3.5 Expansion and contraction of gene families and positively selected genes
To investigate the adaptive evolution of C. undulatus , we estimated the expansion and contraction of gene families. A total of 560 expanded gene families and 1,848 contracted gene families were identified in the C. undulatus genome (Fig. 6B). A total of 430 genes belonging to 199 significantly expanded gene families (P< 0.05) were subjected to a GO term enrichment analysis. We found that the significantly expanded gene families were mainly associated with oxidation- reduction processes, oxidoreductase activity, and catalytic activity (Fig. S2). According to the branch site ofC. undulatus in the evolutionary tree, we found that 46 genes were subjected to significantly positive selection (P<= 0.05), and these genes function in fatty acid synthase, elongator complex protein 1 (ELP1), and hepatocyte growth factor-like protein, identified by SWISS-PROT for function annotation (Table S10). Interestingly, ELP1 is the largest subunit of the evolutionarily conserved elongator complex, which catalyzes translational elongation, and the loss of function variants leads to protein misfolding and aggregation, which predisposes tumor development (Waszak et al.2020). Positively selected ELP1 in C. undulatus may play an important role in development, and its impact could be investigated in the future.