Abstract
In the RNA world hypothesis, RNA(-like) self-replicators are suggested as the central player of prebiotic evolution. However, there is a serious problem in the evolution of complexity in such replicators, i.e., the problem of parasites. Parasites, which are replicated by catalytic replicators (catalysts), but do not replicate the others, can destroy a whole replicator system by exploitation. Recently, a theoretical study underlined complex formation between replicators—an often neglected but realistic process—as a stabilizing factor in a replicator system by demonstrating that complex formation can shift the viable range of diffusion intensity to higher values. In the current study, we extend the previous study of complex formation. Firstly, by investigating a well-mixed replicator system, we establish that complex formation gives parasites an implicit advantage over catalysts, which makes the system significantly more vulnerable to parasites. Secondly, by investigating a spatially extended replicator system, we show that the formation of traveling wave patterns plays a crucial role in the stability of the system against parasites, and that because of this the effect of complex formation is not straightforward; i.e., whether complex formation stabilizes or destabilizes the spatial system is a complex function of other parameters. We give a detailed analysis of the spatial system by considering the pattern dynamics of waves. Furthermore, we investigate the effect of deleterious mutations. Surprisingly, high mutation rates can weaken the exploitation of the catalyst by the parasite.
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Notes
This problem is often referred to as the error threshold or information threshold, although the problem does not necessarily exhibit threshold-like behavior (Takeuchi and Hogeweg 2007).
In the genomic tag hypothesis, Maizels and Weiner (1998) suggested a tRNA-like structure as such a tag motif in ancient RNA genomes.
If it were used to obtain the dynamics of the replicators, θ would cancel out in the growth term κc xx θ, and thus there would be no limitation to the growth, which is not sensible. According to numerical solutions of Eq. (5), when κ \( \gg \) b i , c xx θ suddenly becomes zero at θ = 0 (i.e., x t + y t = 1) in a singular-like manner. This behavior cannot be captured if one takes the limit of κ i → ∞ (i = x, y) because the term κ i θ appears in Eq. (5).
E.g. for the replication reaction to happen, one cell must be empty, and the other must be a complex.
Strictly speaking, θ = 1−x−y−2c xx −2c xy in the CA model because a complex occupies twice as much space as a single molecule, but this hardly affects the results.
The template activity of parasites was 1% higher than that of the catalyst.
See also footnote 8.
R was set small in attempt to make β max comparable to that for R = 0. However, it turns out that a slight mutation can significantly increase β max when a parameter set allows the formation of wave patterns. An increase can be as much as 20% for D = 0.01. This increase of β max can be understood in the light of pattern dynamics of waves, which is developed later in this paper. Mutation inoculates a small number of parasites in the traveling front of waves (see Fig. 5, D = 0.1), and thereby, the parasite can split a wave in a few parts, giving rise to more waves. Thus, a slight mutation can enhance the stability of the system.
In a standard predator–prey (host–parasite) system predators (parasites) can directly kill preys (host), whereas in the current model, such a direct killing does not happen. To replace a catalyst population, parasites must wait until the catalysts disappear via intrinsic decay.
The following two sets of parameters were examined: (1) log10 D ∞ = −1.15 (D 2 ≈ 0.053), k 2 = 1, k 1 = 0.085, k −1 = 0.001, d = 0.00125; and (2) log10 D ∞ = −1.075 (D 2 ≈ 0.063), k 2 = 1, k 1 = 0.085, k −1 = 0.001, d = 0.015. See Füchslin et al. (2004) for the notation.
Note that M max is not exactly the same as the error threshold: while the error-threshold phenomenon—or the breakdown of Darwinian optimization—happens because of the competition between the fittest and the mutants (Eigen et al. 1989; see also Takeuchi and Hogeweg 2007), M max exists because X cannot grow faster than it decays for a sufficiently large M.
Suppose β = 1 is tolerable, and the next examined value β = 2 is not tolerable, but the third examined value β = 1.5 is tolerable. Then, it might be that β = 2 is tolerable if the initial condition is taken from the system with β = 1.5 instead from that with β = 1.
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Acknowledgments
We express our gratitude to the associate editor Dr. N. Lehman and the three anonymous reviewers for their constructive criticisms on our manuscript. The research was supported by NWO exact sciences 612.060.522.
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Takeuchi, N., Hogeweg, P. The Role of Complex Formation and Deleterious Mutations for the Stability of RNA-Like Replicator Systems. J Mol Evol 65, 668–686 (2007). https://doi.org/10.1007/s00239-007-9044-6
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DOI: https://doi.org/10.1007/s00239-007-9044-6