Mitochondria rule the cell.
Are we humans merely the extended phenotype of a bacterium?
Introduction:
After nearly forty years I have re-read Richard Dawkins’ 1982 book ‘The Extended Phenotype’. The academically detailed sequel to ‘The Selfish Gene’ of 1976 is for professional biologists really and, for those that have not ploughed through the arguments therein, it makes the case once again for the ‘gene’ being the replicating ‘unit’ that is acted upon by natural selection. But there is a twist. The outward physical expression of a gene, or rather the collection of genes is the classic so-called phenotype. A bird is a bird with a beak and so on but he extends the idea beyond appearance to the products of the organism. An abstraction that extends the phenotype to the ‘works’ of the organism. So a bird’s nest is part of the extended phenotype. In the same way our AI systems would be part of our extended phenotypes as much as our behaviours and building.
Dawkins’ ideas have survived and replicated just like one of his memes and is gaining currency again. For example in popular science reading this is well exemplified in ‘The Entangled Life’ by Merlin Sheldrake. To cut a very tangled long story short, the once sacrosanct idea of organisms as individuals, ‘them and us’ is melting away as the interconnectedness of all life is becoming more apparent, understood and elucidated. So, maybe now is the time to look at the extended phenotype and the dissolving individualism of the organism together in a new, mitochondrial, context.
Mitochondria (and Chloroplast)s … rather special organelles.
As usual, these posts are about mitochondria and my drift has been towards what I shall call ‘intracellular ecology’. I may have invented this term but I doubt it. Mitochondria and their plant counterparts chloroplasts are accepted today as ex-free-living bacteria-like organisms that came to be part of the modern eukaryotic cell by some ancient happenstance probably involving phagocytosis by a larger prokaryote.
Although mitochondria and chloroplasts are able to replicate within the cell their small circular DNA is regarded as pretty much vestigial. It has few genes and limited internal apparatus for the synthesis of proteins and certainly not enough to build a replica of themselves. Nearly everything is carried out by genes found in the nucleus and its proteins are synthesised in the cytoplasm. Given these facts I always feel a sense of disappointment. It’s as if my beloved organelles ‘sold out’, and are now enslaved by a vast nuclear arsenal of genes, doing their work powering the cell and providing enough surplus energy to power multicellular evolution.
Maybe I was wrong, they didn’t get taken over.. We already know mitochondria have the power of life and death of a cell through the process called apoptosis. When a mitochondrion depolarises and starts to leak Cytochrome C, a cell is doomed. This is a big ‘responsibility’. Why cannot the nucleus take care of monitoring the cell’s health and initiate the process of apoptosis?
The answer, maybe, is that there is but one ‘organism’ and it’s inside the cell, the mitochondrion. The rest of the cell is ‘the environment’ … from its perspective. And it’s this unit of life which today is still exploiting its ancient ecosystem.
The question is quickly begged as if this is so, then why do mitochondria have such vestigial DNA, so little that they are such a long way from self-sufficient, let alone qualifying as a traditional organism.
But, what if mitochondria ( and chloroplasts ) have simply outsourced their genes to the nucleus?
The analogy with a first world power outsourcing its basic production facilities and data storage to other countries is obvious. No one expects the UK or even the US to be self-sufficient. So is that what mitochondria do?
Support to this idea is given by the discovery and exploration of so-called ‘retrograde signalling’. Here is an extract by Wang in 2020* to give a flavour of the work.
‘The biochemical, cellular and physiological reasons why mitochondrial and chloroplast retrograde signalling may be linked at several levels are worth considering. Organelle retrograde signalling provides feedback to anterograde signals that alters gene expression encoding proteins located in a variety of locations in the cell. Thus, chloroplasts and mitochondria have emerged as environmental sensors to ensure that the pipeline from gene expression to protein function is optimized relative to organelle function.’
‘Retrograde signalling’ Essentially this phrase, though cryptic, is well described above.
Signalling is the same as ‘outsourcing’. Mitochondria may have simply outsourced their genes whilst retaining control of their new intracellular world. Dawkins’ extended phenotype notions would be entirely comfortable with this scenario. Put simply mitochondria can ‘message’ the nuclear DNA store and get manufactured what they need when they need.
This may, ironically, be a good strategy for actually preserving mitochondrial genes. Inside the mitochondrion is a ferment of deadly free radicals spewed from the process of oxidative phosphorylation which provides the vast amount of energy consumed by eukaryote cells. So it would make sense for a gene to be stored somewhere safer to do its replication courtesy of energy provided by the ‘mothership’.
As an aside, I think, as do others, that back in the mists of time the ‘nucleus’ was a mere bag of junk DNA, probably viral in origin, and parcelled up somewhere where it could do no harm. So called horizontal transfer of plasmid DNA is normal in the prokaryote world and this store would be a good place to keep DNA away from damage. It would need to be supplied with energy and a signalling system that’s all.
The non-mitochondrial genes in the bag were then the accidental beneficiaries of the enormous supply of energy from the mitochondria and their own replication journey took off. You cannot help but think this when you see a nucleus undergoing division today literally wrapped in a reticulate net of fused tubular mitochondria. Replication requires energy.
An unparalleled source of energy and a bucket of random genes would be a great precursor for and explosion of forms, random, mad, hopeless but some ultimately successful.
Finally.
If the constructs of humans, physical or mimetic or computational can be regarded as part of man’s extended phenotype and that the organism’ man’ is the extended phenotype of his genetic replicators.
Then it is perfectly ok to regard an organism as the extended phenotype of a humble prokaryote!
What a journey. From the cell’s batteries to their master. Long live retrograde signalling
*Philos Trans R Soc Lond B Biol Sci. 2020 Jun 22; 375(1801): 20190410.
Published online 2020 May 4. doi: 10.1098/rstb.2019.0410Linking mitochondrial and chloroplast retrograde signalling in plants
