Minimum viable genome?


(James McKay) #1

Here’s an interesting article that cropped up on my Twitter feed about half an hour ago:

I’ve come across this before – apparently there’s quite a bit of research going on into the smallest possible genome that will allow cells to survive (and by extension, reproduce and evolve).

Seems like an interesting talking point, but surprisingly I’ve seen very little discussion about it with respect to the creation and evolution debate. Even the creationist and ID organisations don’t seem to have much to say about it as far as I can tell.

Any thoughts on what we should make of it?


(Stephen Matheson) #2

I think the question is too complex to answer, because it’s too hard to be clear about what is “needed” to build a cell that can survive and reproduce. But we can probably start with the smallest Mycoplasma genomes, which contain less than 600 genes. As the Quanta article describes, you can maintain viability with as few as 473 of those genes. According to the article below, comparison of the Mycoplasma genome with other bacterial genomes yielded an estimate of about 250 genes as a “theoretical minimum.”

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4879981/

Even the highest of those numbers (about 580) is less than a tenth of the number of genes in the yeast genome. To me, this means that the “smallest possible genome” will differ substantially in bacteria vs. eukaryotes like yeast. Nevertheless, there are some very small compact genomes in some weird parasitic eukaryotes that are thought to be highly modified fungi:

The smallest one is a little more than 1800 genes (!!). The Nature Communications paper is cool because they discuss the possibility that one of these microsporidia genomes has reached minimum non-coding DNA.


(James McKay) #3

Thanks Stephen. It’s interesting to see some more information about this line of research.

The reason I’m asking about this though is because I wondered how a minimum viable genome might tie in with the debate around the concept of irreducible complexity. If it can be shown that there’s a minimum viable size for a genome that can survive, let alone replicate and evolve, would that be an example of an irreducibly complex structure? If not, how could it have arisen in the first place?

Unfortunately I haven’t found much in the way of discussion about this particular question. My Google searches have come up with this article on Evolution News which says that it is, but that’s about it.


(Stephen Matheson) #4

Oh, I see. There are plenty of examples of biological things that are “irreducibly complex,” in the sense that they rely on a full set of components for some kind of minimal function. The IC challenge is, in my experience, mostly a lack of attention to the history of development of such things. So, to ask about the minimal genome (or proteome) to support a free-living cell is to ask an interesting question; to assume that this means or could mean that the precursors had to have all those same forms is not. Among many things missing from the analysis: the environment in which the components, or first “cells” would have existed.

In short, a minimal viable genome, or a minimalist cell, is just another IC system (by the most basic definition) in a world full of them.


(Larry Bunce) #5

The first living cells would not be able to survive in today’s world. If a new cell arose today that was identical to the first cell that started all life, it would be eaten by something that already exists. The first life did not have to have defenses against other forms of life, so it could be much simpler than anything that could survive today.
DNA evidence suggests that the cell nucleus began as an independent life form that became a parasite on a primitive bacterium. The symbiotic relationship was so beneficial to each organism that their DNA fused, and the modern cell came into existence.


(Stephen Matheson) #6

There are a few different hypotheses about the origins of the nucleus. I don’t know whether any of them is strongly motivated by “DNA evidence” but I’d be interested to see your sources.


(Phil) #7

I suspect that as @Larry_Bunce said, early cells would not need what modern ones find essential, just because of interdependent systems. A modern car cannot function without a battery, but early cars did fine with magneto ignitions and hand cranks.


(Larry Bunce) #8

I believe that the hypotheses on cell nucleus origin predate current DNA analysis. I looked up “origin of cell nucleus” and found this article that was the basis of my post:
https://www.cell.com/trends/cell-biology/pdf/S0962-8924(01)01951-1.pdf


(Stephen Matheson) #9

I see. That’s a news blurb and it’s pretty old. Here’s something more up to date that will give you a picture of where things stand. You will see that endosymbiotic hypotheses are one major class of hypotheses, but that there are competing hypotheses with different strengths.

https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(15)00238-4


(system) #10

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