Positively selected genes
Species often face various several environmental pressures, which may explain the presence of several rapidly evolving, positively selected genes. Some specific gene loci may also undergo nucleotide substitutions. We determined positively selected genes in B. l. tsinlingensis and in the salmonid linage with E. lucius as the only outgroup species. B. l. tsinlingensis has the highest Ka/Ks value among these species, indicating that B. l. tsinlingensispossessed much more rapidly evolving genes (Figure 4E ). This result is consistent with the findings above indicating that B. l. tsinlingensis has the fastest evolutionary rate among salmonids. There were 36 positively selected genes inB. l. tsinlingensis and 14 positively selected genes in salmonids. Four positively selected genes in B. l. tsinlingensis were related to the immune response, includingc1qb ,cebpb ,tnfa , and psme1 (Table S21 ). c1qb is associated with the brain immune system and social behavior (Ma et al. 2015), cebpb is a key immune-related gene that may be involved in sepsis (Xu et al. 2020),tnfaplays a vital role in the immune response by regulating several pathways that produce an immediate inflammatory reaction (Holbrook et al. 2019), andpsme1may play an important role in the progression of Fanconi anemia to acute myeloid leukemia (Hou et al.2020). For the salmonid line, two positively selected genes were related to cell division and muscle development. btg3 may play an important role in tumor suppression and is a key effector kinase in the cell cycle checkpoint response (Chenget al.2013).rapsn has been shown to be associated with congenital myasthenic syndromes.cld10can form paracellular channels with ion selectivity; its variant causes anhidrosis and kidney damage (Joakimet al. 2017) (Table S22 ). Both muscle function and kidney function are important for the migration of salmonids, as salmonids need to be able to inhabit freshwater and seawater, which requires adaptation to both types of environments. The long-distance migration of salmonids also requires strong muscles. Both of these genes were positively selected in the salmonid line, which may be associated with the migratory habits of these species.