Virus controls wing growth in aphids

The periodic development of wings among a normally wingless species of aphid is caused by a virus and its evolutionary drive to extend its own range.

Researchers led by Jennifer Brisson from the UK’s University of Rochester set out to determine the genetic mechanism behind a peculiar trait found in pea aphids (Acyrthosiphon pisum) and all too familiar to gardeners and farmers alike.

The insects feed by sucking sap from legumes, including valuable crops such as peas, alfalfa, broad beans and clover. The aphids are normally wingless, but as populations on any plant grow to crowded proportions, some of the females start to produce winged young – which then fly off to colonise new areas. 

Once the winged insects land on fresh plants, their own offspring revert to the standard wingless model. In the jargon of evolutionary biology, this ability to swap features in and out over generations is known as “phenotypic plasticity”.

Brisson and colleagues decided to discover just which genes were responsible for the trick.

“Aphids have been doing this trick for millions of years,” Brisson says.

“But some aphids are more sensitive to crowding than others. Figuring out why is key to understanding how this textbook example of phenotypic plasticity works.”

To do this, the researchers trawled through the aphid genome using techniques from genetics and molecular biology. What they found came as a surprise. 

The genes that form the on-off switch for wings belong to a virus, a member of the insect-specific group known as densoviruses, the genome of which has become incorporated wholly into that of that of the aphid.

The researchers suggest that the virus – resident in the aphid genome across deep time – induces wing development in order to spread itself around.

“Microbial genes can become incorporated into animal genomes, and this process is important to evolution,” explains co-author Benjamin Parker. Despite this, he describes the aphid example as “a novel role for viral genes”.

Most virus genes that end up being incorporated into animal genomes – including some examples in humans – end up inert, effectively only adding bulk.

In a few cases, however, the process leads to the addition of useful functionality, in which case having the acquired genes remain active represents an example of positive selection.

“Even in ancient traits like the one studied here, new genes can start to play a role in shaping plastic traits and can help organisms cope with an unpredictable world,” says Brisson.

The research is published in the journal Current Biology.

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