Maternal inheritance of deltamethrin resistance in the salmon louse Lepeophtheirus salmonis (Krøyer) is associated with unique mtDNA haplotypes

Citation

Carmona-Antoñanzas G, Bekaert M, Humble JL, Boyd S, Roy W, Bassett DI, Houston RD, Gharbi K, Bron JE & Sturm A (2017) Maternal inheritance of deltamethrin resistance in the salmon louse Lepeophtheirus salmonis (Krøyer) is associated with unique mtDNA haplotypes. PLoS ONE, 12 (7), Art. No.: e0180625. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0180625; https://doi.org/10.1371/journal.pone.0180625

Abstract
Parasitic infections by the salmon louse, Lepeophtheirus salmonis (Krøyer), cause huge economic damage in salmon farming in the northern hemisphere, with combined treatment costs and production losses in 2014 having been estimated at US$ 350 million for Norway (annual production 1.25 million tonnes). The control of L. salmonis relies significantly on medicinal treatments, supplemented by non-pharmacological approaches. However, efficacy losses have been reported for several delousing agents, including the pyrethroid deltamethrin. The aim of the present study was to analyse the genetic basis of deltamethrin resistance in L. salmonis. Deltamethrin median effective concentrations (EC50) were 0.28 μg L-1 in the drug susceptible L. salmonis strain IoA-00 and 40.1 μg L-1 in the pyrethroid resistant strain IoA-02. IoA-00 and IoA-02 were crossed to produce families spanning one parental and three filial generations (P0, F1-F3). In three families derived from P0 crosses between an IoA-00 sire and an IoA-02 dam, 98.8% of F2 parasites (n = 173) were resistant, i.e. remained unaffected after exposure to 2.0 μg L-1 deltamethrin. F3 parasites from these crosses showed a deltamethrin EC50 of 9.66 μg L-1. In two families of the inverse orientation at P0 (IoA-02 sire x IoA-00 dam), 16.7% of F2 parasites were resistant (n = 84), while the deltamethrin EC50 in F3 animals was 0.26 μg L-1. The results revealed a predominantly maternal inheritance of deltamethrin resistance. The 15,947-nt mitochondrial genome was sequenced and compared among six unrelated L. salmonis strains and parasites sampled from wild salmon in 2010. IoA-02 and three further deltamethrin resistant strains, established from isolates originating from different regions of Scotland, showed almost identical mitochondrial haplotypes. In contrast, the mitochondrial genome was variable among susceptible strains and L. salmonis from wild hosts. Deltamethrin caused toxicity and depletion of whole body ATP levels in IoA-00 but not IoA-02 parasites. The maternal inheritance of deltamethrin resistance and its association with mitochondrial haplotypes suggests that pyrethroid toxicity in L. salmonis may involve molecular targets encoded by mitochondrial genes.

Journal
PLoS ONE: Volume 12, Issue 7

People

Dr David Bassett

Senior Animal Technician, Machrihanish

Professor James Bron

Professor, Institute of Aquaculture

Dr Armin Sturm

Senior Lecturer, Institute of Aquaculture