Sex Determination in Birds - How Does It Work?

More than 60% of adult birds and offspring of many avian species are monomorphic, meaning they do not show any morphological differences between adult males and adult females at such a level that excludes reliable sex determination. Since the usual gender determination in birds includes methods like vent sexing that usually represent a high risk of bird injury and mistakes, alternative non-invasive genetic methods, with high accuracy were developed.

Birds have a ZW sex-determination system, which is different from the XY system found in humans and mammals. Male birds have two Z chromosomes (ZZ), while female birds have one Z and one W chromosome (ZW). The W chromosome is larger than the Z chromosome and carries more genetic information. Therefore, the sex determination in an unknown bird is based on proving the absence or presence of a W-chromosome or a specific sequence of W-chromosome.

Genetic tests are used to determine the sex of birds in various ways. One common method is polymerase chain reaction (PCR), which amplifies specific regions of the bird's DNA and analyzes the amplified DNA for the presence of sex-linked genes. Another method is sequencing, which reads the entire DNA sequence of the bird and identifies the presence of sex-linked genes. Both methods are suitable for the examination of nestlings, in which the sex organs are not still developed and they can be also used for the determination of sex on the shell membrane immediately after the hatching of the chick.

The first gene described to be specific to sex-chromosome was the CHD gene (chromo-helicase DNA binding protein). One copy of CHD1 was located on the W chromosome (abbreviated as CHD1W) and another on the Z chromosome (abbreviated as CHD1Z). The sequence of the CHD1 gene is interrupted by intron and the length of the intron differs for CHD1W-gene and CHD1Z-gene. Hence, the sex determination in birds is based on the amplification of homological sequences of CHD1Z and CHD1W genes including the inserted intron that determines the length of PRC products. Males will give results only for CHD-Z while females show two different products (from CHD1W and CHD1Z).

These genetic tests are useful for several reasons. In commercial poultry production, for example, knowing the sex of chicks early on can improve efficiency and reduce waste. In avian conservation, genetic tests can be used to determine the sex of rare or endangered birds, which is important for breeding and management purposes. The main benefits of determining the gender of a bird by genetic testing are quickness, reliability, and non-invasiveness. And finally, even more sex-chromosome specific genes are being discovered making them potential targets for sex determination in birds.

References:

Kuroiwa, A. (2018). Sex Determination and Differentiation in Birds. In: Kobayashi, K., Kitano, T., Iwao, Y., Kondo, M. (eds) Reproductive and Developmental Strategies. Diversity and Commonality in Animals. Springer, Tokyo. https://doi.org/10.1007/978-4-431-56609-0_19

Fridolfsson, A.-K., & Ellegren, H. (1999). A Simple and Universal Method for Molecular Sexing of Non-Ratite Birds. Journal of Avian Biology, 30(1), 116–121. https://doi.org/10.2307/3677252

Smith, C. A. (2007). Sex Determination in Birds: HINTs from the W Sex Chromosome? Sexual Development, 1(5), 279–285. doi:10.1159/000108934