Aphids have evolved a unique ability to create their own carotene, a pigment thought to be made only by plants, by stealing the relevant genes.

Researchers at the University of Arizona conducted a genetic study of the pea aphid and found the insects make their own carotenoids, nutrients that give plants a red-orange colour.

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Pea aphids have both red and the green forms. Researchers have found the insects have the ability to make their own red-orange pigments, called carotenoids, making them unique among animals. ((Charles Hedgcock R.B.P.))

Beta-carotene, for example, is the pigment that makes carrots orange and is a building block for Vitamin A.

"The yellow colour in egg yolks, the pink in shrimp and salmon, the pink in flamingos, tomatoes, carrots, peppers, Mexican poppies, marigolds — the yellow, orange, and red are all carotenoids," Nancy Moran of the University of Arizona, said in a statement.

Carotenoids are important nutrients for immune systems, vision, skin and bone growth. They are produced naturally in plants, as well as some photosynthetic algae, fungi and bacteria.

All animals get their carotenoids from their food, except for these aphids, which have found a way to make their own.

Specifically, they found the gene for making carotenoids in the genome of another organism.

"What happened is a fungal gene got into an aphid and was copied," Moran said.

While it's common for bacteria to acquire working genes from other bacteria, this is the first time such as transfer has been seen to occur between a fungus and an animal.

The researchers say an ancestor of the aphid must have gained the genes from a fungus millions of years ago. The researchers conducted experiments to eradicate the bacteria in the aphids' guts to eliminate the possibility the bacteria were creating the nutrients.

Moran said the source of the gene might have been a fungus closely associated with insects, such as one that causes disease.

"By recognizing the horizontal transfer of nutritionally important carotenoid genes, Nancy Moran and her colleagues are the first to discover that gene transfer can occur between very distantly related groups of higher, multi-cellular organisms such as fungi and insects," said Matt Kane of the National Science Foundation.

The research was published this week in the journal Science.