Structure, function and evolution of sex-determining systems in Dipteran insects

Nature has evolved an astonishing variety of genetic and epigenetic sex-det ermining systems which all achieve the same result, the generation of two s exes. Genetic and molecular analyses, mainly performed during the last 20 y ears, have gradually revealed the mechanisms that govern sexual differentia tion in a few model organisms. In this review, we will introduce the sex-de termining system of Drosophila and compare the fruitfly to the housefly Mus ca domestica and other Dipteran insects. Despite the ostensible variety, al l these insects use the same basic strategy: a primary genetic signal that is different in males and females, a key gene that responds to the primary signal, and a double-switch gene that eventually selects between two altern ative sexual programmes. These parallels, however, do not extend to the mol ecular level. Except for the double-switch gene doublesex at the end of the cascade, no functional homologies were found between more distantly relate d insects, In particular, Sex-lethal, the key gene that controls sexual dif ferentiation in Drosophila, does not have a sex-determining function in any other genus studied so far. These results show that sex-determining cascad es, in comparison to other regulatory pathways, evolve much more rapidly.