FIG. 3.

Hypothetical scenario for the evolution of CoV cluster 1a. (a) Rooted neighbor-joining tree inferred from multiple amino acid sequence alignments of the M and N proteins, illustrating the evolutionary relationships between members of phylogroup 1a. Human CoV 229e served as an outgroup. Support from bifurcations from 100 bootstraps is indicated. PRCoV, porcine respiratory CoV. (b) CCoV-I and FCoV-I apparently arose from a common ancestor by linear descent. As these viruses diverged, several distinct RNA recombination events led to the emergence of CCoV-II, FCoV-II, and TGEV. Details are explained in the text. Question marks indicate steps that are not yet completely understood.