The seminal proteome of a monandrous fly, Drosophila subobscura

When females mate with multiple males, ejaculates from different partners compete within the female reproductive tract. Males gain a competitive advantage by transferring seminal fluid proteins (Sfps) that manipulate female post-mating responses, such as reducing female propensity to remate. But in monandrous species, where females typically mate only once, post-mating sexual selection is absent, raising key questions about the complexity and identity of Sfps in monandrous seminal proteomes. To address this, we characterised the seminal proteome of the monandrous fly Drosophila subobscura. Using long-read RNA-seq, we created improved gene models for the D. subobscura accessory gland and ejaculatory duct, then, using a label-free quantitative proteomics approach, we identified 172 Sfps. This number is comparable to the 153 Sfps previously reported in polyandrous D. melanogaster using a similar approach. The D. subobscura seminal proteome contains all the D. melanogaster sex peptide network – proteins that control the female post-mating response – and many proteins known to be involved in post-copulatory sexual selection. We also found a surprising over-abundance of proteins previously shown to be down-regulated under monogamy selection (where sexual conflict is removed) in D. melanogaster. Collectively, the patterns suggest that the D. subobscura seminal proteome is characterised more by conflict than cooperation, and despite differences in mating systems, is compositionally similar to that of D. melanogaster. Hence, sexual conflict could be pervasive in the seminal fluid of monandrous species, shaping a complex proteome rich in receptivity-inhibiting Sfps.