In many species, both morphological and molecular traits related to sex and reproduction evolve faster in males than in females. Ultimately, rapid male evolution relies on the acquisition of genetic variation associated with differential reproductive success. Many newly evolved genes are associated with novel functions that might enhance male fitness. However, functional evidence of the adaptive role of recently originated genes in males is still lacking. The Sperm dynein intermediate chain multigene family, which encodes a Sperm dynein intermediate chain presumably involved in sperm motility, originated from complex genetic rearrangements in the lineage that leads to Drosophila melanogaster within the last 5.4 million years since its split from Drosophila simulans. We deleted all the members of this multigene family resident on the X chromosome of D. melanogaster by chromosome engineering and found that, although the deletion does not result in a reduction of progeny number, it impairs the competence of the sperm in the presence of sperm from wild-type males. Therefore, the Sperm dynein intermediate chain multigene family contributes to the differential reproductive success among males and illustrates precisely how quickly a new gene function can be incorporated into the genetic network of a species.

Offense double-mating experiments for duplication-bearing males were performed as reported (Yeh, et al. 2013), and concomitantly with those for other male genotypes whose results were already published (Clifton et al. 2020).  Briefly, sperm competitive ability for any given male genotype was calculated with the P2 metric, which measures the relative contribution of the second male to mate to the total progeny of doubly-mated females.  The angular transformation was applied to the P2 values (Sokal and Rohlf 1994).