From: A plant virus that switched to vertebrates (Virology Blog)
Viruses can be transmitted to completely new host species that they have not previously infected. Usually host defenses stop the infection before any replication and adaptation can take place. On rare occasions, a novel population of viruses arises in the new host. These interspecies infections can sometimes be deduced by sequence analyses, providing a glimpse of the amazing and unpredictable paths of virus evolution. One example is a plant virus that switched hosts and infected vertebrates.
Circoviruses infect vertebrates and have small, circular, single-stranded DNA genomes. Nanoviruses have the same genome structure, but infect plants. The genes encoding one of the viral proteins – called the Rep protein – appear to be hybrids, and share significant sequence similarity. They also exhibit homology with a protein encoded by caliciviruses, which are RNA viruses that infect many different vertebrates.
Analysis of the viral DNA sequences suggests that two remarkable events occurred during the evolution of circoviruses and nanoviruses. Not long ago, a nanovirus was transmitted from a plant to a vertebrate. This event might have occurred when a vertebrate fed on an infected plant. The virus adapted to vertebrates, and the circovirus family was established. After the host switch from plants to vertebrates, recombination took place between the circovirus and a vertebrate calicivirus. A reverse transcriptase probably converted the circovirus RNA genome to DNA to allow recombination to occur. (…)