Abstract
Understanding the genetic basis of adaptation is one of the main enigmas of evolutionary biology. Among vertebrates, hemoglobin has been well documented as a key trait for adaptation to different environments. Here, we investigate the role of hemoglobins in adaptation to ocean depth in the diverse teleost order Gadiformes, with species distributed at a wide range of depths varying in temperature, hydrostatic pressure and oxygen levels. Using genomic data we characterized the full hemoglobin (Hb) gene repertoire for subset of species within this lineage. We discovered a correlation between expanded numbers of Hb genes and ocean depth, with the highest numbers in species occupying shallower, epipelagic regions. Moreover, we demonstrate that the Hb genes have functionally diverged through diversifying selection. Our results suggest that the more variable environment in shallower water has led to selection for a larger Hb gene repertoire and that Hbs have a key role in adaptive processes in marine environments.
Highlights
Understanding the genetic basis for how organisms adapt to specific environments is a fundamental challenge within evolutionary biology
The foundation for this diverse repertoire is partially associated with the teleost whole-genome duplication (TGD) 320–400 Ma12, which resulted in two Hb clusters located on different chromosomes; the LA cluster and the MN cluster, respectively[11, 13]
Hb could be important for adaptation to the variety of environments occupied by the other gadiform species
Summary
HelleTessand Baalsrud[1], Kjetil LysneVoje[1], Ole KristianTørresen[1], Monica Hongrø Solbakken 1, Michael Matschiner[1,2], Martin Malmstrøm[1], Reinhold Hanel[3], Walter Salzburger[1,2], Kjetill S. Many sites were consistently reported using either a species tree or a gene tree by two or all of the three tests These sites are likely to be the most important for the evolution of divergent functions (Fig. 4, Table S3). To further evaluate whether the sites identified to be under diversifying selection have any potential impact on the actual function of the Hb tetramer, we constructed a structural protein model based on G. morhua hemoglobin sequences and plotted these sites onto the structure (Fig. 4). Most of the sites in gadiform Hb genes appear to be on the surface of the tetramer (Fig. 4), which again supports sub-functionalization, as substitutions on the surface of the protein affect Hb subunit interactions and affinities, and are likely to influence its O2 binding capacity under different environmental conditions We chose to use maximum depth (Table 1) as an indicator of depth of usual occurrence
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