Increased Thermal Tolerance of Rainbow Trout by Selective Breeding at High Temperatures

There is an increasing concern about the negative impact of global warning on salmonids, representative cold water fish. Rainbow trout Oncorhynchus mykiss is one of salmonid of great economic importance in aquaculture. Many efforts have been conducted to enhance thermal tolerance of rainbow trout, resulting in the establishment of several strains endowed with a high-temperature resistance. Recently a number of quantitative trait loci (QTL) linked with thermal tolerance of rainbow trout have been reported by using these strains. However, responsible genes have not yet been identified and thus molecular mechanisms involved in their thermal tolerance remained unclear.

In Japan, a strain of rainbow trout with thermal tolerance has been established by selecting fish which survived in high temperature water at 20-27°C in summer for 4 month with occasional exposure to 30-35°C for 1-5 min through more than 15 generations in the Miyazaki Prefectural Fisheries Experimental Station (Ineno et al., 2005, 2008). This strain is featured by growing and feeding even at 24°C and by developing and hatching of embryos even at 17°C. In general, rainbow trout is unable to survive at 24°C, and its embryo terminates development at 17°C. While cytosolic proteins were released to the circulation system for a normal strain after exposure to high temperatures, this thermally selected strain showed no change after the same high temperature treatment (Itoi et al., 2001). We also examined the differences in the gene expression pattern of embryos between thermally selected and normal strains by mRNA arbitrarily primed reverse transcription-polymerase chain reaction (RAP RT-PCR) following a high temperature treatment. One of the genes upregulated in the thermally selected strain encoded cytochrome c oxidase subunit II, which is located in the mitochondrial genome (Ikeguchi et al., 2006). Further expression analysis of mitcondorial and nuclear genes revealed that the genes encoding ATP synthesis in mitochondria were constitutively upregulated in embryos of the thermally selected strain.

Based on the above-mentioned findings, we have started genome-wide screening of genes responsible for the thermal tolerance of rainbow trout. In addition to traditional QTL mapping, transcriptomal analyses and a direct comparison of the whole genome sequences between thermally selected and normal strains are now in progress by using next generation sequencers. In this presentation, we introduce biological and biochemical features of our thermally selected strain, as well as our ongoing project to identify genes responsible for thermal tolerance of rainbow trout.