Research Highlights

Written by Cameron A. Schmidt

Phenotypic variation in sperm has generally been explained using a game-theoretic competition model in which males adopt evolutionarily stable strategies that maximize fitness under sexual selection[1]. For example, in mammals, phenotypic variation that enhances swimming velocity and/or sperm number per ejaculate are stabilized in scenarios where there is strong inter-male competition for mates[2]. Following from these observations, motility patterns and counts have been used as heuristic guides for clinical sperm selection, under the straightforward assumption that the highest quality sperm can be identified from an idealized set of competitive features. However, during assisted reproductive technology (ART) applications only intra-male variation in sperm phenotype is under consideration, and the mathematical predictions of sperm competition theory apply only to inter-male variation. Despite the reliance on heuristic methods, there is no unifying theory of sperm selection. This is a particularly salient issue because sperm competition theory addresses the positive or stabilizing selection of sperm phenotypes, but largely ignores the developmental relevance of sperm traits that are under negative or purifying selection, which is of primary concern in ART applications. 

In our research, we are applying fundamental concepts from complex systems theory to explore the statistical implications of intra-male phenotypic variation among sperm cell populations. Additionally, we are developing computational and experimental tools aimed to unify microscale (single cell) and macroscale (cell population) aspects of sperm physiology. Agent based models (ABMs) of computer aided sperm motility analysis (CASA; above) are being used to construct theoretical strategies for quantifying the change in phenotypic variety that sperm populations experience under microenvironmental constraint. The basis of our theoretical framework is that physiological characteristics of individual spermatozoa can be quantified as state vectors that contain degrees of freedom in their phenotypic variety. The reproductive microenvironment (either in vivo or in vitro) acts as a ‘regulator’ by constraining the phenotypic features of the sperm that ultimately find and fertilize the egg. Information about these quantities (i.e., sperm variety and microenvironmental constraint) can be leveraged to develop precision in vitro selection strategies that may be tailored to specific patients, species, or other contexts to optimize sperm selection for ART.  

[1] Parker GA. SPERM COMPETITION AND ITS EVOLUTIONARY CONSEQUENCES IN THE INSECTS. Biol Rev 1970; 45:525–567.

[2] Tourmente M, Gomendio M, Roldan ERS. Sperm competition and the evolution of sperm design in mammals. BMC Evol Biol 2011; 11.