Effects of endocrine modulators on sex differentiation in birds

This mini-review focuses on sexual differentiation of the reproductive organs and the brain in birds and the effects of endocrine modulators on these processes. Sex determination in birds is genetically controlled, but the genetic events implicated are largely unknown. Female birds have one Z and one W sex chromosome, while males have two Z sex chromosomes. It is not clear whether it is the presence of the W chromosome in females, the double dose of the Z chromosome in males vis-a-vis females, or both of these characteristics that are crucial for the determination of sex in birds. Oestradiol directs sexual differentiation in birds during critical periods of development. Consequently, exogenous compounds that interfere with the endogenous oestrogen balance can disrupt sexual differentiation of the reproductive organs and the brain. Therefore, sexual differentiation in birds provides a good model for studying the effects of endocrine modulators at various biological levels from gene expression to behaviour. Some compounds known to be present in the environment can alter endocrine function and have adverse effects when administered during development, resulting in alterations in gonads, accessory sexual organs, and behaviour. Data reviewed in this paper are mostly from laboratory studies on endocrine modulators with oestrogenic activity, whereas evidence for adverse effects of pollutants on sexual differentiation in avian wildlife is scarce.