Exponential growth for self-reproduction in a catalytic reaction network: relevance of a minority molecular species and crowdedness

TL;DR

This study demonstrates that exponential self-reproduction in catalytic networks requires a minority molecular species and crowding effects, highlighting their importance for the origins of life and evolutionary potential.

Contribution

It reveals that a slow-replicating minority molecule is essential for exponential growth in catalytic networks, emphasizing the role of molecular discreteness and crowding.

Findings

Exponential growth occurs only with a minority molecular species.

Crowding and molecular discreteness are crucial for exponential growth.

Superexponential or subexponential growth occurs without minority species.

Abstract

Explanation of exponential growth in self-reproduction is an important step toward elucidation of the origins of life because optimization of the growth potential across rounds of selection is necessary for Darwinian evolution. To produce another copy with approximately the same composition, the exponential growth rates for all components have to be equal. How such balanced growth is achieved, however, is not a trivial question, because this kind of growth requires orchestrated replication of the components in stochastic and nonlinear catalytic reactions. By considering a mutually catalyzing reaction in two- and three-dimensional lattices, as represented by a cellular automaton model, we show that self-reproduction with exponential growth is possible only when the replication and degradation of one molecular species is much slower than those of the others, i.e., when there is a minority molecule. Here, the synergetic effect of molecular discreteness and crowding is necessary to produce the exponential growth. Otherwise, the growth curves show superexponential growth because of nonlinearity of the catalytic reactions or subexponential growth due to replication inhibition by overcrowding of molecules. Our study emphasizes that the minority molecular species in a catalytic reaction network is necessary to acquire evolvability at the primitive stage of life.