3.4 Gene family expansion and contraction
This approach revealed 1,191 contracted gene families in the S. peregrina , and only 568 gene families had a greater degree of expansion(Fig. 3, Additional file 1: Table S22) . The corresponding genes were identified from these gene families, including 367 contracted genes and 2,805 expanded genes, which were utilized for enrichment analyses of KEGG and GO, respectively. They mainly encoded synthesis of hydrolases, amino acid metabolism, fatty acid synthase activity, olfactory receptor activity, fibroblast growth factor receptor activity as well as chorionic membrane formation, etc. (Additional file 2: Tables S24 and S25). Besides, a total of 692 positively selected genes were identified and then analyzed by KEGG and GO enrichment analyses (Additional file 2: Table S26 and S27). These genes encoded fatty acid alpha-hydroxylase activity, epidermis morphogenesis, transferase activity, etc.