Rapid divergence in sperm morphology and transgressive segregation of hybrid sperm kinematics in a young pupfish radiation

Understanding how divergence in reproductive traits can promote speciation remains a fundamental question in evolutionary biology. Sperm morphology and kinematics diverge rapidly across species. However, the effect of hybridization between recently diverged species on sperm traits remains unclear, limiting our understanding of how reproductive isolation evolves. Here, we evaluated sperm morphology, kinematics, and trait integration in species of a young (~10,000 years), sympatric Cyprinodon pupfish radiation from San Salvador Island, Bahamas, as well as fertile advanced-generation hybrids between two of these species. We found significant divergence in flagellum length, midpiece area, and sperm velocity among species. In contrast, hybrids displayed transgressive kinematic profiles defined by high velocities, reduced path curvature, and distinct patterns of sperm kinematic integration. Our findings suggest that hybridization between recently diverged species may reorganize the underlying control of sperm locomotor mechanisms, generating novel phenotypes that could contribute to reproductive isolation in the early stages of speciation.