Do Bacteria Age? Biologists Discover the Answer Follows Simple Economics

Do bacterial cells get “older” after repeated divisions? What would that even mean, once an original “mother” cell has divided into two “daughters”? Well, there are certain things one might look for as indicators of aging. For example, accumulation of molecular damage and slower rates of division in succeeding generations.

Previous studies have given conflicting results. One one hand it seems that many bacterial cells in later generations do show signs of having aged. But on the other, if aging is measured over the average of the whole population, it isn’t found.

The latest study found that, just as a population genetic model predicted would be optimal, half the daughter cells in a new generation appeared to be young and rejuventated, while the other half had accumulated the signs of aging.

Do Bacteria Age? Biologists Discover the Answer Follows Simple Economics

“Aging in organisms is often caused by the accumulation of non-genetic damage, such as proteins that become oxidized over time,” said Lin Chao, a professor of biology at UC San Diego who headed the study. “So for a single celled organism that has acquired damage that cannot be repaired, which of the two alternatives is better—to split the cellular damage in equal amounts between the two daughters or to give one daughter all of the damage and the other none?”

The UC San Diego biologists’ answer—that bacteria appear to give more of the cellular damage to one daughter, the one that has “aged,” and less to the other, which the biologists term “rejuvenation”—resulted from a computer analysis Chao and colleagues Camilla Rang and Annie Peng conducted on two experimental studies.

Further reading:

Temporal Dynamics of Bacterial Aging and Rejuvenation

Giant viruses may have evolved from cellular organisms, not the other way around

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

Further reading:

Megavirus claims ‘biggest genome’ crown

World’s largest virus proves giants came from cells

Ocean hunt nets world’s biggest virus

Ocean trawl reveals ‘megavirus’