Evolutionary relationships among bluetongue and related orbiviruses

Polymerase chain reaction (PCR) sequencing of specific viral gene segments was used to investigate the phylogenetic relationships among the orbiviruses. Sequence comparisons of the bluetongue virus (BTV) RNA3 from different regions of the world (North America, South Africa, India, Indonesian, Malaysia, Australia and the Caribbean region) showed that geographic separation had resulted in significant divergence, consistent with the evolution of distinct viral populations. There were at least 3 topotypes (Gould, 1987); the Australasian, African - American and another topotype represented by BTV 15 isolated in Australia in 1986. The topotypes of BTV had RNA3 nucleotide sequences that differed by approximately 20 per cent. Analysis of BTV-specific gene segments from animal and insect specimens showed that bluetongue viruses had entered northern Australia from South East Asia, possibly by wind-borne vectors. Nucleotide sequence comparisons were used to show the close genetic relationship between BTV 2 (Ona-A strain) from Florida and BTV 12 from Jamaica, and to investigate the reassortment of BTV genome segments in nature. The mutation rates of the BTV RNA2 and RNA3 segments were estimated to be of the order of 10(-4) nucleotide changes/site/year, similar in magnitude to that reported for other RNA viruses.