Evolution: special genes for symbiosis? (Evolution)

by David Turell , Tuesday, April 26, 2022, 19:03 (940 days ago) @ David Turell

A new bacterial study:

https://www.quantamagazine.org/ancient-genes-for-symbiosis-hint-at-mitochondrias-origin...

"Was the addition of mitochondria a first step in the formation of complex cells or one of the last? A new study of bacteria tries to answer this contentious question in evolutionary biology.

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"Scientists still understand relatively little about what happened during that transformation. One of the central conundrums is how and when our eukaryotic ancestor acquired its mitochondria, the powerhouse organelles that generate the cell’s energy. The mitochondrion was clearly once an independent bacterium, until some host cell (an archaeon or descendant of one, from all the evidence) engulfed it and turned it into a permanent symbiotic partner.

"But the way eukaryotic cells engulf bacteria is energetically costly; it involves extensive and rapid remodeling of the cytoskeleton, the protein scaffolding beneath the cell membrane. A cell almost needs to possess mitochondria to do it, since mitochondria can wring about 18 times as much energy from a molecule of glucose as glycolysis and fermentation, the alternative metabolic processes. So scientists debate which came first: the mitochondrion or the engulfment process, known as phagocytosis.

"The two options suggest vastly different origin stories for eukaryotes: Was the mitochondrion an afterthought, a late arrival in the evolution of the first eukaryote? Or did it come early, with its spectacular energy-generating powers, and drive the changes in our ancestor?

"A recent paper in Molecular Biology and Evolution provides an intriguing peek into what might have happened when this chicken-or-egg dilemma played out more than 1.5 billion years ago. The researchers sequenced the DNA of more than 30 species of parasitic and symbiotic bacteria that, when engulfed by eukaryotic cells, avoid being digested and live off the resources of their hosts. The ability to dwell within eukaryotic cells, the scientists realized, seemed strikingly older than expected. It suggests — with some important caveats — that some version of phagocytosis predated mitochondria, setting the stage for the revolution to come.

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"Guy and his colleagues sequenced the genes of members of Legionellales ranging from the Legionnaires’ disease pathogen to bacteria discovered by expeditions trawling the ocean for exotic microbes. Unsurprisingly, the proteins that the parasites used to shroud their presence and extract nutrients from their hosts varied substantially. But the researchers also found that nearly all the Legionellales deployed these proteins with fundamentally the same molecular machinery, called the Type IVB secretion system, which they seemingly inherited from a shared ancestor countless generations ago.

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"... Guy and his colleagues tentatively dated the origin of the Legionellales group to 1.9 billion years ago.

"That’s a provocative number. If the group is that old and it inherited its machinery for hijacking eukaryotic cells from its earliest shared ancestor, this suggests that there were eukaryotic cells capable of phagocytosis to infect.

"But many current estimates, based on fossils and chemical evidence, place the first appearance of cells with mitochondria nearly half a billion years later. The timing lends credence, the researchers suggest, to the theory that mitochondria were a late addition to evolving eukaryotes.

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“'The history of life is written in genomes; we just have to learn how to read it,” said Martin.

"To understand how this fundamental transition happened, scientists may have to look at the problem from new angles. For instance, says Snel, it’s commonly thought that the first big change in the evolution of eukaryotes, whatever it was, set the stage for everything else.

"But the last addition to our ancestors’ toolkit may prove even more essential to understanding eukaryotes. “Whatever the final step was, it caused the extinction of all the other competitors. Maybe that was the most important,” said Snel.

“'Scientists like a good mystery,” he said. “I just want to know what happened.'”

Comment: An intersting approach to a tough problem. Looking at God, perhaps this is a sign of pre-planning for a future necessary event.