Screening of hypoxia-tolerance related SNP in a selectively bred F5 strain of blunt snout bream (Megalobrama amblycephala)

Abstract

Blunt snout bream (Megalobrama amblycephala) is a typical hypoxia-sensitive freshwater fish species. The breeding of new hypoxia-tolerant strains is important for blunt snout bream aquaculture. Starting in 2007, selective breeding under hypoxic stress has been performed in a wild-caught blunt snout bream population from Poyang Lake, China. In 2017, the selectively bred F5 population was obtained, which showed markedly improved tolerance toward hypoxic stress. In this study, we identified two completed linked SNP sites at 397 and 715 within exons of the Egln2 gene. We identified these SNP sites after examination of transcriptome data from blunt snout bream of the hypoxia-tolerant strain, which can be formed using two haplotypes, namely, haplotype 1 (C397G715) and 2 (T397T715). Haplotype 1 occurred mainly in hypoxia sensitive control blunt snout bream, whereas haplotype 2 was predominantly found in blunt snout bream of the hypoxia-tolerant strain. The enzyme activities of CAT, SOD, and Na+/K+-ATP related to hypoxia tolerance in blunt snout bream of diplotype II (T397T397T715T715) were significantly higher than those in blunt snout bream with other genetic combinations. The homozygous diplotype II blunt snout bream, in particular, had a significantly (p < 0 .01) lower critical oxygen tension at which they lost their equilibrium (LOEcrit). The average protruding lamella heights and mean lamellar area of gills of blunt snout bream of the diplotype II strain were also significantly (p < 0.01) smaller than those of homozygous diplotype I (C397C397G715G715) or heterozygous diplotype III (T397C397T715G715) blunt snout bream exposed to 4d or 7d of hypoxia treatment. Additionally, the blood erythrocyte count and hemoglobin (Hb) concentration of blunt snout bream of the diplotype II strain increased significantly (p < .01) during hypoxia treatment. The results indicate that blunt snout bream with the Egln2 type II genotype had a significant correlation with hypoxia-tolerant traits. Thus, the haplotype 2 can be used as a marker for future practical programs aimed at selective breeding of hypoxia-tolerant blunt snout bream.