Phenotypic manifestations during the development of the dominant and default gonads in mammals and birds

The dominant embryonic gonad--testis in mammals and ovary in birds--secretes one or more morphogenetic substances that exert a major effect on the phenotype of the embryo. When deprived of their gonads, mammalian embryos develop into females, and avian embryos assume predominantly male characteristics, although retaining both oviducts. In order to fulfill their task of masculinizing the reproductive tract, mammalian testes grow and differentiate faster than ovaries. In birds the pattern is less straightforward. In 5-day-old embryos of White Leghorn chickens, sexual differentiation manifests itself in two different ways: (1) the gonads of ZZ embryos are larger, and on day 6 contain more protein and DNA than those of ZW embryos; (2) in both sexes, left gonads are larger than right gonads and contain a thick "germinal epithelium" capable of giving rise to an ovarian cortex under the influence of oestrogen. The pattern changes in embryos aged between 7 and 8 days, when the left gonad of ZW embryos outgrows all others, developing into an ovary, and when the bilateral asymmetry between left and right gonads increases in female embryos. A remnant of gonadal bilateral asymmetry is seen in the distribution of gonads in cases of true hermaphroditism in humans and other mammals. Whereas the initial fast growth of the mammalian testis is assumed to be due to one or more Y-chromosomal genes, that of the early avian testis is mostly simply explained as the effect in the disomic state of one or more genes on the Z chromosome. However, the later growth of the avian ovary is more likely to be due to oestrogen than to a direct gene effect. It is postulated that oestrogen has lost its power to determine ovarian development in mammals, in which both sexes are exposed to the oestrogen-rich environment of the uterus. Hence, the task of sex determination devolves on the fetal testis, whose early development and hormonal function are required to induce the male phenotype, the female phenotype arising in default mode.