The Soup in My Fly: Evolution, Form and Function of Seminal Fluid Proteins

The seminal fluid of males from vertebrate and invertebrate taxa is a complex mixture of biologically potent molecules and is far more than a medium to support the successful transit of sperm. But the complexity of this mixture, even in the fly, is only now being fully realised. Recent research is highlighting extraordinarily high evolutionary lability within the genes that encode seminal fluid proteins and is revealing an almost bewildering variety of fitness-related functions. Hence the study of the chemical messages passed from males to females at mating provides a unique window through which to view evolution in action. It was in the 1960s that details of the nature of the seminal fluid substances that transform the behaviour of female Drosophila melanogaster following mating first started to emerge [1–3]. This elegant work showed that the characteristic refusal of recently mated females to mate again was caused by mating or seminal fluid in the short term and by seminal fluid together with sperm in the longer term [1,2]. The “sex peptide” that was responsible for this effect was identified in 1988 [4]. There then followed first a trickle [5,6] and then an ever increasing stream [7,8] of identifications of the non-sperm seminal fluid proteins [9]. Findlay et al. [10] now provide a tour de force demonstration of the identification of a further 63 proteins, bringing the total so far to 133 proteins confirmed as transferred during mating along with sperm (Figure 1). To put this into perspective, this number represents 35% of the number of proteins (381) found in the Drosophila sperm proteome [11]. What then is the purpose behind such a diverse and biologically active soup?