Some remarks on the female and male Keimbahn in the light of evolution and history

Abstract

From the existence of two types of cells for reproduction—the female and male germ cells (GCs)—and by recombination of the genome, evolution proceeded dramatically. Unicellular and multicellular plants frequently are characterized by a sequence of haploid and diploid phases, or generations, with gametes and spores as reproductive cells. Isogamy, anisogamy, and oogamy can be distinguished depending on the GCs that correspond, differ in size, or impose as egg cell and sperm cell. In protozoans, too, species are found in which GCs differ clearly from each other. In the female lineage of angiosperms, a “Keimbahn chain” consisting of five successive germ line cells can be observed. Oogenesis and spermatogenesis are complete in coelenterates and similar in mammals. However, the controlling mechanisms are by far more complex in the latter. This means that the balance of hormonal and vegetative nervous influences (stimulation, inhibition) on gametogenesis is not primarily orientated on the germ line cells themselves, but mostly on the structural and functional situation of the gonads and the individual carriers. This becomes particularly evident in insects, where gametogenesis, on the one side, depends on the development of the rest of the organism but on the other side represents an independent developmental process. The point at which germ line cells and somatic cells separate correlates more or less with the degree of phylogenetic development. In worms, insects, and up to the anurans, a part of the cytoplasm, the so‐called germ plasma, is separated for the development of GCs during oogenesis (preformistic development). However, in urodeles, reptiles, birds, and mammals, GCs and somatic cells cannot be distinguished before gastrulation (epigenetic development). In various species (e.g., in some oligochaetes and snails), there exist “double spermatogenic lines.” In mammals (probably in other vertebrates and perhaps in various phyla of animals, too), the female Keimbahn is provided with only one proliferation system. The male gametogenesis is equipped with two systems: the first corresponds to the female germ line, the second is responsible for the immense number of gametes produced in the mature testes. In mammals the message to become male lies on the Y‐chromosome (on its short arm in man and mouse) and was identified as the gene SRY in human and Sry in mouse. The fertility genes that are responsible for an uninterrupted spermatogenesis, up to fertilizing spermatozoa, are sitting on the long arm of the human Y‐chromosome. J. Exp. Zool. (Mol. Dev. Evol.) 285:197–214, 1999. © 1999 Wiley‐Liss, Inc.