EVOLUTION OF ALPHAVIRUSES IN THE EASTERN EQUINE ENCEPHALOMYELITIS COMPLEX

Evolution of viruses in the eastern equine encephalomyelitis (EEE) com plex was studied by analyzing RNA sequences and oligonucleotide finger prints from isolates representing the North and South American antigen ic varieties. By using homologous sequences of Venezuelan equine encep halomyelitis virus as an outgroup, phylogenetic trees revealed three m ain EEE virus monophyletic groups. A North American variety group incl uded all isolates from North America and the Caribbean. One South Amer ican variety group included isolates from the Amazon basin in Brazil a nd Peru, while the other included strains from Argentina, Guyana, Ecua dor, Panama, Trinidad, and Venezuela. No evidence of heterologous reco mbination was obtained when three separate regions of the EEE virus ge nome were analyzed independently. Estimates of the overall rate of EEE virus evolution (nucleotide substitution) were 1.6 X 10(-4) substitut ion per nucleotide per year for the North American group and 4.3 x 10( -4) for the Argentina-Panama South American group. Evolutionary rate e stimates for the North American group increased over 10-fold (from abo ut 2 x 10(-5) to 4 x 10(-4)) concurrent with divergence of two monophy letic groups during the early 1970s. The North and South American anti genic varieties diverged roughly 1,000 years ago, while the two main S outh American groups diverged about 450 years ago. Analysis of multipl e strains isolated from an upstate New York transmission focus during the same years suggested that, in certain locations, EEE virus may be relatively isolated for short time periods.