International Journal of Marine Science, 2024, Vol.14, No.5, 295-303 http://www.aquapublisher.com/index.php/ijms 298 underscores the genetic mechanisms that enable S. clava to thrive in fluctuating thermal environments (Wei et al., 2020). Similarly, the ascidian Pyura chilensis demonstrates significant genetic differentiation across environmental gradients, particularly around the 30 °S transition zone of the Humboldt Current System. This differentiation is driven by adaptive loci correlated with sea surface temperature and other environmental variables, highlighting the role of local adaptation in maintaining genetic structure despite potential gene flow (Segovia et al., 2020). 4.2 Role of horizontal gene transfer in habitat specialization Horizontal gene transfer (HGT) plays a crucial role in the genomic adaptation of Ascidians to their environments. In Styela clava, the acquisition of cold-shock protein genes from bacteria exemplifies how HGT can introduce novel genetic material that enhances environmental resilience. This genetic exchange allows S. clava to better cope with cold stress, which is particularly advantageous in temperate and polar regions (Valero et al., 2021). The integration of these horizontally transferred genes into the host genome and their subsequent functional assimilation illustrate the dynamic nature of ascidian genomes in response to environmental pressures (Li, 2024). 4.3 Case study: genomic insights from polar and tropical Ascidians The study of Ascidians from polar and tropical regions provides valuable insights into the genomic basis of environmental adaptation. For instance, the genomic analysis of Styela clava reveals significant expansions in gene families associated with stress responses, which are critical for survival in both cold polar waters and warmer temperate zones (Feng et al., 2021). Additionally, the research on Pyura chilensis along the southeast Pacific coast demonstrates how local environmental factors, such as temperature and productivity, drive adaptive genetic differentiation (Figure 2). This differentiation is crucial for the species' ability to inhabit diverse ecological niches within the Humboldt Current System (Segovia et al., 2020). Figure 2 Redundancy analysis (RDA) showing the relative contributions of oceanographic variables to the genetic structure of outlier and neutral genotypes. SNP genotypes in gray; individuals are represented by different colors according to their location according to the map in the right panel. Plot shows the most relevant variables obtained with ordistep and ordiR2step functions (Adopted from Segovia et al., 2020) 5 Case Study: Ascidians in Extreme Environments 5.1 Adaptation mechanisms in deep-sea Ascidians Deep-sea environments present unique challenges such as high hydrostatic pressure, low temperatures, and limited light. While specific studies on deep-sea Ascidians are limited, insights can be drawn from related marine organisms. For instance, the genomic basis of adaptation in Arctic Charr (Salvelinus alpinus) to deep-water habitats has been explored, revealing significant genetic divergence between deep and shallow water morphs (Wang et al., 2024). Genes involved in processes such as gene expression, DNA repair, cardiac function, and
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