Competitive Exclusion Among Self-Replicating Molecules Curtails The Tendency Of Chemistry To Diversify
The transition of chemistry into biology is poorly understood. One of the key questions in this transition is how the inherently divergent nature of chemical reactions can be curtailed, allowing product mixtures to become enriched in only a limited subset of all possible reaction products.
Another seemingly unrelated question is whether Darwinian principles from biology extend to chemistry. Addressing both questions simultaneously, we now show that the evolutionary principle of competitive exclusion, which states that a single niche can only be stably occupied by one species, also applies to self-replicating chemical systems, and that this principle diminishes the tendency of chemistry to diversify.
Specifically, we report two systems in which three different self-replicator quasi-species emerge in a largely stochastic fashion from a mixture of two building blocks (resources). To enable their evolution, these replicator mixtures were subjected to an out-of-equilibrium replication-destruction regime, implemented by serial transfer.
Out of the many different products initially produced, competitive exclusion leads to the selection of only a single quasi-species when all replicators rely to the same extent on both resources. When, on the other hand, one of the quasi-species preferentially uses one resource and another quasi-species specializes in the other (resource partitioning), these replicator quasi-species effectively occupy different niches and were found to coexist in an evolutionary stable steady state.
The ability to escape from competitive exclusion through resource partitioning is important for future efforts on addressing a major evolutionary challenge on the path to life’s emergence: Eigen’s paradox, which requires evolutionary stable communities of co-existing replicators with specific community dynamics.
Figure 1. (a) Structures and graphical representation of building blocks. System A consists of equimolar amounts of 1 and 2, while system B consists of equimolar amounts of 1 and 3. (b) When thiols are slowly oxidized to disulfides by oxygen from the air, a DCL of macrocycles is formed that constantly exchange building blocks through thiol-disulfide exchange. Separate nucleation events can cause macrocycles of specific sizes (pentamers, hexamers or octamers) to self-assemble into fibres (shown for the pentamer in panel c), shifting the equilibrium of the DCL to produce more of the very macrocycle that self-assembles. (c) Fibers of a sufficient length are fragmented by mechanical agitation through stirring, resulting in exponential growth of the fibres. The elongation/fragmentation mechanism is only shown for the pentamers, but the other self-replicators also replicate using this mechanism. (d) Procedure of serial transfer. After oxidation is essentially completed 10% of the DCL is transferred to a vial containing fresh building blocks (1.0 mM total concentration; 25 mM B2O3-buffer; pH 8.18) that had been rapidly oxidized to 50% disulfides using sodium perborate (NaBO3; 40 mM) prior to transfer. The new DCL was stirred (1200 rpm) for 4 days at 45 oC to allow essentially complete conversion of all thiols to disulfides. Transfer to a fresh solution was performed several times generating a total of n generations. (e) In the absence of resource partitioning (system A) the fittest replicator displaces all others when they have to compete for their common resources in an out-of-equilibrium regime, implemented by serial transfer. Resource partitioning (occurring in system B) allows different replicators to coexist in an out-of-equilibrium regime. — chemrxiv.org
Competitive Exclusion among Self-Replicating Molecules Curtails the Tendency of Chemistry to Diversify chemrxiv.org
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