Nudging Mitochondria Along

It the previous posting, we saw there was good reason to think mitochondria were a necessary prerequisite for the evolutionary emergence of metazoan-type complexity.  Again, as Lane and Martin clearly point out:

Our considerations reveal why the exploration of protein sequence space en route to eukaryotic complexity required mitochondria. Without mitochondria, prokaryotes—even giant polyploids—cannot pay the energetic price of complexity; the lack of true intermediates in the prokaryote-to-eukaryote transition has a bioenergetic cause.

So we can see that natural selection, functioning as a designer-mimic, is, like other designers, constrained (and thus guided) by the materials used to express the design.  Just as there is no reason to think natural selection could craft something as complex and sophisticated as the prokaryotic cell without proteins, natural selection apparently cannot craft something as complex as a mouse or squid without the eukaryotic cell plan.  That’s why cells had to be first re-tooled through an endosymbiotic relation.

But why haven’t bacteria, after billions of years, ever been able to discover a method of evolving something mitochondrial-like without relying on endosymbiosis?   At first, it might seem to be simply an issue of scale, as the typical mitochondrion is roughly the same size as the typical bacteria.  But there are bacteria that are as large as some eukaryotic cells and it looks like they try to mimic mitochondria, but never quite make it.  One such bacterium is Thiomargarita.  

Lane and Martin discuss this bacterium and Myers summarizes their argument as follows:

the largest known bacterium, Thiomargarita, can reach a diameter of a half-millimeter. It gets more metabolic power in a similar way to how eukaryotes do it: we eukaryotes carry a population of mitochondria with convoluted membranes and a dedicated strand of DNA, all to produce energy, and the larger the cell, the more mitochondria are present. Thiomargarita doesn’t have mitochondria, but it instead duplicates its own genome many times over, with 6,000-17,000 nucleoids distributed around the cell, each regulating its own patch of energy-producing membrane. It’s functionally equivalent to the eukaryotic mitochondrial array then, right?

Wrong. There’s a catch. Mitochondria have grossly stripped down genomes, carrying just a small cluster of genes essential for ATP production. One hypothesis for why this mitochondrial genome is maintained is that it acts as a local control module, rapidly responding to changes in the local membrane to regulate the structure. In Thiomargarita, in order to get this fine-tuned local control, the whole genome is replicated, dragging along all the baggage, and metabolic expense, of all of those non-metabolic genes.

Because it is amplifying the entire genomic package instead of just an essential metabolic subset, Thiomargarita’s energy output per gene plummets in comparison.

Myers then adds:

So, what Lane and Martin argue is that the segregation of energy production into functional modules with an independent and minimal genetic control mechanism, mitochondria with mitochondrial DNA, was the essential precursor to the evolution of multicellular complexity — it’s what gave the cell the energy surplus to expand the genome and explore large-scale innovation.

So the trick is to get functional modules of energy production that can be distributed throughout the cell, each under some form of genetic control.  Endosymbiosis provides a clear and easy route to that objective – engulf a symbiont that encodes an electron transport chain and then strip out its genes that would not be needed to serve these metabolic ends.  In other words, the membrane and metabolic proteins are delivered in one package and the blind watchmaker just needs to remove the superfluous information and material over time.

The alternative is for the blind watchmaker to build these energy modules with independent genetic control from the bottom-up, by duplicating and changing material already present in the cell that would be host in an endosymbiotic relation.  But this route appears to be closed off, as no species of bacteria has ever been able to build a mitochondrion-like structure like this.  In fact, Lane and Martin explain this in more detail:

The main difference between endosymbiosis and polyploidy relates to the size and distribution of genomes over evolutionary time. In endosymbiosis, surplus organelle genes are lost or transferred to the host’s chromosomes, streamlining endosymbiont replication via cytoplasmic inheritance. The outcome is a massive reduction in genome size, both in prokaryotic endosymbionts and organelles, with a reciprocal relocation of genes in low copy number to nuclear chromosomes in the latter. By contrast, in giant polyploid prokaryotes, all genomes are essentially the same. Without cytoplasmic inheritance, no genomic specialization ensues.

In principle, prokaryotes could control respiration using specialized, membrane-associated plasmids that emulate organelle genomes in gene content and function. In practice, such plasmids are not found. Bacteria usually have small, high-copy-number plasmids that segregate randomly at cell division, or very few giant plasmids that co-segregate with chromosomes on filaments from midpoint. For plasmids in a prokaryote to support electron flux as organelle genomes do, high-copy-number giant plasmids encoding components of the electron-transport chain would need to associate with the plasma membrane, and evolve counter to the tendency to segregate with size rather than function. That no mtDNA-like plasmids are known indicates that high energetic barriers preclude their evolution: unlike organelles, which pay back energetically from the start, substantial energetic costs must be paid up front (high copy number of the correct plasmids, and the machinery to associate them with the membrane at regular intervals) before any energetic advantage can accrue.

What’s more, if we consider the origin of chloroplasts this point becomes even stronger, as their origin as yet another form of energy module under independent genetic control again depended on an endosymbiotic union rather than the gradual and incremental reshaping of the would-be-host cell itself.  It’s as if the endosymbiotic strategy was repeated and re-used because there is no other viable strategy.  So it turns out Margulis might not only have identified the event behind the origin of the mitochondria, but also a necessary prerequisite event for the origin of complex eukaryotes.

What all this means is that we are to facilitate the emergence of mitochondria to facilitate the emergence of metazoan-type complexity, we’ll need a method to turn a symbiont into an organelle.  We have already seen that the bacterial cells would come equipped with machinery that would pre-adapt transport of the metabolic machinery back into the symbiont-turned-mitochondria. But all this assumes the symbiont’s useful genes would not merely be stripped from its genome, but also moved and stored into the host’s genome.  In other words, what would help facilitate this re-tooling is if the host had a genome that was segregated from the symbiont – a place to store the useful symbiont genes as single copies so the two genomes could evolve together.  In other words, a nucleus.  If the host cell already had a nucleus, then a separate membrane-bound compartment could house the symbiont genes in a fashion where they were completely isolated from the symbiont.

Given that the nucleus is the repository for many of the symbiont’s genes and thus put the symbiont in a state of dependency, we can think of the nucleus itself as something as another preadaptation to facilitate the endosymbiotic emergence of mitochondria.

And that will take us to the next posting.


One response to “Nudging Mitochondria Along

  1. So it’s endosymbiosis (to the rescue) because the blind watchmaker couldn’t construct such an organelle from scratch?

    We need an objective way to test the endosymbiotic scenario beyond jst saying “it looks like endosymbiosis”…

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