Female preference for red plumage drives hybridization in tinkerbirds

But does this red plumage also honestly signal male quality?

Despite four million years of divergence, Red-fronted Tinkerbird (Pogoniulus pusillus) and Yellow-fronted Tinkerbird still hybridize. Why has reproductive isolation not solidified between these two African species? The answer might be connected with female choice. In a 2019 study, Emmanuel Nwankwo and his colleagues documented extensive introgression between Red-fronted and Yellow-fronted Tinkerbird. Their analyses revealed that hybridization was asymmetrical with a high proportion of individuals with Yellow-fronted genotypes that sported a red forecrown. This finding suggests that the red plumage trait is spreading in the Yellow-fronted Tinkerbirds, possibly because females prefer males with a red forecrown.

However, for a trait to be involved in mate choice, it needs to send out an honest signal. This means that females should be able to judge the quality of a male by that trait. Red plumage tends to be the product of carotenoids which individuals acquire in their diet. Some species use carotenoids unchanged from dietary components while others biochemically convert them before deposition in feathers. A large-scale analysis on feather coloration suggested that converted carotenoids better reflect individual quality because of the physiological links between cellular function and carotenoid metabolism. So, understanding the genetic basis of the red forecrown in tinkerbirds can show if this trait is an honest signal.

Red-fronted and Yellow-fronted Tinkerbirds differ in the color of their forecrown (figures c and d). The researchers collected samples across a hybrid zone in South Africa (figure e). From: Kirschel et al. (2020) Molecular Ecology.

Candidate Gene

In a recent Molecular Ecology paper, Alexander Kirschel and his colleagues analyzed the genetic make-up of 85 individuals that were collected across a hybrid zone in South Africa. A genome-wide association study (GWAS) pointed to a significant connection between the red forecrown and a genomic region on chromosome 8. Close inspection of this region revealed the gene CYP2J19 which is known to be involved in red coloration in other bird species, such as canaries and zebra finches. In the tinkerbirds, the different variants of this gene nicely explained the color of the forecrown. All individuals with two copies of the yellow-variant had a yellow forecrown and all individuals with two copies of the red-variant showed a red forecrown. Moreover, birds with a yellow-variant and a red-variant had either red, reddish or orange crowns.

But what about the function of CYP2J19? This gene codes for a ketolase, an enzyme that convert dietary yellow carotenoids into red ketocarotenoids. As I explained above, converted carotenoids might reflect individual quality better and can thus be used as an honest signal in mate choice. However, the exact mechanism underlying the workings of CYP2J19 remain to be unraveled. The GWAS uncovered other genomic regions that might contain the regulatory switches that control the expression of CYP2J19 (perhaps similar to the situation in Hooded Crows). More research is thus needed to understand how evolution is tinkering with the tinkerbirds.

The GWAS uncovered a genomic region on chromosome 8 (indicated with an arrow) which contains the gene CYP2J19. From: Kirschel et al. (2020) Molecular Ecology.

References

Kirschel, A. N., Nwankwo, E. C., Pierce, D. K., Lukhele, S. M., Moysi, M., Ogolowa, B. O., Hayes, S. C., Monadjem, A. & Brelsford, A. (2020). CYP2J19 mediates carotenoid colour introgression across a natural avian hybrid zone. Molecular Ecology29(24), 4970-4984.

Featured image: Red-fronted Tinkerbird (Pogoniulus pusillus) © Alandmanson | Wikimedia Commons

This paper has been added to the Piciformes page.