Stolen genes give spider mites super-pest abilities
Scientists have uncovered how it is that a tiny pest called the two-spotted spider mite can damage such an enormous variety of plants — it uses genes stolen from bacteria, fungi and even plants.
"They can change the repertoire [of genes] that they're using in order to be able to feed on hosts that they would not be adapted to," said Miodrag Grbic, a University of Western Ontario biologist, who led an international project to sequence the spider mite's genome. The results were published this week in the journal Nature.
Spider mites are eight-legged pests less than a millimetre long that can feed on 1,000 different plants, including 150 of agricultural importance, such as tomatoes, cucumbers, peppers, maize, soy, apples, pears, strawberries and Clementine oranges.
The fast-breeding pests, which reach adulthood just seven days after hatching, destroy 13 per cent of all potential crops worldwide, causing $1 billion in damage annually, a news release from the University of Western Ontario reported.
"What makes them an especially big problem in agriculture is actually [their] ability to develop resistance to pesticides," Grbic told CBC's Quirks & Quarks in an interview set to air Saturday.
The mites' ability to feed on so many different plants is also highly unusual, he added.
Typically, plants protect themselves against specific pests by producing toxins.
So, in order for pest species to feed on these plants, Grbic said, "they have actually to detoxify and break down these toxic products."
Most pests can only break down a limited range of toxins. Spider mites, on the other hand, are champion detoxifiers. A close look at their genome revealed the source of that unusual ability.
While the mite genome had just 90 million base pairs of DNA — making it 30 times smaller than the human genome — half of those code for genes, instructions for making proteins. That means spider mites can make many more kinds of proteins than the size of their genome might indicate. (In humans, only 1.5 per cent of the genome codes for genes.)
Spider mites genes include a far greater number that deal with the detoxification of plant products compared to the usual number found in insects, Grbic said. They also include genes that one wouldn't expect to find in a spider mite.
"Spider mites not only proliferate known genes families involved in detoxification, but they also grab genes from other organisms, like for example bacteria, fungi and even plants and incorporate that in their genome," Grbic said.
When mites reared on a particular kind of plant, such as beans, and then were transferred to a different kind of plant, such as tomatoes, the researcher found that they turned on a completely different set of genes, including many borrowed from other organisms.
Grbic suggested they may use a similar strategy to become resistant to pesticides.
The researchers hope that by studying the genome of the mite as it interacts with different plants, they will be able to figure out how to isolate toxins that are actually effective against the pests.