The origin and evolution of porcine reproductive and respiratory syndrome viruses

Porcine reproductive and respiratory syndrome viruses (PRRSV) are divided into North American and European types, which show about 40% difference in their amino acid sequences. The divergence time of these two types has been estimated to be about 1980 from epidemiological data. This suggested that PRRSV have evolved at a higher evolutionary rate (order of 10(-2)/site/year) compared with other RNA viruses of 10(-3) to 10(-5)/site/year. Here, to test the evolutionary history of PRRSV speculated by the epidemiological background, we estimated the divergence time and evolutionary rate of PRRSV with molecular evolutionary analysis. Estimated divergence time (1972-1988) corresponded well to that estimated by the epidemiological data, and the evolutionary rate (4.71-9.8) x 10(-2) of PRRSV was indeed the highest among RNA viruses so far reported. Furthermore, we inferred important sites for the adaptation in order to examine how PRRSV have adapted to swine since they emerged. The adaptive sites were located not only in the epitopes related to immunity but also in the transmembrane regions including a signal peptide. In particular, the adaptive sites in the transmembrane regions were considered to affect compatibility to the host cell membrane. We conclude that PRRSV were transmitted from another host species to swine in about 1980 and have adapted to swine by altering the transmembrane regions.