Sex from W to Z: Evolution of vertebrate sex chromosomes and sex determining genes

Sex determination in major vertebrate groups appears to be very variable, i ncluding systems of male heterogamety, female heterogamety and a variety of genetic and environmental sex determining systems. Yet comparative studies of sex chromosomes and sex determining genes now suggest that these differ ences are more apparent than real. The sex chromosomes of even widely diver gent groups now appear to have changed very little over the last 300+ milli on years, and even independently derived sex chromosomes seem to have follo wed the same set of evolutionary rules. The sex determining pathway seems t o be extremely conserved, although the control of the genes in this pathway is vested in different elements. We present a scenario for the independent evolution of XY male heterogamety in mammals and ZW female heterogamety in birds and some reptiles. We suggest that sex determining genes can be made redundant, and replaced by control at another step of a conserved sex dete rmining pathway, and how choice of a gene as a sex switch has led to the ev olution of new sex chromosome systems. J. Exp. Zool. 290:449-462, 2001. (C) 2001 Wiley-Liss, Inc.