NEW RESEARCH HORIZON Review Sperm competition and the evolution of spermatogenesis

Spermatogenesis is a long andcomplexprocess that, despite the sharedoverall goal of producing themale gamete, displays striking amounts of interspecific diversity. In this review, we argue that sperm competition has been an important selection pressure acting on multiple aspects of spermatogenesis, causing variation in the number andmorphology of sperm produced, and in themolecular and cellular processes by which this happens. We begin by reviewing the basic biology of spermatogenesis in some of the main animal model systems to illustrate this diversity, and then ask towhatextent this variationarises fromthe evolutionary forces acting on spermatogenesis,mostnotably spermcompetition. Weexplorefivespecific aspectsof spermatogenesis fromanevolutionaryperspective, namely: (i) interspecificdiversity in thenumberandmorphology of spermproduced; (ii) the testicular organizations and stem cell systems used to produce them; (iii) the large number and high evolutionary rate of genes underpinning spermatogenesis; (iv) the repression of transcription during spermiogenesis and its link to the potential for haploid selection; and (v) the phenomenon of selection acting at the level of the germline.Overall we conclude that adopting an evolutionary perspective can shed light onmanyotherwise opaque features of spermatogenesis, and help to explain the diversity of ways inwhichmales of different species perform this fundamentally important process.