Aphids make their own bright colors

Insects borrowed genes from fungi to make nutritious pigments

Aphids may be the only animals that can get a colorful type of nutrient without having to eat their veggies.

TRICOLOR Pea aphids of all colors make their own carotenoid pigments, the first animals shown to do so. Body color depends on gene variations that create blends of different pigments. Charles Hedgcock, RBP

NATURAL COLOR Red and green forms of the pea aphid occur naturally and mingle in the same populations. The yellow form arose spontaneously out of a mutation in a red lineage. Charles Hedgcock, RBP

The sap-sucking insects manage this unexpected feat thanks to ancestors that incorporated genes from a fungus into their own DNA more than 100 million years ago, says Nancy Moran of the University of Arizona in Tucson. Both pea aphids and peach aphids carry genes that make nutrients called carotenoids, she and Arizona colleague Tyler Jarvik report in the April 30 Science.

“To my knowledge, this is the first report of an animal that can synthesize its own carotenoids,” says evolutionary biologist Takema Fukatsu of the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan.

Carotenoids, a brightly colored group of molecules including beta-carotene and lycopene, are powerful antioxidants and immune-system boosters. Carotenoid pigments can put the reds and yellows into feathers and other show-off tissues for courting displays, and they catch light in the human retina. Yet in essence, “there’s only one way to make carotenoids in all of nature,” Moran says, and animals apparently lost the basic toolkit long ago in their evolutionary history.

Many microorganisms, fungi and plants still have the power, however. Carotenoids put the blush in tomatoes, and the fiery range of yellows, reds and oranges into flowers.

They give pea aphids a range of hues; the critters naturally show up in reds or greens, or even a rare mutant yellow depending on which carotenoids their genes encode, Moran and Jarvik found. And among aphids, color has consequence: Ladybird beetles are more likely to prey on red aphids, but parasitoid wasps are more likely to attack green forms.

An animal taking up a fungus gene could also be a first, according to microbiologist Julie Dunning Hotopp of the Institute for Genome Sciences at the University of Maryland School of Medicine in Baltimore. She has identified dozens of genes that animals have incorporated from bacteria, including a complete set of bacterial genes inside a fruit fly chromosome. Biologists don’t know enough yet to make a fair estimate of the chances of fungus-to-animal transfer, she says, but she can’t think of another case so far.

Ironically, the discovery hasn’t changed much about researchers’ basic view of the pigment’s evolutionary history. The fact that aphids had to pick up a fungal gene in order to make carotenoids “actually reinforces the conventional wisdom that animals can’t manufacture such pigments,” says Geoffrey Hill of Auburn University in Alabama, who studies bird coloration.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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