Evidence for avian cell autonomous sex identity (CASI) and implications for the sex-determination process?

For the majority of animals, males and females are obviously different in terms of appearance, behaviour and physiology, and until recently, these differences were considered to be the result of hormone actions. However, there is now considerable evidence that the development of some sexually dimorphic structures/behaviours is a function of properties inherent to male and female cells (hormone independent). The relative contribution of hormones and cellular identity to the development of the phenotype is not clear and is likely to vary from species to species. The study of gynandromorph birds and chimeric embryos has greatly assisted efforts to distinguish between the effects of hormones and inherent cellular factors on phenotype. It is now clear that in birds, male/female differences are not primarily the result of hormone action and that male and female somatic cells possess a cell autonomous sex identity (CASI). Here, we review evidence for CASI in birds and discuss the implications for the process of sex determination.