Unraveling the sperm bauplan: relations between sperm head morphology and aperm function in rodents

María Varea-Sánchez; Maximiliano Tourmente; Markus Bastir; Eduardo R.S. Roldan

Department of Biodiversity and Evolutionary Biology, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain; Department of Paleobiology, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain.

Rodents have spermatozoa with features not seen in other species. Sperm heads in many rodent species bear one or more apical extensions, known as « hook(s) ». The process by which hooks have evolved, together with their adaptive significance, are still controversial issues. In order to improve our understanding of the biological meaning of these sperm head adaptations, we analyzed hook curvature angles, hook length and overall hook shape in muroid rodents by using geometric morphometrics. We also searched for relationships between hook design and measures of inter-male competition to assess if postcopulatory sexual selection is an important selective force driving changes in this sperm structure. Finally, we sought possible links between aspects of sperm hook design and sperm velocity as a measure of sperm performance. Results showed that one hook curvature angle is under strong selective pressure. Similarly, hook length appears to be strongly selected by sexual selection with this selective force also exhibiting a stabilizing role reducing inter-male variation in this trait. The adaptive significance of changes in hook structure was underlied by the finding that there is a strong and significant covariation
between hook dimensions and shape and between hook design and sperm swimming velocity.
Overall, this study strongly suggests that postcopulatory sexual selection has an important effect on the design of the sperm head that, in turn, is important for enhancing sperm velocity, a function crucial to reaching the vicinity of the female gamete, and winning fertilizations under competitive situations.

doi: 10.1095/biolreprod.115.138008