Virus genes help determine if pea aphids have their wings

Many traits of an organism are influenced by signals from one's environment. These features are called phenotypically plastic characteristics and are important to enable an organism to cope with unpredictable environments.

But what are the genetic mechanisms underlying these traits?

Jennifer Brisson, an associate professor of biology at the University of Rochester, and her former postdoctoral fellow, Benjamin Parker, today an assistant professor of microbiology at the University of Tennessee, have been studying the phenotypically plastic traits of Pea aphids and found, for the first time, genes that influence, that aphids produce wingless or wingless offspring in response to their environment. In a new article in the newspaper Current biology, the researchers shed light on the evolution of phenotypically plastic traits and address crucial issues related to the evolution of environmentally sensitive traits.

Pea aphids are fast-growing insects that usually give birth to wingless offspring. As many gardeners know, aphids can quickly submerge and kill host plants on which they live and feed. When an environment becomes overcrowded with other aphids, females begin to produce offspring that have wings, rather than typical offspring without wings. The winged offspring can then fly and colonize new, less populated plants.

"Aphids have been doing this for millions of years," says Brisson. "But some aphids are more susceptible to crowding than others." Understanding why is the key to understanding how this classic example of phenotypic plasticity works. "

Researchers used evolutionary genetics and molecular biology techniques to identify genes that determine the degree of aphid response to overpopulation. Surprisingly, the genes they discovered come from a virus that has been incorporated into the genome of the aphid. The virus, which belongs to a group of insect viruses called densoviruses, forces its host to produce offspring with wings. The researchers believe that the virus acts in this way to facilitate its own dispersion. As Brisson and Parker have discovered, the virus gene has retained the same function of producing winged offspring even after its transfer and incorporation into the aphid genome.

"It's a new role for viral genes co-opted by the genome for other purposes, such as modulating plastic phenotypes," Parker said. "Microbial genes can be incorporated into animal genomes and this process is important for evolution."

Most of the DNA laterally transferred – the DNA inherited from other organisms, such as viruses – is not expressed by its hosts because it is rapidly inactivated or eliminated. However, there are examples in most organisms, even humans, where genomes coopt genes laterally; in humans, for example, the gene that creates a membrane between the placenta and the fetus was co-opted from a retrovirus.

Brisson and Parker have found a clear case in which genes outside an organism have been co-opted by the genome of the body to alter the strength of a plastic response to environmental cues. Microbial genes, like those of viruses, can therefore play an important role in the evolution of insects and animals, explains Brisson. "Even in ancient traits like the one studied here, new genes can begin to play a role in the formation of plastic traits and can help organisms cope with an unpredictable world."