Effect of selection for muscle lipid content on body shape, fat deposition and dressing yields in rainbow trout

Comparison of lines or strains differentially susceptible to diseases is a powerful tool to identify the genetic elements that regulate resistance to a disease and the key defence mechanisms and pathways. In fish, the production of individuals with extreme phenotypes may be accelerated using chromosome manipulation techniques. Doubled-haploid homozygous clones which present many interesting features for genetic and immunological studies can be produced after two successive generations of gynogenesis. We used nine of those clones and the control (original population) in a study of the resistance to two rhabdoviruses, VHSV (Viral Haemorrhagic Scepticemia Virus) and IHNV (Infectious Hematopoietic Necrosis Virus). Water route infection and intraperitoneal injection were tested. For VHSV, survival after waterborne challenge was 17% in control and ranged from 0 to almost 100% among clones. Resistant clones remained resistant when challenged in cohabitationwith susceptible ones, an encouraging result in view of ‘on field’ applications. For IHNV (five clones only and control tested), survival after waterborne challenge increased with age/weight and significantly depended on clones. Correlation between resistance to both diseases was borderline significant (Spearman coefficient 0.77,P=0.07). For both viruses, injection provoked complete mortality whatever the group. Implications of the results in terms of mechanisms of resistance will be discussed. Extreme clones will provide key support for further genetic (search for QTL and candidate genes) and immunopathological (specific candidate molecules or gene expression profiling) studies aiming at the identification of the immune pathways and genetic factors regulating susceptibility to the viruses.