Selection for plaque variants of two California group arboviruses (Jamestown canyon and La Crosse) by passage in natural vertebrate hosts.

The plaque size and distribution of prototype La Crosse (LAC) and Jamestown Canyon (JC) viruses were investigated in Vero cell cultures. The effect of serial passage of the viruses in their natural vertebrate hosts - the chipmunk and grey squirrel for LAC and the white-tailed deer for JC virus - was studied. Prototype JC virus was predominately a pinpoint plaque type (about 0.3 mm in diameter). A large plaque variant (about 1.0 mm in diameter), which was only 1 percent of prototype JC virus, increased to over 80 percent of the resultant virus after one passage in white-tailed deer. The large plaque type (about 1.0 mm in diameter) also predominated in JC isolates from mosquitoes, biting flies, and a white-tailed deer in Wisconsin. Prototype LAC virus consisted of a variety of plaque sizes, ranging in diameter from 0.3 mm to 3.0 mm. Passage through chipmunks increased the mean plaque diameter almost twofold, whereas passage in the grey squirrel resulted in a more uniform small plaque population practically eliminating the largest plaques. Preliminary results suggest that the resultant viruses are antigenically different. If these selective processes occur in nature, they man explain how four serologically related California group arboviruses with distinct vector-host cycles in nature could have evolved and have sympatric distribution.