The coevolution of ova defensiveness with sperm competitiveness in house mice.

Theoretical models have suggested that sperm competition can lead to increased ova resistance to fertilization. While there is some comparative evidence that this might be true, there is no experimental evidence to show that ova defensiveness evolves in response to sperm competition. We performed a series of in vitro fertilization assays to gauge the fertilizability of ova produced by female house mice from experimental populations that evolved either with or without sperm competition. Our analysis revealed that after 24 generations of experimental evolution, females that evolved under a polygamous regime produced more defensive ova than females that evolved under a monogamous regime. We therefore provide the first direct line of evidence that sperm competition can generate sexual conflict at the gametic level and lead to asymmetries in fertilization rates among populations. Our results show that females respond to sperm competition via fertilization barriers that have the potential to mediate sperm entry.