Evolution and spermatogenesis

Abstract

Sexual reproduction depends on the production of haploid gametes, and their fusion to form diploid zygotes. Here, we discuss sperm production and function in a molecular and functional evolutionary context, drawing predominantly from studies in model organisms (mice, Drosophila, Caenorhabditis elegans). We consider the mechanisms involved in establishing and maintaining a germline stem cell population in testes, as well as the factors that regulate their contribution to the pool of differentiating cells. These processes involve considerable interaction between the germline and the soma, and we focus on regulatory signalling events in a variety of organisms. The male germline has a unique transcriptional profile, including expression of many testis-specific genes. The evolutionary pressures associated with gene duplication and acquisition of testis function are discussed in the context of genome organization and transcriptional regulation. Post-meiotic differentiation of spermatids involves very dramatic changes in cell shape and acquisition of highly specialized features. We discuss the variety of sperm motility mechanisms and how various reproductive strategies are associated with the diversity of sperm forms found in animals.

Figure 1.

Stem cell niche organization and signalling. (a) In the proximal arm of the male gonad of C. elegans, the DTC provides the necessary cues for stem cell maintenance to the closest germline stem cells. The outline on the right indicates the signalling molecules involved. (b) In the Drosophila testis, a somatic cluster of cells called the hub acts as stem cell niche for the closely associated germline and somatic stem cells. The signalling pathways involved in the decision between self-renewal and differentiation are outlined on the right. (c) In the mouse testis, the stem cell niche comprises different factors; Sertoli cells, Leydig cells and vascularization seem to play a role. The outline on the right depicts the factors promoting self-renewal of the germline stem cells.