Life with others:Function and mechanisms of social modulation of behaviour and physiology in <i>Drosophila melanogaster</i>

Group-living facilitates cooperation between individuals, but also creates competition for limited resources. Individuals should thus modulate their reproductive output in response to the presence of others. However, mechanisms that lead to such social context-dependent modulation of reproduction are poorly understood. In the fruit fly Drosophila melanogaster, females actively attract conspecifics to lay eggs on the same resources, generating groups in which individuals may cooperate or compete. The genetic tractability of this species gave me an opportunity to dissect the mechanisms by which females adjust their behaviour and physiology to their social environment.I discovered social facilitation of oogenesis and egg-laying in Drosophila melanogaster and described the neuro-hormonal mechanisms underlying this modulation. I demonstrated that social environment affects ovarian activity of females in a density-dependent manner. I showed that these responses to the presence of others are mediated by vision through the motion detection pathway and are the result of the stimulation of the juvenile hormone pathway. While social modulation of reproduction is considered a sign of sociality and is mostly expressed in social hymenopterans, my findings thus demonstrate that such phenomenon can already be found in rather solitary species such as D. melanogaster, making the concept of solitary vs social species a bit artificial. I moreover proposed a new approach to explore the variation in sociability among D. melanogaster through the quantification of multiple sociability traits. I found significant variation between the flies in the strength of responses to others, demonstrating the existence of a sociability spectrum in this species