Effects of Hybridization and Triploidization on Gene Expression in Rainbow Trout (Oncorhynchus mykiss)♀ × Brook Trout (Salvelinus fontinalis)♂ Hybrids

The transcriptomic effects of hybridization and triploidization were investigated in the diploid and triploid rainbow trout, diploid brook trout and in the triploid hybrids of rainbow trout and brook trout. The examined fish were reared under identical conditions for about two and a half years after hatching. Expression of ten genes involved in cellular respiration (Atp5bp, Slc25a5), mitochondrial functioning (Mrpl28, Micu2), ribosome biogenesis (Rpl24, Rps24), proteasome-mediated protein turnover (Derl1, Psmc2) and protein chaperoning (Hsp90B1, Pdia4) were studied in liver and muscle tissues. Most of the analyzed genes displayed comparable expression levels across all the fish groups, with triploid hybrids showing expression patterns more similar to the purebred diploid trout. Compared to all other fish groups, purebred triploid rainbow trout exhibited significant upregulation of Slc25a5, Derl1, Rps24 and Rpl24 genes in liver and downregulation of Micu2 gene muscles. In turn, triploid hybrids showed marked upregulation of Atp5pb, Slc25a5, Rpl24, Rps24 and Pdia4 genes in muscle and downregulation of Hsp90B1 gene in both liver and muscle when compared to all the fish groups examined. Although protein synthesis- and energy-related genes were upregulated in the muscles of triploid hybrids, the recoded growth performance data did not indicate clear evidence of growth heterosis, suggesting that the potential benefits of increased heterozygosity in this cross may be counterbalanced by metabolic inefficiencies. Three- to fourfold downregulation of heat-shock protein genes was observed in both tissues of triploid hybrids compared with purebred diploid and triploid trout, indicating possible maladaptive genomic incompatibilities usually observed in salmonid fish hybrids that may reduce their heat tolerance under aquaculture conditions. The obtained results also revealed significant upregulation of genes linked to liver protein synthesis and energy production, along with muscle protein turnover in the triploid rainbow trout, supporting the hypothesis that the higher energy demands for maintaining proper protein concentrations in the larger cytoplasmic cell volume of triploid fishes may be met through enhanced protein metabolism.