Institute of Neuroscience, University of Oregon, Eugene, OR 97403, USA.
Wellcome Sanger Institute, Tree of Life, Wellcome Genome Campus, Hinxton CB10 1SA, United Kingdom.
Mol Biol Evol. 2023 Nov 3;40(11). doi: 10.1093/molbev/msad236.
Expression of multiple hemoglobin isoforms with differing physiochemical properties likely helps species adapt to different environmental and physiological conditions. Antarctic notothenioid fishes inhabit the icy Southern Ocean and display fewer hemoglobin isoforms, each with less affinity for oxygen than temperate relatives. Reduced hemoglobin multiplicity was proposed to result from relaxed selective pressure in the cold, thermally stable, and highly oxygenated Antarctic waters. These conditions also permitted the survival and diversification of white-blooded icefishes, the only vertebrates living without hemoglobin. To understand hemoglobin evolution during adaptation to freezing water, we analyzed hemoglobin genes from 36 notothenioid genome assemblies. Results showed that adaptation to frigid conditions shaped hemoglobin gene evolution by episodic diversifying selection concomitant with cold adaptation and by pervasive evolution in Antarctic notothenioids compared to temperate relatives, likely a continuing adaptation to Antarctic conditions. Analysis of hemoglobin gene expression in adult hematopoietic organs in various temperate and Antarctic species further revealed a switch in hemoglobin gene expression underlying hemoglobin multiplicity reduction in Antarctic fish, leading to a single hemoglobin isoform in adult plunderfishes and dragonfishes, the sister groups to icefishes. The predicted high hemoglobin multiplicity in Antarctic fish embryos based on transcriptomic data, however, raises questions about the molecular bases and physiological implications of diverse hemoglobin isoforms in embryos compared to adults. This analysis supports the hypothesis that the last common icefish ancestor was vulnerable to detrimental mutations affecting the single ancestral expressed alpha- and beta-globin gene pair, potentially predisposing their subsequent loss.
具有不同生理化学性质的多种血红蛋白同工型的表达可能有助于物种适应不同的环境和生理条件。南极鳕鱼生活在冰冷的南大洋，其血红蛋白同工型较少，每种同工型对氧气的亲和力都比温带亲属低。血红蛋白多样性的减少被认为是由于寒冷、热稳定和高含氧的南极水域选择性压力的放松。这些条件还允许白血冰鱼的生存和多样化，冰鱼是唯一生活中没有血红蛋白的脊椎动物。为了了解血红蛋白在适应冷冻水时的进化，我们分析了来自 36 种南极鳕鱼基因组组装的血红蛋白基因。结果表明，适应寒冷条件通过与寒冷适应同时发生的间断性多样化选择以及与温带亲属相比在南极鳕鱼中的普遍进化来塑造血红蛋白基因的进化，这可能是对南极条件的持续适应。在各种温带和南极物种的成年造血器官中分析血红蛋白基因表达，进一步揭示了血红蛋白基因表达的转变，这是南极鱼类血红蛋白多样性减少的基础，导致成年掠夺鱼和飞龙鱼（冰鱼的姐妹群）中只有一种血红蛋白同工型。然而，基于转录组数据预测的南极鱼类胚胎中高血红蛋白多样性，提出了关于胚胎中不同血红蛋白同工型与成年鱼类相比的分子基础和生理意义的问题。这项分析支持了这样一种假说，即最后一个共同的冰鱼祖先易受影响单一祖先表达的α-和β-珠蛋白基因对的有害突变的影响，可能使它们随后丢失。
