The cell's power package has a more complicated origin than we thought

The cell’s power package has a more complicated origin than we thought

For billions years after the origin of lifethe only living beings on earth were small, primitive cells similar to those of today bacterium. But then, more than 1.5 billion years ago, something remarkable happened: one of these primitive cells, belonging to a group known as archaeaswallowed another – a bacterium.

Instead of being digested, the bacterium took a permanent break in the other organism in what biologists call an endosymbiont. Eventually, it was fully integrated into its archaeal host cell, becoming what we know today as mitochondria, the crucial energy-producing component of the cell.

Its acquisition has long been seen as the key step in what is undoubtedly the most important evolutionary leap since the very beginning of life: the transition from early primitive cells, or prokaryotes, to the more sophisticated cells of higher organisms, or eukaryotes, including ourselves.

It’s a nice story that you will find in most biology textbooks – but is it that simple? In recent years, new evidence has challenged the notion that mitochondria played a crucial role in this transition. Researchers sequencing the genomes of modern relatives of the first eukaryotes have found many unexpected genes that do not appear to come from either host or endosymbiont. And that, according to some researchers, may mean that the development of the first eukaryotes involved more than two partners and took place more gradually than was suspected.

Others still see no reason to abandon the theory that the acquisition of mitochondria was the spark that ignited the rapid development of eukaryotes – which gave rise to eons later in plants, animals, vertebrates and humans. Recent evidence from genomics and cell biology can help resolve the debate, while pointing to knowledge gaps that still need to be filled to understand one of the fundamental events of our own lineage, the origin of complex cells.

Mysterious extras

The mystery genes appeared in the last decade when researchers, among others Toni Gabaldónan evolutionary genomicist at the Barcelona Supercomputing Center, and his colleagues used today’s inexpensive gene sequencing technology to explore the genomes of a wide range of eukaryotes, including several obscure, primitive, modern relatives of early eukaryotes.

They expected to find genes whose lineage is traced back to either the archaeal host or the mitochondrial ancestor, a member of a group called alphaproteobacteria. But to their surprise, the researchers also found genes that appeared to come from a wide range of other bacteria.

Gabaldón and colleagues assumed that the cellular ancestor of eukaryotes had acquired the genes from a variety of partners. These partners could have been additional endosymbionts that were later lost or free-living bacteria that sent one or more of their genes to the ancestral host in a common process called horizontal gene transfer. However, the tango that led to eukaryotes involved more than two dancers, they suggested.

“It is clear now that there are additional contributions from additional partners,” said Gabaldón, who wrote about the early development of eukaryotes during 2021 Annual review of microbiology.

It is difficult to know exactly where the old foreign genes came from because so much time has passed. But there are many newer, looser endosymbiosis where the origin of foreign genes is easier to identify, says John McCutcheon, an evolutionary cell biologist at Arizona State University in Tempe who wrote about endosymbiont evolution during 2021 Annual review of cell and developmental biology. Studying these can, by analogy, give us a chance to understand how mitochondria and the first eukaryotes could have evolved, he says.

Mealybugs, which share a symbiotic relationship with certain microbes. vinisouza128 / 500px / 500Px Plus / Getty Images

Cell mates

An excellent example is an approximately 100 million year old partnership between insects called aphids and two bacterial endosymbionts, one encapsulated inside the other in the aphids’ cells. (The endosymbionts make essential amino acids that the flour belly cannot get from its diet.)

Based on genomic analysis, McCutcheon and his colleagues found that the metabolic pathways of mealworms are now a mosaic consisting of genes that originated with the insects themselves, entered with their endosymbionts or were picked up by horizontal transmission from other microbes in the environment.

To make this work, McCutcheon’s team showed, mealworm cells had to develop a device that transports proteins back and forth between what were once independent organisms – allowing them from the mealworm cell to travel across two sets of endosymbione membranes to use the innermost endosymbionten.

Something similar occurs in a single-celled, amoeba-like eukaryote called Paulinella. Paulinella has an endosymbiont, engulfed tens of millions of years ago, which allows it to harvest energy from sunlight without chloroplast organelles which usually drives photosynthesis. Eva Nowackleading a lab at the University of Dusseldorf in Germany, discovered it Paulinellas genome now contains genes from endosymbiont along with others acquired through horizontal gene transfer.

Remarkably, endosymbiont import more than 400 proteins from the host, so it must also have developed a complex protein transport system such as mealybugs. “It’s pretty exciting,” says the molecular evolutionist Andrew Rogerwho is studying the development of organelles at Dalhousie University in Halifax, Canada, as it suggests that it is not as difficult to develop these transport systems as previously thought.

These examples illustrate how endosymbionts become integrated with their hosts and suggest that horizontal gene transfers from different sources could also have been quite frequent early in the development of eukaryotes. “It does not show that this is what happened in the formation of the mitochondria, but it does show that it is possible,” McCutcheon said.

Others agree. “There is a lot of strong evidence for horizontal gene transfer in eukaryotes, so there is really no reason to say that it could not have happened during that period of transition between the prokaryote and the eukaryote. In fact, it almost certainly did,” says Roger.

Shop genes

This means that the ancient host could have gradually acquired eukaryotic properties one at a time, such as a shopper putting things in a shopping bag, via horizontal gene transfers or by devouring a series of endosymbionts, explains John Archibald, a comparative genomicist at Dalhousie University. Some of these newly acquired genes could have been useful for the host as it developed the rest of the machinery found in modern eukaryotic cells.

If so, when the ancient host engulfed the precursor to the mitochondria, it would already have had many eukaryotic properties, perhaps including some organelles, the inner compartments surrounded by membranes – meaning that the mitochondria would not have been the main driver of eukaryotic evolution, but a late addition.

But despite all the evidence supporting a gradual hypothesis for the development of eukaryotes, there are some reasons for doubt. The first is that these newer endosymbiosis may not tell us much about what happened during the origin of the eukaryotes – after all, in these latter cases, the modern host cells were already eukaryotes.

“These examples tell you how easy it is, once you have a eukaryotic cell, to establish intracellular endosymbiosis,” says Bill Martin, an evolutionary biologist studying the origins of eukaryotes at the University of Düsseldorf. But eukaryotes already have all the intracellular machines needed to engulf another cell. It is not at all clear that the ancestor’s proto-eukaryote had that ability, says Martin – which would make the barrier for the first endosymbiosis much higher. It, for him, speaks against a gradual development of the eukaryotic cell.

In fact, some evidence suggests that important eukaryotic properties were acquired at once, rather than gradually. All eukaryotes have exactly the same set of organelles that are familiar to anyone who has studied cell biology: nucleus, nucleolus, ribosomes, coarse and smooth endoplasmic reticulum, Golgi apparatus, cytoskeleton, lysosome and centriole. (Plants and some other photosynthetic eukaryotes have an extra, the chloroplast, which everyone agrees was created by a separate endosymbiosis.)

There are strong indications that the other organelles all originated at about the same time – if they did not, different eukaryotic lines should have different mixtures of organelles, says Jennifer Lippincott-Schwartza cell biologist at the Howard Hughes Medical Institute’s Janelia Research Campus in Virginia.

Some biochemical evidence also points to this. The ancestor’s values ​​and endosymbiont belonged to different branches of the tree of life – archaea and bacteria, respectively – that use different molecules to build their membranes. None of the membranes in eukaryotic organelles have only an archeal structure, so it is unlikely that they came from the ancestral host cell. Instead, this suggests that the archaeal host was a relatively simple cell that developed its other organelles only after the arrival of the mitochondrial ancestor.

But what about all these mysterious alien genes recently found in the eukaryotic family tree? There is another possible explanation, says Martin. All of these foreign genes could have come in a single package of endosymbionts that evolved into mitochondria. Later – for 1.5 billion years after that event – these genes could have been spread among many bacterial groups, thanks to how easily the bacteria change genes. It would give the false impression that several partners contributed genes to the early eukaryote.

In addition, Martin adds, if the gradual idea is correct, different lines of eukaryotes should have fundamentally and measurably different collections of genes, but he has shown they do not. “There is no evidence to suggest that it was a serial acquisition,” says Martin. “A single acquisition of mitochondria at the origin of eukaryotes is sufficient.”

The debate is unlikely to be resolved soon. “It is very difficult to find data that allows us to clearly distinguish between these alternatives,” says Roger. However, if further studies of obscure, primitive eukaryotes revealed some that have only a subset of eukaryotic organelles, this could give weight to the gradual hypothesis. On the other hand, if evidence was found for a way that a simple archeal cell could acquire an endosymbiont, it would make the “early” mitochondrial hypothesis more plausible.

“People are drawn to big questions, and the harder they are to answer, the more people are drawn to them and discuss them,” says Archibald. “That’s what makes it fun.”

This article originally appeared in Knowledgeable newspaper, an independent journalistic endeavor from Annual Reviews. sign up for newsletter.

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