Sympatric Drosophilid species melanogaster and ananassae differ in temporal patterns of activity.

The fruit fly Drosophila melanogaster has long served as a model system for circadian rhythm research. Various aspects of its genetic, molecular, and circuit-level properties are the subject of investigation, based on which several circadian behaviors and their neuronal controls have been unraveled. In an attempt to address the question of functional significance of circadian organization using a comparative approach, we studied activity/rest rhythm of wild-caught D. melanogaster (DM) and its close relative, Drosophila ananassae (DA). We compared features of the rhythm such as the ability to anticipate morning and evening transitions, presence or absence of morning-associated or evening-associated activity peaks, and phase of these peaks in both species. We found that these 2 sympatric species are different from each other in several aspects of activity/rest rhythm. Unlike DM, which showed a distinct bimodal activity pattern with both morning and evening peaks and a midday interval of relative inactivity under a 12:12-h light/dark regime, DA exhibited unimodal activity with a predominant morning peak, restricting most of its activity to the light phase with no apparent "siesta" during midday. While daytime sleep levels were not different between the 2 species, DA exhibited significantly lesser nighttime activity and higher, more consolidated sleep. This predominant morning activity of DA was also reflected in persistence and phasing of the morning peak under a range of photoperiods. Both under long and short days, the morning peak was the most dominant and persistent peak of DA, whereas the evening peak was more dominant in DM. In addition, DA had a significantly faster circadian clock and more consolidated activity compared with DM. Hence, we hypothesize that these recently diverged sympatric species of fruit flies occupy distinct temporal niches due to differences in their underlying circadian clocks and speculate that they occupy different spatial microenvironments in the wild.