Mechanistic underpinnings of dehydration stress in the American dog tick revealed through RNA-Seq and metabolomics

Ticks are obligate blood feeders but spend the majority of their lifetime off-host where they must contend with a multitude of environmental stresses. Survival under desiccating conditions is a determinant for habitats where ticks can become established, and water-balance characteristics of ticks have been extensively studied. However, little is known about the underlying aspects associated with dehydration stress in ticks. In this study, we examined the response of male American dog ticks, Dermacentor variabilis, to dehydration using a combined transcriptomics and metabolomics approach. During dehydration, 497 genes were differentially expressed, including an up-regulation of stress-response and protein-catabolism genes and concurrent down-regulation of several energetically expensive biological processes. Accumulation of several metabolites, including specific amino acids, glycerol and gamma aminobutyric acid (GABA), and transcript shifts in the associated pathways for generating these metabolites indicated congruence between changes in the metabolome and gene expression. Ticks treated with exogenous glycerol and GABA demonstrated altered water-balance characteristics; specifically, increased water absorption at high relative humidity. Finally, we observed changes in locomotor activity in response to dehydration, but this change was not influenced by the accumulation of GABA. Overall, the responses to dehydration by these ticks were similar to those observed in other dehydration-tolerant arthropods, but several molecular and behavioral responses are distinct from those associated with other taxa.