Why mosaic canaries have sexual dichromatism

One feature you see in lots of birds is called sexual dichromatism - the scientific way of saying females are differently coloured to males. Think of mallard ducks, for example, or compare colourful peacocks to brown peahens. There’s a type of canary that has this too - it’s called the mosaic canary, and it’s a crossbreed between other canaries and a bird called a red siskin; and now scientists from the University of Porto in Portugal have found a gene that seems to be regulating the sexual dichromatism. Malgorzata Gazda explained to Phil Sansom…

Malgorzata - We found the gene that regulates the sexual dichromatism, differences in the colouration between males and females in birds.

Phil - That's something that's really common, right? A lot of birds have different coloured males and females.

Malgorzata - Yes. And some of them will have carotenoid colouration, so red, orange, or yellow colouration; and some of them will also have melanin colouration, so more brown and black.

Phil - Now what birds exactly are you looking at here?

Malgorzata - Domestic canaries. But to get them to sexual dichromatism, the breeders crossed the canaries with the red siskin, and then they got the breed we just called mosaic canaries.

Phil - This mosaic canary then, that's a crossbreed with the red siskins. This has sexual dichromatism which the other canaries don't?

Malgorzata - Yes. The wild canaries, they have a very slight sexual dichromatism. But then most of the domestic canaries, usually the male and female look the same.

Phil - What do the mosaic canaries look like then?

Malgorzata - Red or yellow. And then the male will have a lot of carotenoid colouration accumulated in the feathers in the face, in the wing, and in the tail.

Phil - Are they pretty then, these red or orange birds?

Malgorzata - Yes. Really pretty. But all canaries are really pretty!

Phil - How did you go about looking into the genes then?

Malgorzata - We sequenced the genomes of a couple of breeds of canaries. We found a very strongly differentiated region in the genome between regular canaries and mosaic canaries. The region was the same as in the red siskin; that confirmed that this region was introduced from the red siskin. This region encodes three genes, and excitingly BCO2 - beta-carotene oxygenase 2 - is the gene that is involved in the carotenoid metabolism. So this gene, basically it removes pigment from the tissue.

Phil - Oh, I thought you were going to say that they got a gene from the red siskins that made the carotenoids, not one that got rid of them.

Malgorzata - Yes. Exciting that it's the other way around.

Phil - That's strange, right?

Malgorzata - Yeah. A little bit. So actually, the females have to put in a lot of effort to remove the pigment and not deposit it in their feathers.

Phil - How can this gene then be doing different things in the female birds than in the male birds?

Malgorzata - It's probably regulated by the hormones. And then when it's active, the females remove the colouration, and in the males, the gene is not active, so they end up depositing the pigmentation in their feathers.

Phil - Does that tell you anything about how these birds evolved?

Malgorzata - Yes. So having one large effect gene that regulates the trait helps evolution to be quite quick, and they can gain or lose the trait pretty quickly.

Phil - So you're saying the fact that it's all controlled by one thing, means you can switch that one thing off and on really easily?

Malgorzata - Yes. Precisely.