<i>Tetranychus urticae</i> mites do not mount an induced immune response against bacteria

Abstract

The genome of the spider mite <i>Tetranychus urticae</i>, a herbivore, is missing important elements of the canonical <i>Drosophila</i> immune pathways necessary to fight bacterial infections. However, it is not known whether spider mites can mount an immune response and survive bacterial infection. In other chelicerates, bacterial infection elicits a response mediated by immune effectors leading to the survival of infected organisms. In <i>T. urticae</i>, infection by either <i>Escherichia coli</i> or <i>Bacillus megaterium</i> did not elicit a response as assessed through genome-wide transcriptomic analysis. In line with this, spider mites died within days even upon injection with low doses of bacteria that are non-pathogenic to <i>Drosophila</i> Moreover, bacterial populations grew exponentially inside the infected spider mites. By contrast, <i>Sancassania berlesei</i>, a litter-dwelling mite, controlled bacterial proliferation and resisted infections with both Gram-negative and Gram-positive bacteria lethal to <i>T. urticae</i> This differential mortality between mite species was absent when mites were infected with heat-killed bacteria. Also, we found that spider mites harbour in their gut 1000-fold less bacteria than <i>S. berlesei</i> We show that <i>T. urticae</i> has lost the capacity to mount an induced immune response against bacteria, in contrast to other mites and chelicerates but similarly to the phloem feeding aphid <i>Acyrthosiphon pisum</i> Hence, our results reinforce the putative evolutionary link between ecological conditions regarding exposure to bacteria and the architecture of the immune response.