International Journal of Marine Science, 2024, Vol.14, No.5, 295-303 http://www.aquapublisher.com/index.php/ijms 300 5.3 Lessons fromAscidians in anthropogenic impacted areas (e.g., pollution) Ascidians in areas impacted by human activities, such as pollution, exhibit notable genomic adaptations. For example, the Ascidian Pyura chilensis shows significant genetic differentiation across environmentally heterogeneous regions, driven by local adaptation to factors such as sea surface temperature and upwelling-associated variables (Segovia et al., 2020). Similarly, the marine invasive Ascidian Molgula manhattensis has demonstrated genomic signatures of local adaptation to salinity-related variables, highlighting the role of environmental selection in driving adaptive divergence (Chen et al., 2021). These studies underscore the importance of understanding the genomic basis of adaptation in Ascidians to predict their responses to anthropogenic changes and to develop effective conservation strategies. 6 Symbiotic Relationships and Their Genomic Impacts 6.1 Symbiosis with microorganisms inAscidians Ascidians, or sea squirts, are known to harbor diverse and host-specific microbial communities. These symbiotic relationships are crucial for the Ascidians' adaptation to various environments. For instance, studies have shown that Ascidians host a wide range of microbial symbionts, including bacteria and archaea, which are distinct from the surrounding seawater bacterioplankton. These microbial communities are involved in essential functions such as ammonia-oxidization, denitrification, and heavy-metal processing, which can enhance the host's tolerance to different environmental conditions (Evans et al., 2017). Additionally, the microbiomes of Ascidians have been found to include bacteria from phyla such as Cyanobacteria, Proteobacteria, Bacteroidetes, Actinobacteria, and Planctomycetes, which contribute to various ecological roles, including UV protection and defense against predators (Matos and Antunes, 2021). 6.2 Genomic modifications influenced by symbiotic interactions Symbiotic interactions can lead to significant genomic modifications in both the host and the symbionts. For example, the genome of the Ascidian Styela clava has undergone expansion due to an increase in transposon numbers and variations in dominant types. This genomic expansion includes the horizontal transfer of cold-shock protein genes from bacteria, which likely aids in the adaptation to cold environments (Wei et al., 2020). Similarly, the genome of the sponge symbiont "Candidatus Synechococcus spongiarum" has undergone streamlining, losing genes related to environmental toxin resistance and polysaccharide biosynthesis, which suggests adaptation to the stable environment within the sponge host (Gao et al., 2014). These genomic changes highlight the dynamic nature of symbiotic relationships and their impact on the genetic architecture of the involved organisms. 6.3 Role of symbiosis inAscidians' adaptation to extreme environments Symbiotic relationships play a crucial role in the adaptation of Ascidians to extreme environments. The presence of specific microbial communities can enhance the host's ability to withstand harsh conditions. For instance, the microbial symbionts in Ascidians have been linked to functions such as heavy-metal processing and bioaccumulation, which can be vital for survival in polluted environments (Knobloch et al., 2019). Moreover, the integration of microbial genes into the host genome, such as the cold-shock protein genes in Styela clava, provides additional mechanisms for coping with extreme temperatures (Rúav et al., 2016). These symbiotic interactions not only facilitate the immediate survival of Ascidians in challenging environments but also drive long-term evolutionary adaptations through genomic modifications. 7 Environmental Stress and Adaptive Evolution inAscidians 7.1 Impact of climate change onAscidians Climate change poses significant challenges to marine organisms, including Ascidians, by altering temperature and salinity levels in their habitats. The leathery sea squirt (Styela clava) has shown remarkable genomic adaptations to these changes, including the expansion of heat-shock protein genes and the horizontal transfer of cold-shock protein genes from bacteria, which help it survive in varied environmental conditions (Wei et al., 2020). Additionally, the estuarine oyster study highlights the role of gene expansion in solute carrier families, which are crucial for temperature and salinity stress responses, suggesting similar mechanisms might be at play in Ascidians (Li et al., 2021).
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