Big pest, small genome: Blueprint of spider mite may yield better pesticides
Nov 23, Biology/Biotechnology
Full size image Scanning electron microscope image of a two-spotted spider mite, which is less than one millimeter long. Richard Clark of the University of Utah helped lead an international team of 55 scientists who deciphered the genome of the two-spotted spider mite in a paper published in the Nov. 24 issue of Nature. Credit: Nature, Vladimir Zhurov, University of Western Ontario (PhysOrg.com) -- An international research team decoded the genetic blueprint of the two-spotted spider mite, raising hope for new ways to attack the major pest, which resists pesticides and destroys crops and ornamental plants worldwide.
The voracious mites, which technically are not insects, can eat more than 1,100 plant species  a rare trait. The mites' newly revealed and sequenced genome contains a variety of genes capable of detoxifying pesticides as well as toxins plants use to defend themselves, the scientists report in the Thursday, Nov. 24 issue of the journal Nature.
"One key thing that makes spider mites unique is they can eat many, many different plant species," says Richard M. Clark, one of five main authors of the study and an assistant professor of biology at the University of Utah. "These mites are often house plant pests  a major cause of people's house plants turning yellow and getting sick. They also are a major problem for agricultural nurseries and greenhouses, and for field crops."
Primary targets are tomatoes, peppers, cucumbers, strawberries, corn, soybeans, apples, grapes and citrus.
Clark says the new study's "importance is largely in understanding how animals eat plants, with the long-term goal of developing effective ways to prevent crop damage from mites and insects. If we can identify the biological pathways mites use to feed on plants, we can potentially identify chemical and biological methods to disrupt those pathways and stop the mites from feeding."
The two-spotted spider mite, which is no more than 1 millimeter long, "is a major global pest, and is predicted to be a growing concern in a warming climate because they multiply extremely fast at high temperatures  90 degrees Fahrenheit or more," he adds. "They do really well in hot and dry climates like Utah."
Yet, the two-spotted spider mite "has been found to rapidly develop resistance to multiple types of pesticides, often within a couple of years after a pesticide is introduced," says Clark. "It is resistant to many common pesticides used against insects."
The Nature study deciphering the genome of Tetranychus urticae, the two-spotted spider mite (which has two red spots), was conducted by an international research team of 55 scientists from North America, Europe and South America.
A normal leaf (left) compared with a leaf (right) attacked by two-spotted spider mites, a major crop and houseplant pest worldwide. The pests newly decoded genetic blueprint reveals an ensemble of genes that allow it to disarm plants' chemical defenses and detoxify pesticides - major reasons the mites can eat more than 1,100 plant species. Credit: Richard Clark, University of Utah
This microscope image shows a spider mite feeding on a leaf while surrounded by several of its eggs. Credit: Joshua Steffen, University of Utah
Uses a different molting hormone to shed its exoskeleton during growth.
Has only eight Hox genes to orchestrate body-plan development, compared with 10 in most other arthropods, and thus has only two main body segments instead of three. There are other cases in which Hox genes were activated differently in different arthropods, "but this is an extreme case," Clark says. "The genes are both gone.
Makes silk that is strong like spider silk but 185 to 435 times thinner. "Spiders spin silk from their abdomens, spider mites from the head region," Clark says. Spider mites use silk to hide from predators, keep themselves warm, and suspend eggs out of predators' reach. Silk from the mites may prove useful as biodegradable surgical sutures and bandages. It "is very thin and very easy to get because you can grow lots of mites on plants," Clark says.
Provided by University of Utah
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A spider mite, roughly 0.5mm long 

A team of 55 researchers led by University of Western Ontario biologist Miodrag Grbic has sequenced the genome of the spider mite '''', a pest that costs over  1 billion to control annually. Their results were published in the journal '''' yesterday.  The genome is the smallest arthropod genome sequenced so far.

The study was funded by the US  Joint Genome Institute programme, Genome Canada, and the European Union.

The pest is resistant to major plant toxins, and it takes a spider mite from two to four years to become resistant to new pesticides. University of Utah assistant professor of biology Richard M. Clark, a study co-author, said, "One key thing that makes spider mites unique is they can eat many, many different plant species... These mites are ... a major cause of people's house plants turning yellow and getting sick. They also are a major problem for agricultural nurseries and greenhouses, and for field crops. ... the spider mite has been found to rapidly develop resistance to multiple types of pesticides, often within a couple of years after a pesticide is introduced."

The study found the mite's genome includes a variety of genes responsible for digestion and toxin degradation, possibly including some genes from bacteria and fungi. Because these bacterial/fungal genes are peculiar to spider mites, they appear to be a rare example of  — gene transfer between distant species.

The study also found 17 web-production genes. The spider mite's webs provide weather- and predator-protection and are somewhat stronger than webs of other species.