| Summary: | Distinct red flesh color is a unique trait of the fish genera Oncorhynchus, Salvelinus, Salmo and Parahucho. In this thesis we have investigated its molecular basis in Atlantic salmon to better understand how and why it developed during the evolution in just these genera. In the aquaculture industry, the red muscle color in salmon is important for consumers perception of filet quality. The red flesh color is mainly caused by accumulation of the carotenoid astaxanthin in muscle tissue, obtained through a crustean rich diet in the wild, or by feed supplementation of syntetic astaxanthin in aquaculture. Individual differences in salmon flesh color intensity in groups fed identical amounts of pigment, indicate variable uptake and metabolic processing between individuals. In order to improve or standardise the flesh coloration, genetic factors and molecular mechanisms responsible for the variation must be identified. Previous studies recognized that low absorption and/or high metabolic transformation rate are critical factors for the observed low retention of astaxanthin in muscle. Previous genome wide association studies have strongly indicated that the β-carotene oxygenases bco1 and bco1l are involved in the flesh coloration. The aim of this thesis was to further investigate how bco1 and its paralogue bco1l influence astaxanthin metabolism and to discover additional genes that might contribute to the flesh color variation. In paper I, the fast freeze substitution tissue preservation method was developed as a useful tool for the experiments performed in paper II and paper III. The main advantage of this preservation method over the standardly used formalin fixation is that fixed tissues are suitable for more than only one type of analysis. Tissue morphology, RNA and proteins are very well preserved with freeze fixation and samples can be used for microscopy, but also for highly sensitive downstream analysis like RNA sequencing and protein and immunoblotting analysis. In paper II, the functional roles of bco1 and ...
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