Host adaptation and specialization in Tetranychidae mites
Composite generalist herbivores are comprised of host-adapted populations that retain the ability to shift hosts. The degree and overlap of mechanisms used by host-adapted generalist and specialist herbivores to overcome the same host plant defenses are largely unknown. Tetranychidae mites are exceptionally suited to address the relationship between host adaptation and specialization in herbivores as this group harbors closely related species with remarkably different host ranges-an extreme generalist the two-spotted spider mite (Tetranychus urticae Koch [Tu]) and the Solanaceous specialist Tetranychus evansi (Te). Here, we used tomato-adapted two-spotted spider mite (Tu-A) and Te populations to compare mechanisms underlying their host adaptation and specialization. We show that both mites attenuate induced tomato defenses, including protease inhibitors (PIs) that target mite cathepsin L digestive proteases. While Te solely relies on transcriptional attenuation of PI induction, Tu and Tu-A have elevated constitutive activity of cathepsin L proteases, making them less susceptible to plant anti-digestive proteins. Tu-A and Te also rely on detoxification of tomato constitutive defenses. Te uses esterase and P450 activities, while Tu-A depends on the activity of all major detoxification enzymatic classes to disarm tomato defensive compounds to a lesser extent. Thus, even though both Tu-A and Te use similar mechanisms to counteract tomato defenses, Te can better cope with them. This finding is congruent with the ecological and evolutionary times required to establish mite adaptation and specialization states, respectively.
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