Transcriptional differences between triploid and diploid Chinook salmon (Oncorhynchus tshawytscha) during live Vibrio anguillarum challenge.

Understanding how organisms function at the level of gene expression is becoming increasingly important for both ecological and evolutionary studies. It is evident that the diversity and complexity of organisms are not dependent solely on their number of genes, but also the variability in gene expression and gene interactions. Furthermore, slight differences in transcription control can fundamentally affect the fitness of the organism in a variable environment or during development. In this study, triploid and diploid Chinook salmon (Oncorhynchus tshawytscha) were used to examine the effects of polyploidy on specific and genome-wide gene expression response using quantitative real-time PCR (qRT-PCR) and microarray technology after an immune challenge with the pathogen Vibrio anguillarum. Although triploid and diploid fish had significant differences in mortality, qRT-PCR revealed no differences in cytokine gene expression response (interleukin-8, interleukin-1, interleukin-8 receptor and tumor necrosis factor), whereas differences were observed in constitutively expressed genes, (immunoglobulin (Ig) M, major histocompatibility complex (MHC) -II and beta-actin) upon live Vibrio anguillarum exposure. Genome-wide microarray analysis revealed that, overall, triploid gene expression is similar to diploids, consistent with their similar phenotypes. This pattern, however, can subtly be altered under stress (for example, handling, V. anguillarum challenge) as we have observed at some housekeeping genes. Our results are the first report of dosage effect on gene transcription in a vertebrate, and they support the observation that diploid and triploid salmon are generally phenotypically indistinguishable, except under stress, when triploids show reduced performance.