Avian sex determination: what, when and where?

Abstract

Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.

All dosage compensation is local: Gene-by-gene regulation of sex-biased expression on the chicken Z chromosome

“…The two unusual genes are located within the male hypermethylated region, and near the DMRT1 locus (at 26.7 Mb), which likely involves in sex determination in birds (Shetty et al, 2002;Smith et al, 2007), and is male biased in our embryonic gonad samples. Although the female-biased genes have no known role in avian sex determination, they must be important in the formation of avian sexual dimorphism, as they are so strongly and consistently female biased through so many time points, and especially in light of the overall male bias of the Z chromosome.…”

Identification of Sexes in Poultry Using DNA Based Methods

“…The Z-linked gene, DMRT1, supports the Z-dosage model of avian sex determination and two W-linked genes, ASW and FET1, represent candidate female determinants [49,50].…”

Genetics in Domestic Animal Reproduction

“…So far diferent hypothesis exist: Avian sex might be determined by dosage of a Z-coded gene or by a W-coded gene or may be both. A SRY gene or homolog is absent in birds and monotremes [22][23][24].…”