Evolution of Hemoglobin Genes in Codfishes Influenced by Ocean Depth

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.

Long‐read sequence capture of the haemoglobin gene clusters across codfish species

“…Pairwise sequence alignment of these paralogous Hb genes from Baalsrud et al. () revealed sequence identities up to 98% (Table S2).…”

“…Gray boxes indicate genes that have been identified in Baalsrud et al. () but are absent in the de novo assemblies. (a) Display the micro‐synteny across the MN region.…”

Long-read sequence capture of the hemoglobin gene clusters across species

“…Furthermore, we found the amino acid positions 55 and 62 in Hbb1, 294 known to be polymorphic in Atlantic cod, to be variable across all codfishes included 295 in this study ( Figure 6). Investigations of the same positions in a number additional 296 codfishes for which we have available gene sequences (Baalsrud et al 2017), revealed 297 that these positions are highly variable across this linage (Table S4). Notably, the 298 most likely ancestral state of codfish Hbb1 is Met55Lys62 ( Supplementary Figures S1 299 and S2).…”

“…and Hbb, has shown to be of importance for ecological adaptation, as environmental 86 factors such as temperature directly influences the ability of Hb to bind O2 at 87 respiratory surfaces and its subsequent release to tissues (Wells 2005). In a recent 88 report, a characterization of the Hb gene repertoire by comparative draft genome 89 analysis uncovered a remarkably high Hb gene copy variation within the codfishes 90 (Baalsrud et al 2017). Based on the gene copy number a negative correlation between 91 the number of Hb genes and depth of which the species occur was observed, as well 92 as signs of diversifying selection on the gene paralogues suggesting that the variable 93 environment in epipelagic waters have facilitated a larger more diverse Hb gene 94 repertoire (Baalsrud et al 2017).…”

“…In a recent 88 report, a characterization of the Hb gene repertoire by comparative draft genome 89 analysis uncovered a remarkably high Hb gene copy variation within the codfishes 90 (Baalsrud et al 2017). Based on the gene copy number a negative correlation between 91 the number of Hb genes and depth of which the species occur was observed, as well 92 as signs of diversifying selection on the gene paralogues suggesting that the variable 93 environment in epipelagic waters have facilitated a larger more diverse Hb gene 94 repertoire (Baalsrud et al 2017). However, the rather fragmented draft genomes did 95 not allow for reconstruction of the gene synteny and a deeper understanding of the 96 evolution of the Hb-clusters.…”