It is not known with certainty when and where the progenitor of the approximately 80 species in the genus Flavivirus first arose, although geographic evidence suggests that this ancestral flavivirus may have first appeared in Africa. Over the course of speciation, the flaviviruses have shown substantial ecological diversification. Most notably, different lineages of flaviviruses adapted to different modes of transmission. A current phylogenetic tree of the genus Flavivirus shows that the basal-most lineages are viruses that have only been isolated from mosquitoes and are not known to infect vertebrates at all. This suggests that the ancestor of the genus may have been a 'mosquito-only' virus that later acquired the ability to infect vertebrates.

The remaining flaviviruses are divided into vector-borne viruses of vertebrates, with major groups using ticks and mosquitoes for horizontal transmission, and another group that infects vertebrates without the use of arthropod vectors. This topology does not suggest whether vector-borne or non-vectorborne transmission was ancestral, but the basal position of the 'mosquito-only' viruses suggests that mosquito-borne transmission among vertebrates may have preceded the loss of vector transmission. Tick-borne transmission may have evolved from a mosquito-borne lineage after the lineage that infects only vertebrates arose.

from Endy et al. in Frontiers in Dengue Virus Research

Labels: flavivirus, Mosquito-borne, Viruses of vertebrates