Monday, December 12, 2016

Early RNA may have used isolation strategy to defeat mutants

Tiny water droplets may have protected the first self-replicating molecules from parasitic mutants. Recent experimental evidence shows that temporary compartmentalization may help RNA molecules resist takeover by shorter, faster replicating mutants.

As the earliest self-replicating RNA strands reproduced, mutations would inevitably arise. Mutations which shortened the molecule, even at the expense of it's function, would have a selective advantage. Based on kinetic chemical principles, shorter and less complex mutants with the capacity to copy themselves more quickly and produce more offspring would push out longer, more advanced RNA molecules. This would cause evolutionary stagnation such that an RNA molecule would be limited in it's ability to grow increasingly more sophisticated. These mutants may therefore be considered parasitic in the sense that they have a high sexual fitness but have lost their genetic "instruction manual" to do anything else.

Mathematical models suggest that having RNA's replicate in many discrete populations instead of one giant gene pool would solve this problem. By simple probability, some pockets would end up with fewer parasites, and in these insulated compartments longer more complex RNA's might be able to get a foothold.

To test this idea in the lab, a German team retrieved a piece of RNA from a bacteriophage and pasted in a catalyst (ribozyme). They then let the RNA duplicate under different conditions with varying degrees of isolation to other RNA molecules -- either freely in a vial or distributed through a million microscopic water droplets in oil. The droplets held populations of RNA molecules together for short periods of time before breaking, letting the RNA mix together again, and then reforming with a different set of RNA inside. Catalytic activity of individual droplets was analyzed throughout the experiment.

Results concluded that after four generations of this repeating cycle, catalytic RNA that wasn't compartmentalized in any way had been completely overrun by parasitic RNA. However, compartmentalized RNA was shown to yield higher complexity, for somewhat surprising reasons. This being that complexity became advantageous due to competition between compartments, such that compartments with too many parasitic mutations died off compared to surrounding compartments with further advancing RNA. After 9 generations, a substantial amount of the functioning RNA still remained.

To quote Brian Paegel, a chemist at the Scripps Research Institute in Jupiter, Fla. "[cellularity] might have been totally central to life as we know it today." He states that it's becoming increasingly clear that compartmentalization helped to shape the emergence of life.

I found this study interesting, because it highlights a certain important biological reality that is often overlooked. This being that increasingly complex and more "advanced" (in the anthropocentric sense of the word) organisms are not the default direction of nature. Evolution will only take pathways which are absolutely necessary, and the most cost efficient, for propagation of a population under given environmental duress and condition. Was it not for the unique environmental conditions and isolation introduced via aerosolized droplets (or a similar container), organic complexity may very well have stagnated, and life itself may never have formed.

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