(September 14th, 2009) Another great mystery in the world of sex has finally been resolved with the identification by an Australian group of the gene that determines whether chicken embryos turn into cockerels or hens. And yes, sex in chickens starts with a Z, not a W, reports Jeremy Garwood.
Not all vertebrates share the familiar XX:XY sex determination system seen in mammals. Rather, in chicken and other birds, sex is determined by a ZZ:ZW sex chromosome system. In this ZW system, it is the egg that determines the sex of the offspring in contrast to the XY system where it is the male’s sperm that says it all - X sperm for a woman, Y for a man.
Both systems have two sex chromosomes. However, in the ZW system, the males are homogametic, with two Z sex chromosomes, while females have a Z and a W chromosome. In birds, the Z chromosome is larger with more genes than the W, making it similar to the X chromosome in the XY system. But surprisingly, there are no genes shared between the avian ZW and mammalian XY chromosomes, indicating different origins for these sex systems from a common ancestor - the respective pairs of sex chromosomes must have originally derived from different autosomal chromosomes.
As such, although present in chicks, the genes critical for mammalian sex determination (for example, the SRY gene on the Y chromosome that initiates male sex determination) have evolved other functions and patterns of expression in ZW animals.
The key question for sex determination in chick has comes down to whether it starts with the Z or the W – Is it the presence of an extra Z chromosome that makes for a male (ZZ), or is there a specific ovary-determining gene on the W chromosome for females (ZW)?
Candidate genes have been found that support both possibilities, but an Australian group has finally succeeded in proving that chicken sex does in fact begin with a Z. In their paper in Nature (vol. 461: 267-71), Craig Smith and his Melbourne colleagues announce that “The avian Z-linked gene ‘DMRT1’ is required for male sex determination in the chicken.”
But as Craig Smith explains, this was not easy to show because unusual chromosome numbers that can occur in mammals (e.g. XYY or XXX) will result in embryonic death in birds before their sex determination occurs. Instead once candidate genes for chick sex determination had been identified on the Z and W sex chromosomes, experimental strategies were required that could either directly enhance or block the expression of these genes in the developing embryo.
Smith’s successful approach used the technique of RNA interference to lower the expression of his candidate gene, the Z-linked ‘DMRT1’ (an abbreviation of its fully inelegant name: ‘Drosophila Doublesex and C. elegans Mab-3 Related Transcription factor, number 1’). Chicken embryos were infected with a modified avian retrovirus that coded for small RNA sequences (microRNA) that could bind to regions of the normal messenger RNA for the ‘DMRT1’ gene, thereby interfering with the normal expression and translation of DMRT1 mRNA resulting in much lower quantities of DMRT1 protein.
With less DMRT1 protein, the chicken embryos developed confused sex organs – males began developing feminised gonads that displayed female-like histology and disorganised testis cords. Meanwhile SOX9, a specific testes protein disappeared from these gonads to be replaced by expression of aromatase, an ovarian marker protein.
Overall, these results support the Z-chromosome dosage hypothesis for sex determination in birds. This is because there is a higher, double ‘dose’ of the DMRT1 protein in males – its gene is present on each of the two Z chromosomes. More DMRT1 protein is sufficient to initiate the differentiation of testes in male embryos. Furthermore, the DMRT1 gene has been found to be conserved on the Z chromosome of all birds so far studied. It is expressed exclusively in the urogenital system of chicken embryos before gonadal sex differentiation.
It may even be important in animals whose sex is determined by temperature (another sex story of its own) – for example, in crocodiles, the DMRT1 protein is upregulated during the thermosensitive period when sex is being determined and only at male-determining temperatures.
Nevertheless, although we are getting closer to understanding the mechanisms that determine the sex of animals, it should be noted in passing that there remains considerable doubt as to the real evolutionary advantages that lead to sex in the first place. This is still to be determined.