Avian sex determination
Avian sex determination is of considerable importance for studies of avian behavior, ecology, evolutionary biology and genetics, and for the conservation of endangered avian species.
More than 50% of bird species are monomorphic, rendering their sexing based on external morphology impossible. Even in dimorphic species, sex determination is problematic in nestlings.
Sex in birds is determined by Z and W sex chromosomes. In birds, unlike in mammals, females are the heterogametic sex i.e. have 2 different sex chromosomes (ZW), while males are the homogametic sex (ZZ). The W chromosome has lost most of the genes during the evolution and is, therefore, reduced in size, while the Z chromosome is highly conserved and, thus, larger. Chromosome examination has developed over time from the cytogenetic to a molecular level.
The discovery of chromo-helicase-DNA-binding protein (CHD) gene that is conserved on an avian W chromosome (CHD1W) enabled sex identification in most avian species. A very closely related copy of this gene was afterwards discovered also on the Z chromosome (CHD1Z). However, these genes cannot be distinguished in ratite birds such as emu, kiwi, rhea, and ostrich due to similarity of CHD gene on W and Z chromosomes.
The molecular sexing (avian sex determination) method based on the amplification of the CHD1W and CHD1Z genes was first successfully established by Griffith et al. Later on, different PCR methods were designed based on the difference in length between introns in the CHD1W and CHD1Z genes.
PCR-based method using specific primers for female fragments SS1 and SS2 has been developed for avian sex determination in ratite birds.
Polymerase chain reaction (PCR) is particularly useful in avian sex determination because of its precision, sensitivity, and efficiency. It requires only a drop of blood, a single feather or an eggshell and can be used in juvenile birds.
Jirajaroenrat, K., Thammakarn, C. (2007). Sex identification of some pet birds by Polymerase Chain Reaction-based methods. International Conference on Integration of Science & Technology for Sustainable Development, Bangkok, Thailand. 26-27 April, 376-379.
Vucicevic, M., Stevanov-Pavlovic, M., Stevanovic, J., Bosnjak, J., Gajic, B., Aleksic, N., and Stanimirovic, Z. (2013). Sex determination in 58 bird species and evaluation of CHD gene as a universal molecular marker in bird sexing. Zoo Biol. 32, 269–276.