M. Dorson

Genetics of resistance to disease in fishes.

Abstract The first part of this paper deals with the identification and exploitation of genetic variations in resistance to diseases. At the interspecific level, hybridization may in some cases allow the exploitation of the character of resistance of one of the parents. Polyploid hybrids represent a particular advantage for this approach. Variations between populations have been treated in many papers. However, the studies are generally limited to F1 crosses and do not allow analysis of genetic determinism of the observed variations. Significant variations are also observed between individuals of one and the same population. Several mass selection results using this variability have been developed. The second part deals with disease resistance mechanisms. At the quantitative level, the studies concern the variability of antibody levels, neutralizing serum and mucus activities and resistance to stress; several cases of correlation between these characters and resistance to diseases are presented. The Mendelian approach is particularly developed in the field of histocompatibility genetics. However, the link between this variability and disease resistance remains to be established. The final discussion deals with the respective advantages of quantitative, Mendelian and molecular biology approaches in order to improve disease resistance in fish.

Interferon production in rainbow trout (Salmo gairdneri) experimentally infected with Egtved virus.

Egtved virus (Zwillenberg, Jensen & Zwillenberg, 1965), a rhabdovirus, is the causative agent of virus haemorrhagic septicemia of trout (V.H.S.), a disease which causes important losses in European hatcheries. This disease occurs mainly when the water temperature is between 6 and 12 °C, and spontaneously disappears when the temperature rises over 14 to 15 °C, in spite of the fact that the latter temperature is the thermal optimum for in vitro virus growth (de Kinkelin & Scherrer, 1970). Although the trout is able to produce circulating antibodies against a variety of antigens at 15 °C (Post 1966, Hodgins, Weiser & Ridgway, 1967; Dorson 1972), there is no evidence of neutralizing antibodies in the serum of trout recovering from an epizootic of V.H.S., and several injections of the virus have been necessary to obtain a low titre of neutralization (Vestergard-Jorgensen, 1971).

Resistance to a rhabdovirus (VHSV) in rainbow trout: identification of a major QTL related to innate mechanisms.

Health control is a major issue in animal breeding and a better knowledge of the genetic bases of resistance to diseases is needed in farm animals including fish. The detection of quantitative trait loci (QTL) will help uncovering the genetic architecture of important traits and understanding the mechanisms involved in resistance to pathogens. We report here the detection of QTL for resistance to Viral Haemorrhagic Septicaemia Virus (VHSV), a major threat for European aquaculture industry. Two induced mitogynogenetic doubled haploid F2 rainbow trout (Oncorhynchus mykiss) families were used. These families combined the genome of susceptible and resistant F0 breeders and contained only fully homozygous individuals. For phenotyping, fish survival after an immersion challenge with the virus was recorded, as well as in vitro virus replication on fin explants. A bidirectional selective genotyping strategy identified seven QTL associated to survival. One of those QTL was significant at the genome-wide level and largely explained both survival and viral replication in fin explants in the different families of the design (up to 65% and 49% of phenotypic variance explained respectively). These results evidence the key role of innate defence in resistance to the virus and pave the way for the identification of the gene(s) responsible for resistance. The identification of a major QTL also opens appealing perspectives for selective breeding of fish with improved resistance.