Genetic variation of hypoxia tolerance in farmed fish: a systematic review for selective breeding purposes

The acceleration of climate change and increasing water pollution have contributed to a global increase in hypoxic events in the oceans. As a result, this environmental stressor has had significant economic repercussions for the marine aquaculture sector. Consequently, selective breeding for hypoxia-tolerant fish is being explored as a promising strategy to mitigate climate change effects. In this context, the present systematic review synthesizes and critically evaluates current knowledge regarding the genetic variation associated with hypoxia tolerance in farmed fish species. A literature search was conducted in Scopus and Web of Science, following the PRISMA 2020 guidelines. In total, 963 articles were identified, of which 40 met the inclusion criteria, encompassing 29 species and three hybrid lines. Among the farmed fish, the blunt snout bream ( Megalobrama amblycephala ), rainbow trout ( Oncorhynchus mykiss ), common carp ( Cyprinus carpio ) and Nile tilapia ( Oreochromis niloticus ) were the most extensively studied. The most commonly used traits to measure hypoxia tolerance included: 1) time of loss of equilibrium (t _LOE ), 2) survival time or status (alive/dead) and 3) critical oxygen partial pressure (P _crit ), measured via respirometry. Notably, 22 studies reported substantial variability in hypoxia tolerance across families, strains, gynogenetic lines, growth-transgenic lines, hybrids, and species. Moreover, 15 studies identified SNP markers significantly associated with hypoxia tolerance; however, heritability estimates, reported in only two studies, ranged from 0.28 to 0.65. Furthermore, candidate genes were frequently identified as downstream effectors of the HIF pathway or as components of signaling pathways such as VEGF and mTOR, which are critical for angiogenesis and energy conservation, respectively. Additionally, genes involved in erythropoiesis, ion regulation, glucose metabolism, DNA repair, and iron metabolism, key processes in the hypoxia response, were identified. Given that aquatic environments are becoming increasingly hypoxic, these findings underscore the potential of the inherent genetic diversity present in farmed fish populations. In this context, genomic selection and gene editing emerge as promising tools for developing hypoxia-tolerant fish lines. Nevertheless, further research is warranted to implement such lines under field conditions, particularly because the correlations between hypoxia tolerance and other economically important traits, such as growth and pathogen resistance, remain largely unknown.