Transcriptomic responses in the spider mite Tetranychus urticae to bacterial infection

Genome annotation of the chelicerate Tetranychus urticae revealed the absence of many canonical immunity genes. T. urticae either does not mount an immune response or it induces uncharacterized immune pathways. To disentangle these two hypotheses, we performed transcriptomic analysis of mites injected with bacteria vs mites injected with LB-buffer. Two types of bacteria were injected: E. coli and B. megaterium and transcriptomes were sampled 3, 6 and 12 hrs after injection. We found no consistent differential expression after bacterial infection, supporting the hypothesis that spider mites do not mount an immune response. We hypothesize that the apparent absence of inducable immunity pathways in T. urticae is a result of relaxed selective pressure due to ecological factors. Six major comparisons were made, having 4 replicates each. Each replicate consisted of 300 injected mites. After 3, 6 and 12 hours of injection, bacteria-injected mites (cy5-labelled) were directly compared to LB buffer-injected mites (cy3-labelled). Samples of non-injected mites are also included.