Mitochondria and the Origin of Eukaryotes

For billions of years after the origin of life, the only living points in the world were tiny, primitive cells resembling today’s bacteria. However, then, more than 1.5 billion years back, something remarkable occurred: One of those primitive cells, pertaining to a group called the archaea, swallowed a different one- a bacterium.

Within the other organism, the bacterium took up permanent residence instead of being digested, the same as what biologists call an endosymbiont. Ultimately, it incorporated fully into its archaeal host cell, turning into what is known today as the mitochondrion (the crucial energy-producing component of the cell).

Its acquisition has long been viewed as the critical step in what is arguably the most crucial evolutionary leap due to the origin of life itself: the transition from early primitive cells, or prokaryotes, to the more cultured cells of higher organisms, or eukaryotes, including ourselves.

The neat story will be discovered in most biology textbooks– however, is it quite that easy? In the last few years, new proof has challenged the notion that mitochondria played a seminal duty in this transition. Researchers sequencing the genomes of modern-day relatives of the first eukaryotes have found numerous unexpected genes that do not seem to come from either the host or the endosymbiont. Moreover, that, some scientists recommend, might mean that the development of the first eukaryotes involved more than two partners and happened more slowly than suspected.

Others do not see a reason yet to quit the theory that the acquisition of the mitochondrion was the spark that ignited the quick evolution of eukaryotes– giving rise, eons later, to plants, animals, and vertebrates, and people. Fresh proof from genomics and cell biology may assist resolve the debate while also indicating knowledge spaces that still need to be filled up to comprehend one of the foundational events in our own origins, the origin of complex cells.

Mysterious extras

The mystery genes showed up in the last decade when researchers, consisting of an evolutionary genomicist (Toni Gabaldón) at the Barcelona Supercomputing Centre, and his colleagues, took advantage of today’s low-cost gene sequencing technology for exploring the genomes of a wide range of eukaryotes, which includes many obscure, primitive, modern-day relatives of early eukaryotes.

They expected to find genes whose descent traced back to either the mitochondrial ancestor or the archaeal host, a member of a group called the alphaproteobacteria. However, to their surprise, the scientists likewise discovered genes that appeared to come from a wide range of other bacteria.

Gabaldón and colleagues assumed that the cellular ancestor of eukaryotes had obtained the genes from various partners. Those partners could have been extra endosymbionts that were later free-living bacteria or lost that passed one or a few of their genes to the ancestral host in a usual process called horizontal gene transfer. Either way, the tango leading to eukaryotes involved more than two dancers, they suggested.

“It is clear since there are additional contributions from additional partners,” states Gabaldón, the writer, about the early eukaryotes evolutions in the 2021 Annual Review of Microbiology.

It is difficult to know exactly where those ancient foreign genes came from because much time has elapsed. However, there are many more recent, looser endosymbioses where the origin of foreign genes is easier to identify, states John McCutcheon, an evolutionary cell biologist at Arizona State University in Tempe. The latter wrote about endosymbiont evolution in the 2021 Annual Review of Cell and Developmental Biology. Studying these might, by comparison, give us a shot at comprehending how mitochondria and the first eukaryotes could have evolved, he states.

Cellmates

A prime example is an about 100-million-year-old partnership between two bacterial endosymbionts and insects called mealybugs, one nested inside the other in the cells of mealybugs. (The endosymbionts make crucial amino acids that the mealybug can not receive from its diet.).

Based upon a genomic analysis, McCutcheon and his colleagues found that the mealybugs’ metabolic pathways are currently a mosaic made of genes that originated with the bugs themselves, came in with their endosymbionts, or were picked up by horizontal transfer in the environment from other microbes.

To make this work out, McCutcheon’s team showed, mealybug cells had to progress an apparatus that carries proteins to and fro between what were once independent organisms– permitting ones from the mealybug cell to travel across two sets of endosymbiont membranes for use by the innermost endosymbiont.

Something similar happens in a single-celled, amoeba-like eukaryote called Paulinella. Paulinella has an endosymbiont, swallowed up tens of millions of years back, that enables it to harvest energy from the sunlight without the chloroplast organelles that generally power photosynthesis. Eva Nowack, leading a laboratory at the University of Dusseldorf in Germany, found that Paulinella’s genome currently contains genes from the endosymbiont together with others that were acquired through horizontal gene transfer.

The endosymbiont remarkably imports more than 400 proteins from the host, so it likewise must have evolved a complex protein transport system like the mealybugs. “That is quite exciting,” says molecular evolutionist Andrew Roger, who studies the evolution of organelles at Dalhousie University in Halifax, Canada, since it recommends that evolving these transport systems anew is not as difficult as previously believed.

These instances show how endosymbionts become incorporated with their hosts and recommend that horizontal gene transfers from numerous sources could have been quite recurring early in the development of eukaryotes, as well. “It does not show that is what happened in the formation of the mitochondria; however, it reveals that it is possible,” states McCutcheon.

Others concur. “There is lots of solid evidence for horizontal gene transfer in eukaryotes, so there is actually no reason to say that it could not have occurred during that period of the prokaryote-eukaryote transition. As a matter of fact, it almost certainly did happen,” Roger says.

Shopping for genes

The effect is that the old host could have slowly obtained eukaryotic characteristics individually, like a shopper placing items in a shopping bag, utilizing a horizontal gene transfer, or swallowing a collection of endosymbionts, describes John Archibald, a comparative genomicist at Dalhousie University. Some of those freshly acquired genes could have been useful to the host as they evolved the remainder of the machinery found in modern eukaryotic cells.

If so, by the time the ancient host swallowed up the precursor of mitochondria, it would have already possessed numerous eukaryotic features, maybe including some organelles, the internal compartments bordered by membranes– implying that mitochondria would have been not the principal driver of eukaryotic development but a late addition.

However, despite all the proof supporting a gradualist hypothesis for the development of eukaryotes, there are some factors for doubt. The first is that these more current endosymbioses may not tell us much about what happened during the beginning of eukaryotes. After all, in these latter situations, the modern host cells were already eukaryotes.

“These examples inform how easy it is in having a eukaryotic cell to establish intracellular endosymbioses,” says Bill Martin, an evolutionary biologist who researches the origins of eukaryotes at the University of Dusseldorf. However, eukaryotes already have all the intracellular machinery required to swallow up another cell. It is not unclear that the ancestral proto-eukaryote had that ability, Martin states– which would make the obstacle to that first endosymbiosis much higher. That, to him, argues against a progressive evolution of the eukaryotic cell.

In fact, some proof recommends that key eukaryotic features were acquired simultaneously rather than slowly. All eukaryotes have the exact same collection of organelles familiar to anyone who has researched cell biology: ribosomes, centriole, nucleolus, nucleus, smooth and rough endoplasmic cytoskeleton, reticulum, Golgi apparatus, and lysosome. (Plants and a few other photosynthetic eukaryotes have one additional, the chloroplast, which everybody agrees arose via a separate endosymbiosis.).

That strongly recommends that the other organelles all originated at concerning the same time. If they did not, different eukaryotic family trees ought to have distinct mixes of organelles, states cell biologist Jennifer Lippincott-Schwartz, at the Howard Hughes Medical Institute’s Janelia Study Campus in Virginia.

Some biochemical proof points by doing this, too. The ancestral host and endosymbiont belonged to distinct branches of the tree of life– archaea and bacteria, respectively– that use distinct molecules to construct their membranes. None of the membranes of eukaryotic organelles are solely archaeal in structure, so it is unlikely they originate from the ancestral host cell. This recommends that, instead, the archaeal host was a relatively easy cell that developed its other organelles only after the mitochondrial ancestor arrived.

However, what about all those mysterious foreign genes recently found in the eukaryotic family tree? There is another possible explanation, Martin states. All those foreign genes could have arrived in a single package with the endosymbiont that developed into the mitochondrion.

Later on in the 1.5 billion years following that occasion– those genes could have been scattered among numerous bacterial groups, courtesy of the ease with which bacteria swap genes. That would provide the wrong impression that numerous partners contributed genes to the early eukaryote.

Moreover, Martin adds, if the gradualist suggestion is correct, distinct lineages of eukaryotes should have measurably and fundamentally different collections of genes, but he has revealed they do not. “There is no proof to suggest that there were serial acquisitions,” Martin says. “A single acquisition of mitochondria at the beginning of eukaryotes suffices.”.

The debate is unlikely to be settled quickly. “It is very hard to find data that is going to make us clearly differentiate these alternatives,” says Roger. However, if further research of obscure, primitive eukaryotes showed some that have simply a subset of eukaryotic organelles, this could lend weight to the gradualist hypothesis. On the other hand, if proof were found for a way that a simple archaeal cell could obtain an endosymbiont, that would make the “mitochondria early” hypothesis more plausible.

“Individuals are drawn to big questions, and the more difficult they are to answer, the more individuals are attracted to them and debate them,” states Archibald. “That is what makes it fun”.

Read the original article on Knowable Magazine.