Abstract
Salmon lice, Lepeophtheirus salmonis (Krøyer, 1837), are fish ectoparasites causing significant economic damage in the mariculture of Atlantic salmon, Salmo salar Linnaeus, 1758. The control of L. salmonis at fish farms relies to a large extent on treatment with anti-parasitic drugs. A problem related to chemical control is the potential for development of resistance, which in L. salmonis is documented for a number of drug classes including organophosphates, pyrethroids and avermectins. The ATP-binding cassette (ABC) gene superfamily is found in all biota and includes a range of drug efflux transporters that can confer drug resistance to cancers and pathogens. Furthermore, some ABC transporters are recognised to be involved in conferral of insecticide resistance. While a number of studies have investigated ABC transporters in L. salmonis, no systematic analysis of the ABC gene family exists for this species. This study presents a genome-wide survey of ABC genes in L. salmonis for which, ABC superfamily members were identified through homology searching of the L. salmonis genome. In addition, ABC proteins were identified in a reference transcriptome of the parasite generated by high-throughput RNA sequencing (RNA-seq) of a multi-stage RNA library. Searches of both genome and transcriptome allowed the identification of a total of 33 genes / transcripts coding for ABC proteins, of which 3 were represented only in the genome and 4 only in the transcriptome. Eighteen sequences were assigned to ABC subfamilies known to contain drug transporters, i.e. subfamilies B (4 sequences), C (11) and G (2). The results suggest that the ABC gene family of L. salmonis possesses fewer members than recorded for other arthropods. The present survey of the L. salmonis ABC gene superfamily will provide the basis for further research into potential roles of ABC transporters in the toxicity of salmon delousing agents and as potential mechanisms of drug resistance.
Highlights
The large gene family of ATP-binding cassette (ABC) proteins has members in all biota
Gene Ontology (GO) annotations were assigned to the assembled L. salmonis transcripts/genes on the basis of refseq_protein annotations
The annotation of the ABC superfamily in L. salmonis represents a significant step towards an improved understanding of potential drug resistance factors in this species and related parasites
Summary
The large gene family of ATP-binding cassette (ABC) proteins has members in all biota. Typical ABC proteins possess transmembrane (TMD) and conserved nucleotide binding domains (NBD) and function as primary transporters in trafficking processes across biological membranes [1]. To form a functional transporter, two TMDs and two NBDs are required, which in full transporters are combined in a single polypeptide. Half-transporters consist of one TMD and one NBD and need to form homo- or heterodimers for transporter function. ABC proteins functioning as drug transporters can contribute to chemical resistance phenotypes in cancers, pathogens and pests. Multidrug resistance (MDR) is defined as the reduced susceptibility of cancer cells to structurally and functionally unrelated cytostatic drugs and can result from the enhanced expression of ABC efflux transporters, which reduces cellular drug accumulation [2]. ABC transporters have been linked to drug resistance in parasitic nematodes [3] and to pesticide resistance in insects and other arthropods [4]
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