Tomato Defence Suppression is a Common Trait in Tetranychus urticae Collected from Field-Grown Tomatoes

Herbivores have evolved several strategies to cope with plant defences. Some herbivores are able to resist toxic compounds, while others can manipulate host immunity. Previously it was found that spider mites collected from non-solanaceous wild host plants consist mostly of individuals that induce tomato defences, but also at low frequencies of individuals that can suppress these defences. Also, it has been shown that mites collected from non-solanaceous plants can adapt to tomato by evolving traits that allow them to suppress defences. Hence, we wanted to know if this trait is also common among mites naturally occurring on tomatoes. Here, we compared three T. urticae populations sampled from tomato at three field sites, and also an outbred population created from them through controlled crosses. We then assessed their fecundity on wild-type (WT) tomato plants and def-1 mutants impaired in inducible JA-defences. We also assessed the magnitude of JA defences they induce in WT plants to discriminate between resistant types (high induction and high performance) and those that can suppress defences (low induction, high performance). We then aligned these data with data on variation in mitochondrial DNA cytochrome oxidase I (COI) and effector 84 (a suppressor of JA-defences in planta), in order to compare genetic diversity patterns among mite lines that induce or suppress defences. We found that suppression is the dominant phenotype in mite strains collected from field-grown tomatoes and that inducers and suppressors predominantly cluster in distinct effector 84 clades. We speculate that effector allelic diversity may be subject to natural selection when these mites colonize a novel host plant, promoting traits to cope with plant resistance, such as defence suppression, which facilitates adaptation.