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

Abstract

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 b-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.

Gene expression regulation and lineage evolution: the North and South tale of the hybrid polyploidSqualius alburnoidescomplex

“…The need for correct gene balance in a polyploid context as a way to tolerate the presence of extra genome copies and to maintain stable aneuploids has been further demonstrated in plants (Birchler & Veita 2007) and discussed also as a possibility in vertebrates (Mable 2007). Recently, dosage effects on transcription levels have been revealed in salmon (Ching et al 2010), and similar levels of gene expression have been reported in triploid and diploid individuals. In the S. alburnoides complex, the necessity for balanced gene expression is apparent in both southern (Pala et al 2008) and northern populations, with higher ploidy forms exhibiting gene expression regulated to the diploid levels.…”

Is more better? Polyploidy and parasite resistance

“…Similarly, nitroblue tetrazolium reaction and hypoferraemic response are reduced in triploid versus diploid goldfish (Carassius auratus) and salmon (Salmo salar), respectively [43,44]. Additionally, while the transcriptional responses of several immunerelated genes to bacterial infection do not differ between diploid and triploid Chinook salmon (Oncorhynchus tshawytscha), some genes show reduced performance in triploids [45]. Polyploidy may thus negatively affect immune defence, or polyploid individuals may not need to mount a strong immune response.…”

Rapid evolution of osmoregulatory function by modification of gene transcription in steelhead trout

“…Other assayed genes included; Elongation factor 1a (EF1a) as the reference for normalization of quantification, b-actin and immunoglobulin M heavy chain (IgM) as ''control'' genes to assess neutral expectations of change between the two populations (since neither gene is expected to be under strong directional selection in either environment). Both b-actin and IgM have been shown to exhibit variable transcription under stress in Pacific salmonids (Ching et al 2009). …”

“…We therefore designed our probes and primers in regions where the isoform sequences are most dissimilar, and that lie across intron-exon boundaries ( Table 2). All assays were developed for this study, except b-actin and IgM which are described in Ching et al (2009). The CFTR I gene of O. mykiss had not been characterized, and thus we amplified and sequenced it using degenerate primers designed from Salmo salar CFTR I & II.…”