By studying such events during mating in the fruit fly, researchers at the University of Oxford have identified a new communication mechanism in which nutrients and signals are combined in fatty droplets that stably store their bioactive cargos in males, until they are transferred to females when they dissipate within minutes. These specialised multi-molecular assemblies called microcarriers, are made by the prostate-like accessory gland of the male and contain a central fatty (lipid) core wrapped with multiple proteins, including a molecule called Sex Peptide. When Sex Peptide is released in the mated female, it stimulates her to produce more progeny and reprogrammes her brain so she rejects other male suitors.
The remarkable role of Sex Peptide in mediating this form of sexual conflict has been recognised for several decades. However, it has been unclear how this peptide can remain stable in the secretions of the male accessory gland, then rapidly become biologically active in the female uterus. The contrasting behaviour of microcarriers in the male and female reproductive tracts provides the answer to that question. Even more surprisingly, analysis of males that lack Sex Peptide reveals that this protein provides an essential part of the wrapping that shapes these reproductive gifts. In its absence, the male accessory gland becomes filled with giant lipid droplets that do not dissipate properly when transferred to females, partly explaining the inability of these males to reprogramme female behaviour after mating.