Comparative Transcriptome Analyses Reveal the Responses to Hypoxia Challenge of Mactra veneriformis

The transcriptomic analysis of Mactra veneriformis under hypoxia stress was carried out in this paper to explore the response mechanism of M. veneriformis under hypoxia stress from the molecular biological level. The results provided a reference for further study on hypoxia tolerance mechanism and creation of new hypoxia tolerant germplasm of Clams quadricornis. The findings suggest that transcriptome sequencing was performed on M. veneriformis branchial filament tissues under normal and hypoxia conditions by high-throughput sequencing technology, and 54.58Gb of clean data were obtained. There were differences in gene expression between the normal-oxygen group and the hypoxia group. Control group A and hypoxia group B had the most differentially expressed genes (7473 genes), among which 4000 up-regulated genes and 3473 down-regulated genes. Secondly, there were 6407 differentially expressed genes between hypoxia group B and hypoxia group C, with 3535 up-regulated genes and 2872 down-regulated genes. There were 5581 differentially expressed genes in the control group A and the hypoxia group C, 2676 up-regulated genes and 2905 down-regulated genes. According to the enrichment of GO functional annotations, differential genes were mainly concentrated in the membrane of cell components, metabolism in biological processes and binding of molecular functions. The enrichment of KEGG pathway showed that multiple metabolic and genetic information processing pathways were activated. Control group (A), treatment group 1 (B) and treatment group 2(C) were set up with dissolved oxygen concentrations of 7.5 mg/L, 4 mg/L and 2 mg/L, respectively. The water temperature was controlled at 20±1 ℃ and the salinity was 27. There were Mactra veneriformis in each group and 40 clams in each parallel. Hypoxia stress is carried out by continuously filling nitrogen into the aquaculture water, and the experiment is carried out when the dissolved oxygen in the water reaches about 2 mg/L and 4mg/L. At the same time, the water surface is covered with plastic wrap to prevent air exchange at the water-air interface, and the dissolved oxygen in the water is maintained at about 2±0.2 and 4±0.2 mg/L. The oxygen concentration in the water body was measured by the dissolved oxygen meter every 4 h, and the low oxygen level was maintained during the whole experiment by adjusting the ventilation volume. In the control group, DO content in water was 7.5±0.2 mg/L after continuous oxygenation. The experiment period was 72 h without changing the water or feeding. During the experiment, the shellfish were observed every 2 hours, and the dead shellfish were picked out in time. The standard four-horn clam double shell was open, and no reaction was found when it touched the axe foot of the clam. The experimental group underwent vivisection of clams tetragonata after 24 h of stress, collected their gill tissues, put them into frozen tubes and quickly put them into liquid nitrogen, and then transferred the gill tissues to the refrigerator at -80 ℃ for storage. Three replicates were set in each group.