Phylogenetic tree of coronaviruses (CoVs) based on the nucleotide sequences of RNA dependent RNA polymerase (RdRp). The Tree, with 1,000 bootstrap values, was constructed by the maximum likelihood method using MEGA 6. The four main phylogenetic clusters correspond to genera alpha-CoV, beta-CoV, gamma-CoV, and delta-CoV. Each CoV genus contains different subgenera. The letters in blue indicate human CoVs.