Spider mite's secrets revealed

The tiny two-spotted spider mite (Tetranychus urticae) causes much anxiety for farmers, and has been, to date, a scientific mystery. It feeds on over 1,100 species of plants, including 150 greenhouse plants and crops, such as maize, soy, tomatoes and citrus.

The cost of chemically controlling damage caused by the spider mite exceeds USD 1 billion per year. In the latest issue of the journal Nature, a multinational consortium of scientists publish the sequenced genome of the spider mite, revealing how it is capable of such feeding frenzy, as well as other secrets of this tiny pest. These findings open the door to new approaches in pest control and crop protection, by allowing greater insight into the biological interactions between plants and herbivores that feed on them.

Élio Sucena and Sara Magalhães, group leaders at the Instituto Gulbenkian de Ciência (IGC) and the Centre for Environmental Biology, University of Lisbon (Portugal), respectively, are part of the 55-strong team of researchers from 10 countries that were involved in this project. Led by Miodrag Grbic (University of Western Ontario, Canada), this team analysed the genome of the spider mite, sequenced with funds from the US Department of Energy (DOE) Joint Genome Institute (JGI) programme, Genome Canada and the European Union.

The spider mite feeds on an astonishingly large number of plants because it withstands the toxins that plants produce. This in itself is an amazing feat. However, among arthropods (animals with exoskeletons, such as spiders, ticks, crustaceans and insects), the spider mite holds first place in the number of pesticides it is resistant to. Mites become resistant to new pesticides within two to four years, meaning that control of multi-resistant spider mites has become increasingly difficult.

Having the sequence of the spider mite genome has shown light on the genetic basis for its feeding flexibility and pesticide resistance. The secret lies in having, on the one hand, more copies of the genes involved in digesting and degrading plant toxins when compared to insects. On the other, the tiny pest seems to have incorporated genes from bacteria and fungi that are involved in digestion and detoxification.

Indeed, the researchers identified two groups of bacterial and fungi genes that are unique to the spider mite, suggesting that the tiny arthropod is adept at making the most of the innovation of transfer of genes between distant species (called lateral gene transfer – a rare occurrence in nature).

Other groups of genes are shared, with aphids, for example (aphids are insects that also feed on crops). By comparing the aphid genome with that of the spider mite, it seems that the bacterial genes moved first into the insects and from these were taken up by the spider mite.

The name gives it away: spider mites make webs, for protection against predators and as a barrier against bad weather. However, their webs are different to those made by spiders: the genome sequence has revealed 17 genes involved in making web proteins. These proteins make thinner fibres, but seem to be slightly more resistant to mechanical forces than other natural materials.

All these secrets came out of a very small genome – only 90 megabases (the fruit fly genome has 180 megabases; the human genome has 3,000 megabases). It is, indeed, the smallest arthropod genome sequenced so far, and reveals a remarkable evolutionary history: the spider mite has lost many genes that are shared amongst arthropods, but has accumulated species-specific genes, such as those that give it the ability to withstand toxins and pesticides.

The Portuguese scientists were involved in analysing immunity-related genes found in the spider mite genome. The spider mite belongs to the Chelicerata family, the second largest group of terrestrial animals. Chelicerates include spiders, scorpions and ticks. Chelicerates and insects make up the Arthropods. The spider mite is the first chelicerate to have its entire genome sequenced and analysed.