Molecular evolution of MERS coronavirus: Dromedaries as a recent intermediate host or long-time animal reservoir?

While dromedary camels are the immediate animal source of MERS coronavirus (MERS-CoV) infection, the evolutionary origin of MERS-CoV remains obscure. We analyzed 219 camel and human MERS-CoV genome sequences available in GenBank. Phylogenetic analysis showed that 5 and 214 strains belong to clade A and B, respectively, with clade A further divided into lineage A1 (3 human strains) and lineage A2 (2 camel strains), and clade B divided into B1 to B6 (each containing both human and camel strains). Recombination analysis showed potential recombination events in five strains from dromedaries in Saudi Arabia, with recombination between lineage B5 and B3 in four strains, and between lineage B3 and B4 in one strain. The spike protein showed the highest number of amino acid substitutions, especially between A2 and other lineages, and contained positively selected codons. Notably, codon 1020 was positively selected among B and B5 strains, and can distinguish between clade A (Q1020) and B (R1020/H1020) strains, suggesting that this residue may play a role in the evolution of S protein during divergence of different lineages. The time of the most recent common ancestor of all MERS-CoV was dated to approximately 2010. The implications on the role of camels in the evolution of MERS-CoV are discussed.