Dr. Rüdiger Schulz
Reproduction is one of the major biological processes characteristic of all living species. In animals, sexual reproduction is the by far dominating mode of reproduction, although only half of the individuals (i.e. mothers) produce offspring. The balance of this rather costly strategy is provided by the evolutionary advantage of the gene shuffle during meiosis in combination with sexual selection that exerts its pressure predominantly on males, the gender often providing little to the next generation beyond genes. The power of sexual selection is exemplified by at times extravagant traits for male-male competition. Seemingly wasteful, these traits reflect the male’s competence to recruit resources from the environment and thus involve several genes spread over the genome. Females make use of these indicators of genome quality during mate choice.
The cellular basis of male reproduction are haploid spermatozoa, highly specialized cells functioning as motile genome vectors. Spermatozoa arise from a developmental process known as spermatogenesis. The process is fuelled by spermatogonial stem cells and goes through three main phases: the initial mitotic phase, during which the number of spermatogonia increases rapidly, doubling with each round of mitosis; the meiotic phase (spermatocytes), during which the genetic information is recombined and reduced to a haploid set; the final spermiogenic phase, during which the haploid spermatids emerging from meiosis differentiate, without further proliferation, into flagellated spermatozoa. Irrespective of the stage of development, germ cells cannot survive unless they receive support from a somatic cell type, known as Sertoli cells in vertebrates, which are in close, physical contact with the germ cells. In vertebrates, the endocrine system has evolved as master control system over spermatogenesis. Since Sertoli cells express the receptors for the most important hormones regulating spermatogenesis, sex steroids produced in the testis and FSH produced in the pituitary, the Sertoli cell population is the main target of the endocrine regulation of spermatogenesis. The dependency on sex steroids is linking spermatogenesis to behavioural and morphological traits relevant in sexual selection.
Basic Research Question
When spermatogonial stem cells (SSCs) divide, they can either self-renew to produce more SSCs, or differentiate during spermatogenesis into spermatozoa. The balance between self-renewal and differentiation is tightly regulated to control fertility, e.g. by preventing SSC differentiation in juveniles, or by allowing/stimulating differentiation during puberty or the beginning of reproductive seasons, but also to avoid tumour formation. Paracrine factors derived from the direct environment of the stem cell determine this balance; Sertoli cells or other somatic cells (e.g. myoid and Leydig cells) in the vicinity of SSCs (forming the stem cell niche) produce these factors and their production and release is modulated by reproductive hormones, such as FSH and sex steroids. Information on the identity of such factors and their mode of action is largely missing in vertebrates. Consequently, it is not clear how hormones regulate spermatogenesis via these factors, and the nutshell-version of our main research question is, “How do hormones and growth factors regulate the proliferation activity of germ cells, in particular of spermatogonial stem cells?” Moreover, we are interested in the regulation of the production/release of the hormones targeting spermatogenesis.
The experimental models used to approach these questions are the zebrafish (Danio rerio), and in collaboration with other research groups (e.g., at the Institute of Marine Research in Bergen, Norway), also economically relevant species, such as the Atlantic salmon (Salmo salar).