Viruses are generally supposed to be the minimal forms of life – if they can even be regarded as “alive” at all. The discovery of very large viruses, as large as bacteria, has suggested that some modification of that view may be necessary.
A few months ago, the discovery was announced of a virus with more than 700,000 base pairs in its genome (more than some bacteria) and that is so large it may host other viruses as parasites.
Before that, another virus (mimivirus) had already been found with a million base pairs in its genome and some genes previously found only in full-fledged cells. A year ago an apparent mimivirus relative was found with 700,000 base pairs. This one also attracts molecular parasites. And though it is related to mimivirus, the two have only about a third of their genes in common – meaning that many of the genes of both came from somewhere else.
Now a virus has been discovered whose genome sets a new record size (for a virus) – 1.26 million base pairs. This one, Megavirus chilensis, seems to be a closer mimivirus relative. It has 1120 protein-coding genes, only 250 of which don’t have a mimivirus equivalent. Of the genes that are shared, about half are transcribed to the same proteins. And a comparison between mimivirus and the new one suggests something rather surprising about virus evolution.
Giant viruses may have evolved from cellular organisms, not the other way around
[The] find supports the view that the virus started out with a much larger complement of genes. For example, Mimivirus has a suite of genes that can help repair DNA. Megavirus has those plus one other that is specialized for the repair of DNA damaged by UV light. The additional gene appears to be functional: Megavirus was able to grow following an exposure to UV that was sufficient to disable Mimivirus.
Both viruses share an identical set of genes involved in transcribing their DNA into RNA, and use an identical set of signals to indicate where the transcripts should start and stop. Mimivirus also contains a number of genes used in the translation of RNA into protein. Megavirus has those plus a few more, including additional genes that attach amino acids (components of proteins) onto RNAs for use in translation.
Clearly, the common genes suggest that the viruses share a common ancestor. This leaves two possibilities for the novel ones: either the ancestral virus had a larger collection and its descendants have lost different ones, or each virus picked up different genes from its hosts through a process called horizontal gene transfer. The authors favor the former explanation, because most of the genes specific to one of the two viruses don’t look like any gene present in their hosts (or any other gene we’ve ever seen, for that matter). This implies that horizontal gene transfer doesn’t seem to have done much to shape the viruses’ genomes.
Given two fairly similar yet different viruses, inferences can be made about their evolution. The surprising conclusion is that these viruses had a common ancestor that was a very early eukaryotic cell. So instead of complex units of life having evolved from simpler forms, the exact opposite seems to have occurred in this case.
Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae
Megavirus exhibits three additional aminoacyl-tRNA synthetase genes (IleRS, TrpRS, and AsnRS) adding strong support to the previous suggestion that the Mimivirus/Megavirus lineage evolved from an ancestral cellular genome by reductive evolution. The main differences in gene content between Mimivirus and Megavirus genomes are due to (i) lineages specific gains or losses of genes, (ii) lineage specific gene family expansion or deletion, and (iii) the insertion/migration of mobile elements (intron, intein).
That evokes eerie echoes of other research reported only a few days ago: Last Universal Common Ancestor had a complex cellular structure
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