Molecular and seasonal basis for freeze resistance in wolffish species (Anarhichas lupus, A. minor and their hybrids)

Desjardins, Mariève (2012) Molecular and seasonal basis for freeze resistance in wolffish species (Anarhichas lupus, A. minor and their hybrids). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

During winter, the coastal waters of Newfoundland can be considered a "freeze risk ecozone" for teleost fishes. The benthic Atlantic (Anarhichas lupus - AW) and spotted wolffish (A. minor - SW) reside at opposite ends of this ecozone, with the AW facing the greatest risk because of its shallower niche. To resist freezing, AW secretes five times the level of plasma antifreeze protein (AFP) activity than does SW. -- The main basis for this difference in plasma AFP levels is gene dosage, as AW has approximately three times more AFP genes than SW. Perhaps as a result, AFP transcript levels in liver (the primary source of circulating AFP's) are several times higher in AW. One explanation for these gene and transcript dosage differences is the presence of tandemly arrayed AFP gene repeats in AW that make up two-thirds of its AFP gene pool. Such repeats are not present in SW. AW and SW diverged from a common ancestor at a time when the ebb and flow of northern glaciations would have resulted in the emergence of "freeze risk ecozones". The duplication/amplification of AFP genes in a subpopulation of ancestral wolffish would have facilitated the exploitation of this high risk habitat, resulting in the divergence and evolution of modem day AW and SW species. -- Investigations on artificially produced AW/SW hybrids showed that all the AFP genes of SW are likely shared with AW, which supports recent gene amplification as an impetus for speciation. The high dynamism of the AW AFP locus (through high variability in dosage and organisation) was best visualized in the single haplotype inherited by the hybrids. Their intermediate levels of plasma AFPs make these fish unfit for survival within the shallowest part of the "freeze risk ecozone". -- While the total levels of AFP gene transcripts varied little from winter to summer in both species (especially in the liver), differential expression of members of the two subfamilies of type III AFPs, the SP- and QAE-type genes (appellation based on the ion-exchange Sephadex resins to which each type of isoform binds), was observed. While the expression of the QAE-type transcripts was ubiquitous (both temporally and spatially), that of the SP-type genes seemed to show some level of tissue specificity, which highlights the complex evolutionary history of these genes.

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