Positive selection
All sequences of these single-copy genes were aligned using MUSCLE software (Edgar 2004a, b) with default parameters. To estimate the lineage-specific evolutionary rate of each branch, the free model (“model = 1, NSsites = 0”) in codeml was used. After we obtained a general evolutionary pattern of selective pressure along the lineages, the branch-site model was used. We used the likelihood ratio test and Chi-square test to calculate the P-value , and any positively selected genes with P-values less than 0.05 were retained.