A genomic approach to investigate resistance mechanisms in the two-spotted spider mite Tetranychus urticae

The two-spotted spider mite (Tetranychus urticae) is a global and important crop pest that is mainly controlled by acaricides. Its rapid development, high fecundity and haplo-diploid reproduction all contribute to the fast development of resistance. Resistance to acaricides can arise due to modification of the target site or by an increased detoxification. Recently, the complete genome of T. urticae was sequenced and used in this work to develop innovative techniques to study resistance in spider mites.
First, using high-throughput sequencing, we developed a bulk segregant mapping method which allowed us to map resistance genes in high resolution. We uncovered that the resistance locus for the mite growth inhibitors etoxazole, hexythiazox and clofentezine maps to a small genomic region coinciding with the chitin synthase-1 (CHS-1) gene. Based on additional genetic and biochemical investigations, we reveal that a point mutation in CHS-1 confers strong resistance, hereby also elucidating the mode of action of an important class of acaricides.
Second, we used a gene expression microarray to compare gene expression levels of susceptible and resistant mites on a genome-wide basis. We investigated resistance to the tetronic acid derivate spirodiclofen, a recently commercialized acaricide. Gene expression data of genetically distinct resistant strains revealed that the expression of a number of detoxification genes was greatly increased. Functional expression of the P450 CYP392E10 confirmed that it hydroxylates spirodiclofen to a non-toxic compound. We further identified and functionally expressed an esterase, CCE04, of which a specific allele is highly overexpressed in resistant strains.