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Recently in our laboratory we have managed to include Turdus philomelos in our species register. In this post we will learn a little more about this bird and the techniques used to determine its sex.
Turdus philomelos, commonly known as the song thrush, is a bird belonging to the Turdidae family of the Passeriform order. The size of this bird is rather small, between 20-22 cm.
It breeds in most of Europe and across the Ukraine and western Russia. However, it is rarely found in the southern areas of Spain, Italy and Greece. It is a species that adapts quite well, since it can nest in parks and city gardens. Its nests which are made of mud are usually located in trees or shrubs. Song thrushes lay between 3 to five eggs per clutch that are of a bluish speckled appearance. Their food is varied, consisting of various plants and insects. Song thrushes are capable of using tools to get food, that is, they can use stones to break the shells of snails.
Like other species of passerine, males and females are difficult to differentiate as they have a similar appearance. Both sexes have a brown back with lower parts in a lighter shade and a yellowish chest. In addition, dark spots can be seen on the neck, chest and belly area.
Is my turd philomelos male or female?
Due to this similarity, the best way to sex song thrushes is to us molecular techniques in the laboratory.
The first step in this process is the extraction of DNA from feathers, blood or eggshell. The polymerase chain reaction (PCR) is then used since it is a fast and reliable method of gene amplification. The results of the PCR are then visualized by agarose gel electrophoresis.
Sexing of Turdus philomelos by DNA
The extraction of DNA consists of collecting genetic material for further study. DNA is a complex molecule that is located in the nucleus of cells. As DNA has such a small size, a large number of cells are required in order to facilitate the amplification of the required gene by PCR. For this reason it is important that the treated feathers are preferably plucked from the bird and not collected from the substrate of the cage, as this increases the number of cells adhered to the calamus. Once the cells have been obtained, they are broken open and the DNA is extracted.
The PCR consists of copying many times a specific region of the genetic material that was obtained from the extraction process, ensuring that a large amount of material is available for visualization. The PCR functions by replicating the system that is used by a live cell during its metabolic processes.
In the case of birds, the area of interest for sex identification is based on the size difference of the CHD genes. These are found in the sex chromosomes of all birds, although ratite birds can not be sexed with these genes.
The sex of a bird can be determined by observing the differences in the morphology of its sex chromosomes. In humans, the differential chromosome is the Y chromosome, men being (XY) and women (XX). In birds however, the distinctive chromosome is the W chromosome, which is found in females, males being (ZZ) and females (ZW). The CHD-W gene is located on the W chromosome, so it is unique to females. On the other hand, the CHD-Z gene is found in the Z chromosome and, therefore, appears in both sexes (Griffiths et al., 1998).
To distinguish between the chromosomes, their size is observed, the Z chromosome being larger than the W chromosome. It should also be borne in mind that the W chromosome carries less genetic information (we refer to information to direct and develop the organism), thereby making this chromosome more susceptible to changes and mutations.
In the final step of the molecular sexing process the PCR products are separated and visualized using agarose gel electrophoresis. This technique consists of separating DNA molecules based on their size and shape. The CHD-W gene and the CHD-Z gene are found in different regions of the gel, thereby indicating the sex of the bird.
● Cortés, O., & Dunner, S. (1998, mayo). sexado de aves mediante técnicas moleculares. Recuperado de https://www.ucm.es/data/cont/docs/345-2013-11-07-sexado-aves.pdf
● Purroy, J., & Purroy, F. J. (2015, 14 mayo). Zorzal común – Turdus philomelos C. L. Brehm, 1831. Recuperado de http://digital.csic.es/bitstream/10261/113687/1/turphi_v1.pdf