Reorganization of the sex-determining pathway with the evolution of placentation

In mammals, the sex determination system has already been unraveled in cons iderable detail, and the genes involved are increasingly used to investigat e this system in non-mammalian vertebrates. Data available so far indicate that many of the genes identified are involved in this pathway throughout t he vertebrate phylum, suggesting that the mechanism of sex determination wa s essentially conserved in this taxonomic category. However, a rather funda mental difference between mammals and non-mammalian vertebrates is the role of steroid sex hormones which are critical for gonadal differentiation in the latter, while during mammalian evolution an innovation occurred, whereb y genes that superseded steroids as factors acting in early gonadogenesis w ere co-opted to the pathway. An intermediate stage of this evolutionary swi tch may still be represented by extant marsupials. Referring to the central role of aromatase in steroid-mediated, and of SRY in eutherian gonadal det ermination/differentiation, it is argued here that the "aromatase system" w as replaced by the "SRY system" as a prerequisite for the evolution of plac entation. It is proposed that in co-evolution with placentation, new specif icities and extensions of the pleiotropic spectrum of sex-determining genes have appeared. The evolutionary innovation of placentation may thus have b een materialized by reorganization of the sex-determining system whereby ge nes were recruited at the top of the pathway and genes at the bottom remain ed rather conservative but became increasingly pleiotropic.