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Can Copies Keep Out the Cold: Characterizing Copy Number of Major Heat Shock Genes in Antarctic Emerald Rockcod, Trematomus bernacchii
The perciform suborder Notothenioidae is comprised of Antarctic and sub-Antarctic fishes, several of which have lost their ability to rapidly upregulate major heat shock proteins (HSP’s) in response to thermal stress. In fact, some notothenioid species have instead adopted a pattern of expression resembling ancestrally constitutive genes. It is possible that evolution in the Southern Ocean has selected for increased expression of molecular chaperones given the denaturing effect extreme cold temperature has on proteins. The selective pressures of the sub-zero water may have also favored duplication events that bolster quantitative output of chaperones via gene dosage effects. Given that newly duplicated genes are under more relaxed selective pressures, it is plausible that gene duplication preceded alterations in the regulatory regions of the inducible HSP genes, enabling altered regulation of such highly conserved gene products. To test for evidence of these mechanisms, gene copy number of ancestrally inducible and constitutive isoforms within the major heat shock gene families were characterized via qPCR and compared for the Antarctic notothen Trematomus bernacchii, which has lost the inducible HSR, and the non-Antarctic notothen, Notothenia angustata, which has maintained an inducible HSR. Results indicate duplication of inducible isoforms within the Hsp70 and Hsp40 superfamily have occurred in the genome of T. bernacchii. The findings from this study suggest adaptive gene duplications may have played a role in maintaining increased protein folding efficiency in the sub-zero waters of the Southern Ocean and provided a mechanism for the evolution of alternative regulation of these conserved gene families.