Ronald B. Johnson

A genetic basis for the phenotypic differentiation between siscowet and lean lake trout (Salvelinus namaycush)

In Lake Superior there are three principal forms of lake trout (Salvelinus namaycush): lean, siscowet and humper. Wild lean and siscowet differ in the shape and relative size of the head, size of the fins, location and size of the eyes, caudal peduncle shape and lipid content of the musculature. To investigate the basis for these phenotypic differences, lean and siscowet lake trout, derived from gametes of wild populations in Lake Superior, were reared communally under identical environmental conditions for 2.5 years. Fish were analysed for growth, morphometry and lipid content, and differences in liver transcriptomics were investigated using Roche 454 GS‐FLX pyrosequencing. The results demonstrate that key phenotypic differences between wild lean and siscowet lake trout such as condition factor, morphometry and lipid levels, persist in these two forms when reared in the laboratory under identical environmental conditions. This strongly suggests that these differences are genetic and not a result of environmental plasticity. Transcriptomic analysis involving the comparison of hepatic gene frequencies (RNA‐seq) and expression (quantitative reverse transcription–polymerase chain reaction (qPCR)) between the two lake trout forms, indicated two primary gene groups that were differentially expressed; those involving lipid synthesis, metabolism and transport (acyl‐CoA desaturase, acyl‐CoA binding protein, peroxisome proliferator‐activated receptor gamma, and apolipoproteins), and those involved with immunity (complement component C3, proteasome, FK506 binding protein 5 and C1q proteins). The results demonstrate that RNA‐seq can be used to identify differentially expressed genes; however, some discrepancies between RNA‐seq analysis and qPCR indicate that methods for deep sequencing may need to be refined and/or different RNA‐seq platforms utilized.

Lipid Deposition in Oocytes of Teleost Fish During Secondary Oocyte Growth

Research pertaining to the deposition of lipids into the maturing oocytes of teleost fish has progressed significantly since the review by Wiegand (1996). Studies on broodstock diets have elucidated the importance of incorporating n-3 and n-6 highly unsaturated fatty acids (HUFAs) into diets at the proper levels and ratios for optimal egg viability and survival of progeny. The vitellogenin receptor, VgR, has been well classified in some teleost species and is now suspected to be evolutionarily linked to the LR8(-) receptor in birds and mammals. The expression of a somatic lipoprotein receptor has been observed in the maturing oocytes of teleost fish. Multiple forms of vitellogenin have been discovered in a number of teleost species. There is also a growing amount of evidence suggesting other plasma lipoproteins may play a major role in the deposition of lipids for some species. Finally, there is additional evidence that vitellogenic oocytes are capable of de novo synthesis of lipids, which provides insight on how the oocyte lipid class composition is regulated within a species. This review attempts to summarize the current knowledge related to the deposition of lipids into the oocytes of teleost fish during oogenesis and collates results from recent studies in this area.

Determination of fat content in fish feed by supercritical fluid extraction and subsequent lipid classification of extract by thin layer chromatography-flame ionization detection

Abstract A supercritical fluid extraction (SFE) method employing carbon dioxide with an alcohol modifier was developed to determine fat content in fish feed. Fat content was determined for 14 dry extruded salmon or trout grower feeds with advertised fat contents ranging from 14% to 33% (w/w). Results with the new method were significantly higher than those obtained from Soxhlet extractions (AOCS Method Ba 3–38) with a difference in mean fat content of 1.9% ( P P P

Transitioning Coho Salmon Broodstock to a Docosahexaenoic Acid (DHA)-Rich Diet during Vitellogenesis: Effects on Egg Composition and Embryo and Fry Quality

Abstract Female coho salmon Oncorhynchus kisutch were fed one of two broodstock diets during vitellogenesis to evaluate the contribution of dietary lipids to egg lipids and to assess any embryo or fry quality differences that were attributable to treatment. The diets contained lipids with different fatty acid profiles and carbon stable isotope signatures (δ13C). Diets were formulated by top-dressing a low-fat salmon broodstock feed with either sardine oil (fish oil feed) or an algal oil–corn oil blend (algal oil feed). The added oils were balanced for total n-3 fatty acid content but contained markedly different levels of linoleic acid (LA), eicosapentaenoic acid, and docosahexaenoic acid (DHA). Analysis of δ13C revealed that dietary lipids were significantly incorporated into egg lipids during vitellogenesis and that polar and neutral lipid fractions were equally affected. Eggs of fish in the algal oil treatment group had significantly higher levels of DHA and significantly lower protein content than the...

Uptake and selective partitioning of dietary lipids to ovarian and muscle tissue of maturing female coho salmon, Oncorhynchus kisutch, during secondary oocyte growth

Female coho salmon, Oncorhynchus kisutch, were fed one of two experimental feeds containing lipids with markedly different stable 13C isotope signatures during the late cortical alveolus, lipid droplet, and vitellogenesis stages of secondary oocyte growth. Ovarian and muscle lipids fatty acid concentrations were significantly affected by treatment during all three stages of development. Stable 13C isotope analyses confirmed that dietary lipids were incorporated into both ovarian and muscle lipids during all three stages and revealed that ovarian lipids were more affected than muscle lipids during vitellogenesis. Arachidonic acid (ARA) was incorporated into ovarian lipids at the highest rate of all fatty acids examined with the greatest uptake observed during the cortical alveolus and lipid droplet stages of development. Docosahexaenoic acid (DHA) was incorporated into ovarian lipids at the next highest rate with the greatest uptake observed during the lipid droplet stage of development. The presence of an ovary specific, fatty acid transfer mechanism is proposed. Results from this study demonstrate the ability to greatly alter the fatty acid composition of ovarian lipids through a dietary change during secondary oocyte growth and may be of great interest to producers of farmed salmon and salmon broodstock programs.