Mitochondrial malic enzyme (MEP-2*) genotype, temperature and growth in juvenile Atlantic salmon (Salmo salar L.)
The genetic variation at the diallelic mitochondrial malic enzyme (MEP-2*) locus represents one of the most widespread polymorphisms detected in the Atlantic salmon (Salmo salar L.). Previous studies of this locus in wild fish populations have found evidence to suggest an adaptive genetic response to local environmental temperature. In both Europe and North America, latitudinal clines in the frequencies of the two MEP-2* alleles exist, that show the same correlation with environmental temperature. In addition, these studies have identified variable associations between MEP-2* genotypes and phenotypic variation in juvenile growth in freshwater. Because growth performance is known to influence survival and reproductive success in Atlantic salmon, it has been postulated that differences in MEP-2* genotype growth performance could be a factor underlying the basis of this apparent adaptive response. This thesis investigates the relationship between MEP-2* genotype, temperature and growth performance in juvenile Atlantic salmon. The thesis aims were (i) to determine whether the genetic variation at the MEP-2* locus in the Atlantic salmon was associated with differences in juvenile growth performance in freshwater, (ii) when any such differences were established, and (iii) to what extent water temperature influenced the association. These were achieved by monitoring MEP-2* genotype growth performance and survival in juvenile Atlantic salmon, hatchery reared under warm (ambient plus ca.4°C), ambient (seasonally variable ambient water temperature) and cold (ambient minus ca. 4°C) water temperature regimens. The results obtained in this study clearly demonstrate that the genetic variation at the MEP-2* locus in the Atlantic salmon is associated with significant differences in juvenile growth performance in freshwater. Associations were observed both early in post-hatch development, and later when the fish had made the transition to exogenous feeding. Furthermore, these associations were found to be strongly influenced by both temperature and feeding ration. These are specific new findings which significantly add to our present knowledge of the MEP-2* polymorphism in the Atlantic salmon. Although these associations did not influence survival in the study, it is postulated that if the same effect were to occur in natural populations, the genetic response indicated could influence fitness. Additionally, it is possible that these genotypic differences in growth performance could be of relevance to selective breeding programs within the salmon farming industry. However, whether the genetic response exhibited was directly due to the MEP-2* locus on its own, or to some other linked locus or loci, remains to be demonstrated.