International Journal of Marine Science, 2024, Vol.14, No.5, 295-303 http://www.aquapublisher.com/index.php/ijms 299 membrane transport were identified as crucial for adaptation to deep-water conditions (Kess et al., 2021). These findings suggest that similar genomic mechanisms may be at play in deep-sea Ascidians, enabling them to thrive in such extreme environments. 5.2 Genomic responses to hypoxia in polar Ascidians Polar regions are characterized by extreme cold and often hypoxic conditions. The leathery sea squirt, Styela clava, has shown significant genomic adaptations to cold environments, including the expansion of gene families related to heat-shock proteins and the horizontal transfer of cold-shock protein genes from bacteria (Wei et al., 2020). These adaptations likely play a crucial role in enabling Ascidians to survive in hypoxic polar waters. Additionally, studies on Ciona robusta and Ciona savignyi have demonstrated species-specific alternative splicing responses to environmental stresses, including temperature variations, which may also contribute to their ability to cope with hypoxic conditions (Figure 3) (Huang et al., 2023). Figure 3 Gene expression and alternative splicing response of serine/arginine-rich splicing factor (SRSF) genes to environmental changes (Adopted from Huang et al., 2023) Image caption: (A) SRSF gene expression changes under recurrent high salinity stresses in C. robusta, of which Cr_SRSF7a and Cr_SRSF6b genes were excluded from differential expression analysis due to their low expression level. The Log2 foldchange values between treatment and control groups were used to draw the heatmap, and the color circles with black borders indicate significantly changed genes (adjusted p value < 0.05). (B) SRSFgene expression changes under low (LT) and high temperature (HT), and low (LS) and high salinity (HS) stresses in C. savignyi. (C) Transcript expression level of two isoforms of alternatively spliced gene (Cs_SRSF12) after 24 h of control and HT stress groups. (D) Percentage of two isoforms of Cs_SRSF12 gene. (E) Conserved domains of isoform1 (upper) and isoform2 (lower) (Adopted from Huang et al., 2023)
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