International Journal of Marine Science, 2024, Vol.14, No.5, 295-303 http://www.aquapublisher.com/index.php/ijms 296 transposon number and variation in dominant types (Chen et al., 2020). Additionally, the genomes of Ascidians like Ciona robusta and Ciona savignyi show differences in exon/intron size distributions, which contribute to variations in alternative splicing responses to environmental changes (Huang et al., 2023). 2.2 Unique genomic elements related to adaptation Ascidians have developed several unique genomic elements that facilitate their adaptation to diverse environments. For example, the genome of Styela clava contains an expanded number of genes encoding heat-shock proteins and members of the complement system (Figure 1), which are crucial for stress responses. Additionally, cold-shock protein genes have been horizontally transferred from bacteria, further aiding in environmental adaptation (Wei et al., 2020). In Ciona robusta, significant genetic differentiation among populations has been observed, with specific loci showing signatures of directional selection in response to local environmental conditions (Lin et al., 2017). Moreover, DNA methylation patterns driven by local environments have been identified, indicating a strong epigenetic component in adaptation (Hoban et al., 2016). Figure 1 Genome map, gene family and phylogenetic position of Styela clava(Adopted from Wei et al., 2020) Image caption: a, Chromosome-level genome map of S. clava; b, Venn diagram of common and unique gene families among five ascidian species; c, The phylogenetic position, divergence time estimation and gene family analysis of S. clava (Adopted from Wei et al., 2020)
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