May 2022 International SAT Reading Test (Q 21-30)

In the 1300s Spanish explorers discovered a small
bird living in the islands off the Western coasts of
Portugal and Morocco, with dull green feathers but a
sweet lyrical voice. The bird became fashionable in
⁵ the courts of Spain and England, and people started
breeding it, gradually changing its plumage to a wide
variety of colors, from dark black to bright yellow.
It’s the latter that the bird is most associated with. It
is, of course, the canary.

¹⁰ Throughout those centuries of breeding
canaries, one color remained elusive—red. The birds
traversed the rainbow, but no hint of red had ever
shown up in their feathers. So in the 1920s, German
breeders decided to cross canaries with a closely
¹⁵ related species—the red siskin of Venezuela. They
then mated the hybrids with more canaries, selecting
offspring with red feathers, but as few other siskin
traits as possible. The result, after many generations,
was the “red-factor canary”—a bird that looks exactly
²⁰ like a typical yellow canary, but with bright red
plumes.
“The canary thus became the first animal that was
purposely genetically modified by moving the genes
from another species into it,” says Geoffrey Hill from
²⁵ Auburn University.

Decades later, his team, including Joseph Corbo
from Washington-University School of Medicine and
Miguel Carneiro from the University of Porto, have
finally learned which gene the breeders moved
³⁰ across.
By sequencing and comparing the genomes of
red siskins, red factor canaries, yellow canaries,
and wild green canaries, the team identified over
15.000 genetic variants that are associated with red
³⁵ color, most of which are confined to two small
stretches of DNA. One of these regions contains a
gene called CYP2J19, which is switched on in the
birds’ skin and liver, and is a thousand times more
active in the red canaries than the yellow ones. That’s
⁴⁰ an important clue.

Red siskins and red factor canaries get their
distinctive colors by converting yellow chemicals
(carotenoids) in their food into red chemicals
(ketocarotenoids) in their feathers. This
⁴⁵ transformation is carried out by an enzyme called a
ketolase, and it takes place mostly in the skin and
liver. Given how active CYP2J19 is in these organs,
it's almost certainly the gene that makes the ketolase.
It's what paints the birds red.
⁵⁰ If that wasn’t evidence enoi^h, another team led
by Nicholas Mundy at the University of Cambridge
independently homed in on the same gene after
studying zebra finches. These birds normally have
red beaks, but some have yellow ones. As Mundy’s
⁵⁵ team discovered, that's likely because of mutations in
CYP2J19 that stop the gene from activating in the
beak.
Natasha Bloch from University College London
notes that neither team actually disabled the CYP2fl9
⁶⁰ gene to see if that de-reddened the birds. But they
"have done all the proper experiments short of that,
and left me with no doubt that they have found the
proper gene responsible for the switch to red color,”
she says.
⁶⁵ “Is the CYP2J19 gene responsible for all
ketocarotenoid-based red coloration across birds,
from flamingos to extinct pink-headed ducks?” asks
Mary Caswell Stoddard from Harvard University.
“Are parrots, which lack carotenoids in their feathers
⁷⁰ and instead use a unique pigmentation system to
make red, missing the CYP2J19 gene or is it merely
turned off ? These are the exciting questions we can
now begin to explore.”

Other genes are probably involved too Hill's team
⁷⁵ found that the red color of siskins and canaries also
depends on a cluster of genes involved in skin
development. And CYP2J19 isn’t just a gene for
making red feathers either. It's switched on in the
retina of many other birds, where it produces a red
⁸⁰ pigment that helps the animals to distinguish between different colors.

So nearly all birds see red, and nearly all could be
red. They have the basis of red feathers and beaks
right there in their eyes, waiting to be repurposed.
⁸⁵ And yet, only a few species have done so. Why?
The answer probably involves mating. Red colors
are a potent signal in the bird world. Their vibrancy
is a reliable indicator of health, and females are most
attracted to the reddest males. Bloch, who studies
⁹⁰ the evolution of color, says.“ Finding the genes
responsible for color brings us one step closer to
understanding how male traits and female
preferences co-evolve and whether they share a
common genetic basis.”