Population genetic structure of the salmon louse,Lepeophtheirus salmonis(Krøyer) on wild and farmed salmonids around the Pacific coast of Canada

Abstract

The population genetic structure of the salmon louse (Lepeophtheirus salmonis: Caligidae) is of considerable interest because of indirect evidence suggesting transmission of this ectoparasite between wild salmon and farmed salmon (reviewed by Boxaspen 2006). Consequently several studies have looked for farm-speci¢c molecular markers that would allow the path of transmission to and from wild ¢sh to be traced. Unfortunately, an early report of RAPD markers that were speci¢c to Scottish salmon farms (Todd,Walker,Woli, Northcott,Walker, Ritchie, Hoskins, Abbott & Hazon 1997) was not found in a second RAPD study (Dixon, Shinn & Sommerville 2004) or in a more extensive study using six microsatellite markers (Todd, Walker, Ritchie, Graves & Walker 2004). Nevertheless, interest in ¢nding farmspeci¢c molecular markers continues. It is still not possible to follow the salmon lice larvae during the 4^14-day period they spend in the plankton before they infect a host (Johnson & Albright 1991). We therefore tested the research hypothesis that population genetic structure existed among salmon lice populations collected from diierent geographical regions of British Columbia, Canada, because of restricted gene £ow. Samples of the salmon louse, L. salmonis, were obtained from three diierent wild Paci¢c salmon species by sports ¢shing and during creel (sports ¢shery catch) surveys and from farmed Atlantic salmon by netting (Table 1). Sea lice were removed from each salmon host with forceps and preserved in 95% ethanol. To test for population genetic structure with the largest sample size possible, we decided to use a fragment of the mitochondrial cytochrome c oxidase subunit 1 (COI) (Folmer, Black, Hoeh, Lutz & Vrijenhoek1994) that has been extensively used for DNA barcoding (Hebert, Cywinska, Ball & deWaard 2003). This allowed us to make use of a large number of existing salmon louse COI sequences and to easily compare the sequence divergence that we observed among diierent clades with that of other species of animals (Hebert et al. 2003). Template DNA was extracted from individual salmon lice using modi¢ed proteinase K methods. A 1mm piece of ethanol-preserved tissue was obtained fromeach specimenand placed directly into 96-well plates containing lysis buier and proteinase K. Subsequent DNA extraction used a glass ¢bre protocol (Ivanova, deWaard & Hebert 2006). A 710-bp fragment of the COI genewas polymerase chain reaction (PCR) ampli¢ed using the primer pairs LCO1490 and HCO2198 (Folmer et al.1994) or LepF1and LepR1 (Hebert, Penton, Burns, Janzen & Hallwachs 2004) and visualized ina 96-well E-Gel (Invitrogen Canada, Burlington, ON, Canada). The PCR mix included 6.25 mL of 10% trehalose, 1.25 mL 10 PCR buier, 0.625 mL (50mM) MgCl2,0.125 mL (10 mm) of each primer, 0.0625 mL (10mM) dNTPs, 0.0625 mL Platinum Aquaculture Research, 2009, 40, 973^979 doi:10.1111/j.1365-2109.2008.02159.x