Validation of QTL associated with resistance to Lernanthropus kroyeri in European seabass

Lernanthropus kroyeri constitutes the most commonly encountered parasitic copepod for European seabass ( Dicentrarchus labrax ). Infection of gills with copepods directly impacts fish survival and growth performance with further economic implications for the aquaculture industry. Earlier studies identified moderate heritability of parasite count and two suggestive candidate QTL that explained 2 % of the total phenotypic variance each. The present study focuses on the two previously identified QTL loci to validate any QTL effect on L. kroyeri parasite resistance. In order to facilitate the validation of healthy and infected fish, individuals were selected based on their resistance to parasites, specifically the presence or absence of L. kroyeri in fish reared in a standard open sea cage. A subset of samples was genotyped using the 30 k MedFISH SNP array and the putative QTL effects were tested for parasite resistance by fitting an animal model in ASREML-R while the AIC criterion was used to assess model fit. The p -values for the two SNP with highest association to parasite resistance AX-373127007 and AX-373218583 were 0.041 (<0.05) and 0.085 (<0.1), respectively. Despite the moderate significance of the second SNP, the lowest AIC score was detected after using both SNPs as fixed effects in the animal model. The findings of the present study using a different population, validate the important role of the detected QTL loci on parasite resistance, highlighting a possible application of a low-cost Marker Assisted Selection (MAS) breeding program in farmed European seabass populations. • Two QTL associated with L. kroyeri resistance in LG7 were validated in European seabass. • The r 2 value was 0.079, indicating that the two QTL are not in linkage disequilibrium. • The most efficient prediction for parasite resistance have been detected by using both QTL in an animal model.