Sex determination: How to become a
 male or a female. Lessons from fish

Manfred Schartl, Ph.D.
Theodor-Boveri-Institut für Biowissenschaften
(Biocenter) University of Würzburg, Germany

Wednesday, September 4, 2002
3:00 p.m.–Pacific Forum

The vast majority of animal species occur in two sexes, and in many cases the decision whether an embryo develops to a female or male is made by the genome. Sex determination genes initiate a series of developmental processes that establish the male or female phenotype. The genetic scenarios of how sex can be determined are extraordinarily diverse.

In most mammals, in several flies, and in the worm Caenorhabditis elegans, the genes that translate the chromosomal system into molecular actions are known and reasonably well understood. Between worms and flies on one side and mammals on the other, there is a large gap in our knowledge about sex determination genes, making it difficult to discuss the evolution of sex from a molecular biological perspective.

Fishes are an attractive group of organisms for studying the evolution of sex determination because members of this class exemplify a broad range of various types of sexuality, from

hermaphrodism to gonochorism and from environmental to genetic sex determination. Comparative studies on sex determination in very closely related fish groups, species, and populations revealed that the molecular mechanisms for sex determination are highly variable, evolved numerous times independently, and may not represent stable situations. In one fish, the medaka, which was the first vertebrate in which the occurrence of crossing over between X and Y chromosomes was shown (in 1921), the male sex determining gene has been cloned. This gene surprisingly is a homologue of a gene that acts in flies, worms, and mammals far downstream in the sex determination cascade. However, in medaka it has been recruited as the master switch gene for male development at the very top of the male determining gene cascade.