In many species of reptiles, sex is determined at fertilization by zygotic sex chromosome composition. In other species, including all crocodilians, most turtles and some lizards, sex is determined by temperature during the earlier stages of gonadal differentiation. The effects of exogenous estrogens, antiestrogens and aromatase inhibitors at different temperatures have unambiguously demonstrated the involvement of estrogens in sexual differentiation of the gonads. Aromatase is the enzyme that converts androgens to estrogens. Gonadal aromatase activity is well correlated with gonadal structure. (...) It increases exponentially in differentiating ovaries, whereas it remains low in differentiating testes. Moreover, there is a high correlation between the thermosensitive periods for both ovary differentiation and increase in aromatase activity. We suggest that a thermosensitive factor intervenes, directly or indirectly, in the transcriptional regulation of the aromatase gene in reptiles with temperature‐dependent sex determination. (shrink)

Sex determination in mammals is mediated via the supporting cell lineage in the fetal gonad. In the very early stages of gonadal development, the fate of the supporting cell population is critically dependent on the expression of the male‐determining gene on the Y chromosome. If this gene is absent or fails to be expressed, or is expressed too late or in too small a number of supporting cells, all supporting cells (XX or XY) differentiate as pre‐follicle cells and development proceeds (...) along the female pathway. Supporting cells in which the male‐determining gene is expressed in a timely manner differentiate as pre‐Sertoli cells; given sufficient such cells, testis cords form and development proceeds in a male direction. If XX supporting cells are also present, a few may be recruited into the pre‐Sertoli population and participate in testis cord formation. The subsequent fate of pre‐follicle cells depends critically on interaction with the germ cell population in the developing gonad: absence of germ cells may lead to partial masculinization of the gonad, and/or to disappearance of the supporting cell component. (shrink)

Mammals have an XX:XY system of chromosomal sex determination in which a small heterochromatic Y controls male development. The Y contains the testis determining factor SRY, as well as several genes important in spermatogenesis. Comparative studies show that the Y was once homologous with the X, but has been progressively degraded, and now consists largely of repeated sequences as well as degraded copies of X linked genes. The small original X and Y have been enlarged by cycles of autosomal addition (...) to one partner, recombination onto the other and continuing attrition of the compound Y. This addition–attrition hypothesis predicts that the pseudoautosomal region of the human X is merely the last relic of the latest addition. Genes (including SRY) on the conserved or added region of the Y evolved functions in male sex determination and differentiation distinct from the general functions of their X‐linked partners. Although the gonadogenesis pathway is highly conserved in vertebrates, its control has probably changed radically and rapidly in vertebrate – even mammalian – evolution. (shrink)