High Gyrodactylus salaris infection rate in triploid Atlantic salmon Salmo salar.

We describe an unusually high infection rate of Gyrodactylus salaris Malmberg in juve- nile Atlantic salmon Salmo salar L. of Baltic Sea origin, which are generally believed to be more resis- tant to G. salaris than East Atlantic salmon populations. Based on analyses of mitochondrial (complete cytochrome oxidase 1 (CO1) gene, 1548 bp) and nuclear (ADNAM1, 435 bp; internal transcribed spacer (ITS) rDNA region, 1232 bp) DNA fragments, the closest relatives of the characterized Eston- ian G. salaris strain were parasites found off the Swedish west coast and in Raasakka hatchery, Iijoki (Baltic Sea, Finland). Analyses of 14 microsatellite loci of the host S. salar revealed that approximately 40% of studied fish were triploids. We subsequently identified triploid Atlantic salmon of Baltic ori- gin as more susceptible to G. salaris infection than their diploid counterparts, possibly due to compro- mised complement-dependent immune pathways in triploid salmon. This is in accordance with ear- lier studies that have shown elevated susceptibility of triploids to various viral or bacterial pathogens, and represents one of the first reports of increased susceptibility of triploid salmonid fish to an ectoparasite. However, further experimental work is needed to determine whether triploid Atlantic salmon is generally more susceptible to G. salaris compared to their diploid counterparts, irrespective of the particular triploidization method and population of origin.