| Summary: | The past major climate changes have disrupted life in the polar regions and triggered different responses in marine organisms. Confronted now with fast-paced environmental changes such as global warming, understanding the adaptations and patterns of diversification in these regions with extreme environmental conditions can help to predict their possible response to future climate change. To do so, the genus Eusirus was chosen as a model organism because: 1) of its worldwide distribution with representatives in both polar regions; 2) local abundance and a large number of species showing high level of endemicity; 3) occupying both deep and shallow marine waters; 4) exhibiting a wide range of morphological variation; and 5) belonging to the Eusiridae family, which is a major family of polar amphipods. By combining molecular, morphological, and ecological data, this thesis aims to understand the genetic adaptations and the evolutionary processes that shaped the current diversity of the Eusirus amphipods in the Arctic and Southern Oceans. First, three novel complete Antarctic mitogenomes were assembled and annotated. Their analysis showed distinct features such as a lower AT-richness in the whole mitogenomes, negative GC-skews on both strands of protein coding genes, and unique gene rearrangements. These mitogenomes also shared characteristics with other amphipod mitogenomes including aberrant tRNA and short rRNA genes, which could be linked to minimalization of mitogenomes. Nucleotide diversity analysis revealed nad6 and atp8 to be the most variable mitochondrial regions of amphipods. In contrast, cox1 showed low nucleotide diversity among closely and more distantly related amphipod species. Second, molecular signatures of cold adaptations were highlighted by comparing different mitogenomic features of amphipods from cold, temperate, and warm regions. Among other results, amphipods living in cold environment possessed mitogenomes with low proportions of charged amino acids and high, average ratios of non-synonymous ...
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