Using comparative genomics to decode the genetics of acaricide resistance

The availability of genome assemblies and other genomic resources is facilitating investigations of complex genetic traits for several species of ticks. Understanding the genetics of acaricide resistance is a priority for tick and tick-borne disease control. The synaptic enzyme acetylcholinesterase (ACE) is recognized as the target of organophosphates (OPs) and carbamates, and mutations in ACE have been tied to resistance. Multiple studies support three ACE (ace) loci in R. microplus but the molecular basis of OP-resistance in this tick remains elusive. Here, we exploited the genome assembly of the black-legged tick Ixodes scapularis and comparative genomic analyses to explore the complement of tick ACEs and their potential roles in OP resistance. We identified eight putative ace loci (IscaACE1a, 1b, 2a-c, 3a-c) in I. scapularis. Molecular analyses and homology modeling suggest ACE activity for IscaACE1a. Our analyses reveal the molecular complexity of the I. scapularis ace gene family, highlight the need for functional studies of ACEs in species of the Ixodidae, and reveal potential challenges to management of OP resistance in ticks.