Frequent haplotypes of caged Caligus rogercresseyi in the austral south of Chile: The result of a long term serial passage experiment?

Abstract

Caligus rogercresseyi is an ectoparasite that causes enormous economic losses to the Chilean salmon aquaculture industry. It has been proposed that native fish are a reservoir of this parasite and that pesticide treatments to control it have led to pharmacological resistance to the chemotherapeutic substances that are broadly used in the austral south of Chile. During 2012–2013, 686bp in the 18s rRNA gene of C. rogercresseyi were sequenced, collected from Salmo salar cultured in five areas between 41°30′S and 45°30′S in Chile. The results show a total of 24 haplotypes, two of which were highly frequent and present in all studied areas (A=36.7% and B=16.3%), as well as in larvae raised under laboratory conditions (A=44% and B=25%). A and B haplotypes were also present in stored sequences of this parasite in the GenBank database from 2003. Seven less frequent haplotypes were found dispersed in four of the areas studied, and the remaining fifteen haplotypes were only present in low frequencies and only in one area. Analysis of the population structure of C. rogercresseyi indicated that the samples collected are part of a panmictic population but with low genetic flow inside the study area, as following a bottleneck event. Hypothetically, after over a decade of this population parasitizing a caged host with low genetic diversity (S. salar), and after repeated exposure to neurotoxic pesticides, the most frequent haplotypes A and B could be the result of drug selective pressure acting inside the cages in a kind of long term serial passage experiment. Finally, the characterization of the molecular footprints of these most frequent haplotypes makes them good candidates for use in experiments to test their resistance to drug treatments. Statement of relevanceThe use of haplotypes described in this study, especially A and B, in experiments of drug resistance in this parasite is relevant compared to the use of unknown haplotypes. This could be important for determining the kind of drugs used against this parasite as well as in the study of resistance mechanisms to the chemotherapeutic substances used in Chilean aquaculture.