Maternally derived egg yolk steroid hormones and sex determination: Review of a paradox in reptiles

Abstract

During the past decade, maternally derived steroid hormones in the egg yolk of oviparous vertebrates have been the focus of attention for their possible role in sex determination and hence, information on the consequences of maternal egg yolk steroids on sex determination has accumulated rapidly in reptiles and birds. Until recently, the observations were dominated by the idea that yolk steroids of maternal origin play an important role in sex determination of oviparous vertebrates. However, more recent studies have cast significant doubt on the above conclusion. These studies suggest instead that steroids may be present in the yolk simply as the byproduct of passive uptake during yolk formation or observed correlations might reflect embryonically produced rather than maternally derived steroids. Thus, the objective of the present review is (i) to provide an overview of such paradoxical observations on the role of maternal yolk steroids in sex determination of reptiles, (ii) to identify and provide brief explanations for the observed paradoxical results, and (iii) to provide some future research directions.