In many insects mating triggers a remarkable change in the behaviour of the female: they are no longer attractive to males, they are not responsive to mating and they increase feeding and egg production (see Chen et al., 1988; Chapman et al., 2003; Kubli, 2003). In Drosophila a main trigger of this post-mating behaviour is sex peptide (SP) that is transferred to the female in the seminal fluid (Chen et al., 1988; Liu and Kubli, 2003). The SP, also known as Acp70A, is encoded on the precursor gene CG8982 and is produced in the male accessory glands. It is, thus, a male specific peptide, and is not found outside the genus Drosophila. SP consists of 36 amino acids with the sequence WEWPWNRKPTKFPIPSPNPRDKWCRLNLGPAWGGRC (Chen et al., 1988). SP acts on a GPCR (SPR; CG16752) shared with myoinhibitory peptides (MIP1-5; also known as allatostatin B1-5) in Drosophila (Kim et al., 2010; Poels et al., 2010). This MIP receptor exists in most studied insects (see entry for myoinhibitory peptides), although the bona fide SP is only found in Drosophila (Kim et al., 2010; Poels et al., 2010; Tsuda and Aigaki, 2016).
SP is produced in the male accessory glands. The SP/MIP receptor is widely distributed in the CNS, but a small set of sensory neurons in the reproductive tract, with axions entering the CNS, is critical in mediating the response to SP (Yapici et al., 2008; Hänsemeyer et al., 2009; Yang et al., 2009).
SP is transferred with the semen, bound to the sperms, and acts on the SP receptor in sensory neurons of the female reproductive tract to trigger a long-lasting response, which alters behaviour and physiology of the fly (Chen et al., 1988; Aigaki et al., 1991; Kubli, 2003; Chapman et al., 2003). The SP mediated switch in behaviour/physiology depends on a network of about 2000 neurons in the CNS that express then transcription factor fruitless (fru; Yapici et al., 2008; Yang et al., 2009; Rezaval et al., 2012). The mated females are no longer receptive to courting males, they increase food intake and lay many eggs. SP also changes sleep behaviour of mated flies (Isaac et al., 2009). Mated females sleep less than male flies, probably to increase foraging to ensure nutrients for egg development and to search for sites for egg-laying. In contrast, virgin females display a male-like sleep pattern with a midday siesta. Thus, SP transferred at mating induces an 8-day-long reduction (by 70%) of this daytime rest (Isaac et al., 2009). Finally, it is known that females develop a strong appetite for sodium after mating. This appetite for salt is caused by a SP-mediated modulation of taste processing (Walker et al., 2015).