From “Could a Newly Discovered Viral Genome Change What We Thought We Knew About Virus Evolution?” (ScienceDaily, Apr. 18, 2012), we learn,
Astonishingly, they found a unique viral genome that has never before been reported — a circular, single-stranded DNA virus encoding a major capsid protein seen previously only in RNA viruses. This unusual genome provides proof that integration of an RNA virus into a DNA virus may have occurred between two unrelated virus groups at some point in evolution — something that has not been observed before. Moreover, this suggests that entirely new virus types may emerge via recombination of functional and structural modules between vastly different viruses, using mechanisms that are as-yet unknown.
The team observed that the Boiling Springs Lake RNA-DNA hybrid virus (BSL RDHV) genome is circular, but its size is roughly double that of typical circoviruses, with the ORFs arranged in an uncommon orientation. They compared the BSL RDHV genome to other metagenomic DNA sequences from the Global Ocean Survey, and found strong evidence to conclude that previously undetected BSL RDHV-like viruses could be widespread in the marine environment and are likely to be found in other environments as well.
No mechanism has been proposed to account for the inferred instances of interviral RNA-DNA recombination, but the team speculate that a DNA circovirus-like progenitor may have acquired a capsid protein gene from a ssRNA virus via reverse transcription and recombination.
A hopeful sign is that no one is yakking up a Darwin tale about the mechanism; it is identified as unknown.