Social modulation of oogenesis and egg-laying in Drosophila melanogaster

Being part of a group facilitates cooperation between group members, but also creates competition for limited resources. This conundrum is problematic for gravid females who benefit from being in a group,53but whose future offspring may struggle for access to nutrition in larger groups. Females should thus modulate their reproductive output depending on their social context. Although social-context dependent modulation of reproduction is documented in a broad range of species, its underlying mechanisms and functions 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 allows dissecting the mechanisms underlying physiological adaptation to social context. Here, we show that females produce eggs increasingly faste ras group size increases. By laying eggs faster in group than alone, females appear to reduce competition between offspring and increase offspring survival. In addition, females in a group lay eggs during the day,while isolated females lay them during the night. We show that responses to the presence of others are determined by vision through the motion detection pathway and that flies from any sex, mating status or species can trigger these responses. The mechanisms of this modulation of egg-laying by group is connected to a lifting of the inhibition of light on oogenesis and egg-laying by stimulating hormonal pathways involving juvenile hormone. Because modulation of reproduction by social context is a hallmark of animals with higher levels of sociality, our findings represent a protosocial mechanism in a species considered solitary that may have been the target of selection for the evolution of more complex social systems