Occurrence of genetic drift and founder effect during quasispecies evolution of the VP2 and NS3/NS3A genes of bluetongue virus upon passage between sheep, cattle, and Culicoides sonorensis

Bluetongue virus (BTV) is the cause of an insect-transmitted virus infectio n of ruminants that occurs throughout much of the world. Individual gene se gments differ between field strains of BTV; thus, we hypothesized that key viral genes undergo genetic drift during alternating passage of BTV in its ruminant and insect hosts. To test this hypothesis, variation in the consen sus sequence and quasispecies heterogeneity of the VP2 and NS3/NS3A genes o f a plaque-purified strain of BTV serotype 10 was determined during alterna ting infection of vector Culicoides sonorensis and a sheep and calf. Consen sus sequences were determined after reverse trans criptase-nested PCR ampli fication of viral RNA directly from ruminant blood and homogenized insects, and quasispecies heterogeneity was determined by the sequencing of clones derived from directly amplified viral RNA. Comparison of these sequences to those of the original BTV inoculum used to initiate the cycle of BTV infec tion demonstrated, for the first time, that individual BTV gene segments ev olve independently of one another by genetic drift in a host-specific fashi on, generating quasispecies populations in both ruminant and insect hosts. Furthermore, a unique viral variant was randomly ingested by C. sonorensis insects that fed on a sheep with low-titer viremia, thereby fixing a novel genotype by founder effect. Thus, we conclude that genetic drift and founde r effect contribute to diversification of individual gene segments of field strains of BTV.