When faced with a challenge, animals have the ability to physically move away. Plants are not so lucky. Their interactions with the soil prevent them from up-rooting and finding a better place. But this does not mean that plants don't fight back. Indeed, plants produce an astonishing diversity of chemicals to ward off invaders, such as the ubiquitous arthropod pests. Many of these naturally produced chemicals are analogous to manufactured pesticides, in that they provide a strong selective pressure for arthropod pests to adapt to. How does this adaptation occur at the molecular level?

Dermauw et al. recently reported on the effect of plant host-switching in the spider mite, Tetranychus urticae. These mites were introduced to tomato plants after being propagated on bean plants. Two strains of mite were already resistant to many pesticides whereas one strain was highly susceptible. For this susceptible strain, the authors measured which genes were induced to be expressed upon shifting the susceptible strain to the new host plant (tomato). Genes that were expressed at a higher level upon introduction to tomato are good candidates for genes that underly the resistance to the toxic chemicals produced by tomato to get rid of the pest. 

Some of the genes induced under the host-shift were those that encode protein families that are know to detoxify nasty plant compounds (e.g., P450 monooxygenases).  But there were also some surprises, the most exciting of which were the ring-splitting-dioxygenases that can degrade complex chemicals with ring structures. The gene(s) encoding dioxygenases were transferred from a microbe to the spider mite, which makes the story even more interesting. Genes originating in a microbe, found their way into a spider mite, and are (probably) at least partially responsible for the adaptability of these mites to new host plant defensive chemicals.

The authors also discovered that, by five generations (80 days) of propagation on tomato plants, the susceptible spider mites were expressing the same genes as resistant spider mites on the same host. Although it wasn't clear from the work presented here whether these expression changes were yet heritable or merely environmentally induced, it was clear that it didn't take long for physiological adaptation to a new plant host. The parallel in pesticide-rich agriculture is astonishing: few generations are needed for these mite arthropods to become resistant to a newly-introduced pesticide. Yikes!