How parrots and finches evolved red and yellow colours

Birds rank among the most vividly coloured animals in the natural world. Yet how did such a striking range of colours emerge across different bird species?

Almost every bird with brilliant red, orange or yellow feathers or bills relies on a family of pigments known as carotenoids for its colouring. Birds cannot produce carotenoids themselves, however, and instead obtain them through their food, ultimately from the plants they consume.

Parrots are unusual in this respect. They evolved a wholly separate method of creating colourful pigments, known as psittacofulvins.

Scientists have long been aware of these distinct pigments, but the biochemical and genetic mechanisms that allow birds to vary in colour have been harder to establish. Two recent independent studies, one involving parrots and the other finches, have now provided important clues.

One investigation, published in Current Biology, was led by one of us (Daniel Hooper). The other, led by Portuguese biologist Roberto Abore, appeared in Science. Collectively, the studies improve our understanding of how birds create colourful displays and how these characteristics evolved.

A single enzyme in parrots

Both new studies brought together large international research teams. Using recent developments in genetic sequencing, they investigated which parts of the genome - an animal's complete collection of DNA - control natural variation from yellow to red in parrots and finches.

Strikingly, although the two bird groups use different pigment types to create their colourful appearances, they seem to have evolved through similar processes.

Abore's research focused on the dusky lory (Pseudeos fuscata), a New Guinea parrot whose feather bands can be yellow, orange or red.

The researchers discovered that changes between yellow and red feathers were linked to an enzyme named ALDH3A2. This enzyme transforms red parrot pigments into yellow pigments.

Feathers developing with high levels of this enzyme become yellow, whereas those with lower levels become red.

The team also found that the ALDH3A2 enzyme accounts for colour differences in numerous other parrot species that independently evolved yellow-to-red colour variation.

Two special genes in finches

The long-tailed finch (Poephila acuticauda) is a songbird found in northern Australia. It has two interbreeding subspecies distinguished by bill colour: one has a yellow bill and the other a red bill.

Most carotenoid pigments available to birds through food are yellow or orange. To create red colours, birds therefore need to alter the pigments' chemistry after they have eaten them.

Hooper's study investigated variation in this feature throughout the wild range of the long-tailed finch, alongside genome variation in the birds measured. It found that bill colour was chiefly associated with two genes: CYP2J19 and TTC39B.

Working together, these genes enable the conversion of yellow carotenoids from the diet into red carotenoids.

In long-tailed finches, yellow colouring seems to be caused by mutations that switch these genes off specifically in the bill, while leaving them active elsewhere in the body, including the eyes.

By comparing the DNA sequences of these colour genes with those of other finch species, the researchers further established that the ancestors of present-day long-tailed finches had red bills. Mutant yellow bills, however, have gradually become more frequent.

Like a dimmer switch

Together, the studies demonstrate how colour can evolve within natural populations.

For both parrots and finches, the mutations producing yellow-to-red colour differences did not alter what the relevant enzymes do. Rather, they affected the places and times at which the enzymes were active.

It is rather like adjusting a room's lighting by fitting a dimmer to an existing switch, instead of taking out the entire light fitting.

The researchers also demonstrated that, in wild parrot and finch populations, mutations in only a small number of genes can drastically change the chemical structure of pigments - sufficiently to determine whether a bird is red or yellow.

The crucial genes alter the pigment molecule's chemical structure through an enzyme that adds only one oxygen atom. In parrots, this changes the pigment from vivid red to bright yellow; in finches, the process works in reverse, converting bright yellow into vivid red.

The wonder of nature

Bird colour evolution has attracted attention since Charles Darwin drew on birds when setting out his theory of evolution by natural selection. Among the most apparent distinctions between closely related bird species around us is their colouring.

These two studies reveal how a handful of genes, along with the addition of a single oxygen atom, can redirect evolution and produce a new form with a dramatically different appearance.

If this gives an animal an evolutionary advantage - perhaps by making it more appealing to potential mates or more conspicuous - it may result in the emergence of a new species.

This research is a reminder of nature's wonder and of the fact that evolution continues.

To conserve species, we must safeguard as much of their genetic complexity as possible. Each individual in a population has a unique genome, and every small variation represents the outcome of millions of years of past evolution. It may also hold the key to a new species developing in the future.

Simon Griffith, Professor of Avian Behavioural Ecology, Macquarie University and Daniel Hooper, Postdoctoral Scholar, Bioinformatics and Computational Biology, American Museum of Natural History

This article is republished from The Conversation under a Creative Commons licence. Read the original article.