An analytical framework for estimating fertilization bias and the fertilization set from multiple sperm-storage organs.

How sperm from competing males are used to fertilize eggs is poorly understood yet has important implications for postcopulatory sexual selection. Sperm may be used in direct proportion to their numerical representation within the fertilization set or with a bias toward one male over another. Previous theoretical treatments have assumed a single sperm-storage organ, but many taxa possess multiple organs or store sperm within multiple regions of the reproductive tract. In Drosophila, females store sperm in two distinct storage organ types: the seminal receptacle (SR) and the paired spermathecae. Here, we expand previous "raffle" models to describe "fertilization bias" independently for sperm within the SR and the spermathecae and estimate the fertilization set based on the relative contribution of sperm from the different sperm-storage organ types. We apply this model to three closely related species to reveal rapid divergence in the fertilization set and the potential for female sperm choice.