Mechanisms of vertebrate sex determination are reviewed in the light of what is known about the origin and evolution of sex chromosomes. All vertebrate males have testes that are similar in anatomy and in spermatogenesis. Despite a variety of sex chromosome systems, the same cascade of genes operate in the differentiation of testes and male genitalia. Only the primary signal or switch that initiates the cascade may be different. Genetic sex determination (GSD) occurs in most species, although temperature-dependent sex determination (TSD) is the switch in some reptiles without GSD. SRY is the genetic switch in eutherians (placental mammals) and DMRT1 may have that function in birds and at least one species of fish. The switch in all other groups of vertebrates with either male or female heterogamety is unknown. DMRT1 is an ancient sex determining gene, found first in invertebrates, and is one of several genes expressed in higher levels in the vertebrate embryonic testis than in the embryonic ovary. It is singled out in this review because of its likely key role in sex determination both in birds and in reptiles with TSD. DMRT1 is one of the few sex genes that have been mapped in representative species of fish, turtles, crocodiles, amphibians, birds and mammals. Greater effort in comparative mapping, sequencing and expression studies is required to discover the other primary switch genes in vertebrates and to answer questions about their evolution.
(c) 2007 S. Karger AG, Basel