Effect of Selection for Low and High Varroa destructor Population Growth Rates on the Honey Bee Transcriptome

Varroa destructor is a major health problem for honey bees (Apis mellifera). Selective breeding of Varroa-resistant bees is a suitable long-term solution to Varroa parasitism. After three generations of selecting honey bees for lower (resistant) and higher (susceptible) V. destructor population growth (LVG and HVG, respectively), LVG bees showed increased behavioral, cellular, and humoral immunity against Varroa. To further analyze resistance, the transcriptomes of both bee genotypes were examined, revealing that parasitized LVG bees had fewer differentially expressed genes (DEGs) than parasitized HVG bees, indicating a reduced impact by Varroa with greater resistance. Annotations of the altered DEGs showed that both genotypes were affected with an increased demand for energy, protein, and repair during parasitism. However, there were also DEGs in LVG bees, possibly related to resistance, such as up-regulation of odorant binding protein genes and down-regulation of the corazonin receptor gene, whereas DEGs in the HVG bees may be more related to stress, such as up-regulation of ATP synthase and down-regulation of the transcription factor dorsal. Overall, this work shows that selection for LVG and HVG bees resulted in genotypes with widespread differences in gene expression during Varroa parasitism, which may be related to resistance and susceptibility.