drawing evolutionary inferences from the effects of light regime on me an or median adult life span alone is difficult and, at best, speculative. We examined the effects of constant Light (LL), alternating Light-dark cycl es (LD 12:12 h), and constant darkness (DD) on the life span of reproducing and virgin flies in four populations of Drosophila melanogaster and also e stimated lifetime fecundity in the three light regimes. The light regime ef fects on life span were further dissected by examining the age-independent mortality and the Gompertz rate of aging under the three light regimes. Whi le mean adult life span of reproducing males and females and vir gin female s was significantly shorter in LL compared to LD 12:12 h and DD, lifetime e gg production was highest in LL. Life span of virgin males was not signific antly affected by light regime. The rate of aging in reproducing females wa s higher in LL as compared to DD, whereas age-independent mortality was hig her in DD. As reproductive output, especially early in life, is a far more significant contributor to fitness than is life span, our results suggest t hat the earlier reported deleterious effects of LL on fitness are partly an artifact of examining life span alone, without considering other component s of adult fitness that trade off with life span. Our results suggest that detailed investigation of the effects of light regime on the physiological and behavioral processes that accompany reproduction is necessary to fully understand the effects of different light regimes on adult fitness in Droso phila.