Gene duplication, conservation and divergence of Heme oxygenase 2 genes in blunt snout bream (Megalobrama amblycephala) and their responses to hypoxia

Heme oxygenase (HO) that catalyzes the degradation of heme, is involved in responding and using oxygen in teleost fish. Physiologic heme degradation can be catalyzed by two isozymes of HO (HO-1 and HO-2). In fish, the molecular constructions, expression characteristics and hypoxic regulation of HO-2 are still not well known. Here, we report the isolation and characterization of duplicated HO-2 genes in blunt snout bream, a hypoxia sensitive fish species. Blunt snout bream HO-2a and -2b genes shared a relatively low sequence identity of 67%. The HO-2a and -2b mRNAs were widely expressed in adult tissues. During embryogenesis, HO-2a mRNAs was significantly upregulated at 16 hpf and then maintained with high lever, while HO-2b mRNAs was gradually increased at 12 hpf and then reduced significantly. Whole-mount in situ hybridization demonstrated that HO-2a and -2b mRNAs mainly detected in brain and eyes at different embryonic stages. The results of acute hypoxia experiment showed that both HO-2a and -2b mRNAs have significant changes in different tissues. Both HO-2a and -2b mRNAs were significantly up-regulated in the brain, but down-regulated in the gill and liver during hypoxia. Under hypoxia, HO-2a mRNA in the heart was significantly increased while HO-2b mRNA was decreased. Embryos in hypoxic conditions at different developmental stages strongly induced the mRNA expression of HO-2a and -2b. These results provide new insights into the functional conservation and divergence of HO-2 genes and improve our understanding of HO-2 responses to hypoxia.