RNA-Seq Reveals Extensive Transcriptional Response to Heat Stress in the Stony Coral Galaxea fascicularis

Abstract

Galaxea fascicularis, a stony coral belonging to family Oculinidae, is widely distributed in Red Sea, the Gulf of Aden and large areas of the Indo-Pacific oceans. So far there is a lack of gene expression knowledge concerning this massive coral. In the present study, G. fascicularis was subjected to heat stress at 32.0 ± 0.5°C in the lab, we found that the density of symbiotic zooxanthellae decreased significantly; meanwhile apparent bleaching and tissue lysing were observed at 10 h and 18 h after heat stress. The transcriptome responses were investigated in the stony coral G. fascicularis during heat bleaching using RNA-seq. A total of 42,028 coral genes were assembled from over 439 million reads. Gene expressions were compared at 10 and 18 h after heat stress. The significantly upregulated genes found in the Control_10h vs. Heat_10h comparison, presented mainly in GO terms related with DNA integration and unfolded protein response; and for the Control_18h vs. Heat_18h comparison, the GO terms include DNA integration. In addition, comparison between groups of Control_10h vs. Heat_10h and Control_18h vs. Heat_18h revealed that 125 genes were significantly upregulated in common between the two groups, whereas 21 genes were significantly downregulated in common, all these differentially expressed genes were found to be involved in stress response, DNA integration and unfolded protein response. Taken together, our results suggest that high temperature could activate the stress response at the early stage, and subsequently induce the bleaching and lysing through DNA integration and unfolded protein response, which are able to disrupt the balance of coral-zooxanthella symbiosis in the stony coral G. fascicularis.

Keywords: DNA damage; RNA-seq; coral reef; heat stress; immunoregulation; unfolded protein response.

Figure 1

Bleaching and lysing in G. fascicularis during heat stress treatment. No apparent bleaching and lysing were observed in control groups (C_10h and C_18h), while in group H_10h (10 h after heat stress treatment), some phenomena of bleaching and lysing initiation began to appear, the cup edges of a few polyps and the coenosarcs turned white (yellow ellipse). As the heat stress continued, in group H_18h (18 h after heat stress treatment), the coenosarcs became lysed and degraded and the tentacles were dissolved into dark brown mucus (red ellipse).

Figure 2

The PCA analysis of the expression level of all genes in four transcriptome groups (Control_10h, Heat_10h, Control_18h, and Heat_18h) of coral G. fascicularis after heat stress.

Figure 3

The matrix of differentially expressed genes between any two groups (Control_10h, Heat_10h, Control_18h, and Heat_18h) of coral Galaxea fascicularis.

Figure 4

The overrepresented GO terms of the significantly upregulated genes in the Control_10h/Heat_10h comparison of coral Galaxea fascicularis.

Figure 5

The overrepresented GO terms of the common significantly upregulated genes in the Control_10h/Heat_10h and Control_18h/Heat_18h comparisons in coral Galaxea fascicularis.

Figure 6

The overrepresented GO terms of the significantly downregulated genes in the Control_18h/Heat_18h comparison of coral Galaxea fascicularis.