How to DNA sex birds.

The males and females of many bird species are difficult to distinguish by their appearance (peacocks are a notable exception). There are many situations where it is useful to know the sex of birds including captive breeding programmes, behavioural studies and even species delimitation in extinct taxa.

DNA sexing provides a simple and quick way to determine which birds are females and which are males. We have been using this technique for some of our bird research projects, including our study of the prion wreck of 2011. For our prion study we want to determine whether there is a gender bias in the birds that were wrecked.

So how does DNA sexing work for birds? By way of background, birds have a different chromosome system to us for determining their sex. In mammals, including us, males have an X and a Y chromosome and females have two X chromosomes. In contrast, birds have a ZW sex-determination system whereby males have two Z chromosomes and females both Z and W chromosomes.

Sex chromosomes in birds and mammals.

Sex chromosomes in birds and mammals. Figure credit: Lara Shepherd

To genetically sex a bird, DNA is first obtained from a blood, feather or tissue sample. We used tongue samples for the prions.

From these DNA samples we made lots of copies of the CHD region, a gene that occurs on both the Z and W chromosomes. Our processing of these gene copies produces a single DNA band for males (because they only have one type of chromosome) and two bands for females (representing the different CHD copies from the Z and W chromosomes).

Prion sex assignment based on the CHD region. Females have two DNA bands – the top band is from the W chromosome and the lower band is from the Z chromosome. Males just have the single Z chromosome band. The lane on the far left with multiple bands contains a size standard  with bands of DNA of known size. Photo credit Lara Shepherd

Prion sex assignment based on the CHD region. Females have two DNA bands – the top band is from the W chromosome and the lower band is from the Z chromosome. Males just have the single Z chromosome band. The lane on the far left with multiple bands contains a size standard with bands of known size. Photo credit Lara Shepherd

DNA sexing is also possible for humans, albeit using a modified method suited to our X/Y chromosome system, and is routinely used in forensics. A recent example is the detection of female DNA on the bombs used in the Boston marathon bombing.

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