Earlier this week, researchers announced the discovery of a distant cousin of Mimivirus, an unusually large viruses of amoebas and possibly of humans discovered in 2003, in the waters of the Pacific Ocean. Named Megavirus chilensis, the virus is notable for its extremely large size and genome, with scientists comparing both to that of a rudimentary bacteria. Genes of note include those encoding for tRNA synthases not seen in Mimivirus, which itself contained the first seen outside of cellular life. This, in combination with a great deal of homology with the complex genome of Mimivirus, lead researchers to speculate that this virus is one of the closest found to approach autonomy and resemble typical life.
The paper announcing this find, which was published in PNAS, goes on to speculate that these properties offer further proof of a reductive evolutionary theory of viruses. This theory postulates that viruses were all once cells themselves that, after falling into an evolutionary niche of exploiting other cells, slowly lost their resemblance to and autonomy from cells in order to streamline their proliferation and cut away all functions that couldn't be taken over from the host. While the complex genome of mimivirus itself could be explained as a fluke of horizontal gene transfer, the paper argues that the homology seen between mimivirus and megavirus in combination with their unique genes points to a common viral ancestor, a cellular organism.
Sorry if this is too broad of a topic seeing as how megavirus was found by proliferating samples through amoebas, but this presents an interesting development in theories about the evolution of viruses.
WIRED article: http://www.wired.com/wiredscience/2011/10/megavirus-cell-evolution/
Distant Mimivirus Relative with a larger genome highlights the fundamental nature of Megaviridae