The seminal odorant binding protein Obp56g is required for mating plug formation and male fertility in .

In and other insects, the seminal fluid proteins (SFPs) and male sex pheromones that enter the female with sperm during mating are essential for fertility and induce profound post-mating effects on female physiology. The SFPs in and other taxa include several members of the large gene family known as odorant binding proteins (Obps). Work in has shown that some genes are highly expressed in the antennae and can mediate behavioral responses to odorants, potentially by binding and carrying these molecules to odorant receptors. These observations have led to the hypothesis that the seminal Obps might act as molecular carriers for pheromones or other compounds important for male fertility, though functional evidence in any species is lacking. Here, we used functional genetics to test the role of the seven seminal Obps in fertility and the post-mating response (PMR). We found that is required for male fertility and the induction of the PMR, whereas the other six genes are dispensable. We found males lacking fail to form a mating plug in the mated female's reproductive tract, leading to ejaculate loss and reduced sperm storage, likely due to its expression in the male ejaculatory bulb. We also examined the evolutionary history of these seminal genes, as several studies have documented rapid evolution and turnover of SFP genes across taxa. We found extensive lability in gene copy number and evidence of positive selection acting on two genes, and . Comparative RNAseq data from the male reproductive tract of multiple species revealed that shows high male reproductive tract expression in a subset of taxa, though conserved head expression across the phylogeny. Together, these functional and expression data suggest that may have been co-opted for a reproductive function over evolutionary time.