The Structure of Autocatalytic Sets: Evolvability, Enablement, and Emergence

Abstract

This paper presents new results from a detailed study of the structure of autocatalytic sets. We show how autocatalytic sets can be decomposed into smaller autocatalytic subsets, and how these subsets can be identified and classified. We then argue how this has important consequences for the evolvability, enablement, and emergence of autocatalytic sets. We end with some speculation on how all this might lead to a generalized theory of autocatalytic sets, which could possibly be applied to entire ecologies or even economies.

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Acknowledgments

This paper was finalized while WH and SK were visiting the Computational Systems Biology Research Group of the Tampere University of Technology, Finland. MS thanks the Royal Society of New Zealand for funding support. We also thank Vera Vasas for helpful and stimulating discussions.

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Correspondence to Wim Hordijk.

Appendix

Appendix

Proof of Theorem 1:

Part 1: First, consider a directed graph G that has 2k vertices r 1r 2,…, r k , and r1r2,…, r k . For each \(i=1,2,\ldots, k-1,\) place a directed edge from r i to r i+1 and also one from r i to r i+1. Next, for each i = 1, 2, …, k − 1, place a directed edge from r i to r i+1 and also one from r i to r i+1. Finally place directed edges from r k back to r 1 and to r1; similarly place directed edges from r k back to r 1 and to r1.

Notice that the number of minimal directed cycles in this digraph is 2k, since we have complete freedom to select r i or r i at each step in the cycle, and we must select one of them (to get a cycle) but not more than one (to get a minimal cycle).

We now use this graph to construct an RAF set that has exponentially many irrRAFs as follows. Associate with r i the reaction \(a_i+b_i \Rightarrow c_i\) and with r i the reaction \(a'_i + b'_i \Rightarrow c_i,\) where:

  1. (i)

    the a i b i a i b i and c i are all distinct from each other (and across different choices of i there is no repetition), and

  2. (ii)

    the a i b i a i b i are all in the food set F (for all i).

For the catalysis set C, we let c i catalyze r i+1 and r i+1 (for \(i=1,2, \ldots, k-1\)). In addition, let c k catalyze r 1 and r1. Figure 3 illustrates this RAF set for the case k = 3.

The irrRAFs in this resulting RAF set are now in one-to-one correspondence with the minimal directed cycles of the graph G described above, and there are 2k such minimal cycles, but only 2k reactions and 5k molecules. So, the number of irrRAFs is exponential in the size of the RAF set. Notice that this construction can be carried out within the binary polymer model.

Part 2: For an arbitrary subset \(\mathcal{R}'' \subseteq \mathcal{R},\) let \(s(\mathcal{R}'')\) denote the (possibly empty) subset of \(\mathcal{R}\) obtained by applying the RAF algorithm to \(\mathcal{R}''\) and F, and let \(\mathcal{R}''_{\neq \emptyset}\) be the set of reactions r in \(\mathcal{R}''\) for which \(s(\mathcal{R}''-\){r}) ≠ ∅. We first establish the following result:

Claim 1: If \(\mathcal{R}'\) is any RAF, then \(\mathcal{R}''\) is a maximal proper subRAF of \(\mathcal{R}'\) if and only if

  1. (a)

    \(\mathcal{R}'' = s(\mathcal{R}'-\{r\})\) for some reaction \(r \in \mathcal{R}'_{\neq \emptyset},\) and

  2. (b)

    \(\mathcal{R}''\) is not strictly contained within any other set of type (a).

To verify this claim, suppose that A is a maximal proper subRAF of \(\mathcal{R}'.\) Then there is at least one reaction \(r \in \mathcal{R}'-A.\) Notice that, since \(A \subseteq \mathcal{R}'-\{r\},\,s(A)=A\) is a non-empty subset of \(s(\mathcal{R}' -\{r\});\) moreover \(s(\mathcal{R}'-\{r\})\) is a strict subRAF of \(\mathcal{R}'\) since \(s(\mathcal{R}'-\{r\})\) does not include r while \(\mathcal{R}'\) does. Thus, since A is a maximal proper subRAF of \(\mathcal{R}\) we have

$$ A= s(A) = s({\mathcal{R}}'-\{r\}), $$

and so (a) holds. Property (b) now follows by the maximality assumption.

Conversely, suppose that (a) and (b) hold for \(\mathcal{R}''.\) Then \(\mathcal{R}''=s(\mathcal{R}'-\{r\})\) is nonempty and so \(s(\mathcal{R}'-\{r\})\) is a proper subRAF of \(\mathcal{R}',\) and if it were not a maximal proper subRAF of \(\mathcal{R}'\) then, from the first part of the proof \(s(\mathcal{R}'-\{r\})\) would need to be strictly contained within \(s(\mathcal{R}' - \{r'\})\) for some reaction \(r' \in \mathcal{R}'_{\neq \emptyset},\) and this is impossible since we are assuming that (b) holds.

From Claim 1, the number of maximal proper subRAFs is at most the number of sets of the form \(s(\mathcal{R}'-\{r\})\) for \(r \in \mathcal{R}',\) and there are at most \(|\mathcal{R}'|\) such sets across the possible choices of r from \(\mathcal{R}.'\)

Part 3: Part (i) follows directly from Claim 1, since the collection of RAF sets \( \{s({\mathcal{R}}'-\{r\}): r \in {\mathcal{R}}'_{\neq \emptyset}\} \) can be computed in polynomial time, and property (b) in Claim 1 can then also be checked in polynomial time.

Part (ii) also follows from Claim 1, since this shows that \(\mathcal{R}'\) is the union of two proper subRAFs if and only if

$$ {\mathcal{R}}' = s({\mathcal{R}}'-\{r_1\}) \cup s({\mathcal{R}}'-\{r_2\}) $$
(1)

for some pair of distinct elements r 1, r 2 of \(\mathcal{R}'_{\neq \emptyset}.\)

From this, it is clear how to obtain a polynomial time algorithm: first construct the set \(\mathcal{R}'_{\neq \emptyset},\) and, provided this set is non-empty, search for all pairs \(r_1, r_2 \in \mathcal{R}'_{\neq \emptyset}\) for which Eqn. (1) holds; for each such pair we can set \(\mathcal{R}_i:=s(\mathcal{R}'-\){r i }), for i = 1, 2 so that \(\mathcal{R}' = \mathcal{R}_1 \cup \mathcal{R}_2.\) If no such pair r 1, r 2 exists (or if \(\mathcal{R}'_{\neq \emptyset}\) is empty), then report that \(\mathcal{R}'\) cannot be decomposed further. This completes the proof of the part (ii).

For part (iii), it suffices to verify the following:

Claim 2: If \(\mathcal{R}'\) is any RAF set and \(\mathcal{R}_0\) is any non-empty subset of \(\mathcal{R}'\) then \(\mathcal{R}_0\) is contained within every subRAF of \(\mathcal{R}'\) if and only if \(s(\mathcal{R}'-\{r\}) = \emptyset\) for all \(r \in \mathcal{R}_0.\)

To verify this claim, first suppose there exists \(r \in \mathcal{R}_0\) with \(s(\mathcal{R}'-\{r\}) \neq \emptyset.\) Then \(s(\mathcal{R}'-\{r\})\) is a subRAF of \(\mathcal{R}'\) and yet RAF \(s(\mathcal{R}'-\{r\})\) does not contain \(\mathcal{R}_0,\) since \(s(\mathcal{R}'-\{r\})\) is a subset of \(\mathcal{R}'-r\) and so does not contain \(r \in \mathcal{R}_0.\) Conversely, suppose there exists a subRAF \(\mathcal{R}''\) of \(\mathcal{R}'\) which does not contain \(\mathcal{R}_0.\) Select any reaction \(r \in \mathcal{R}_0-\mathcal{R}''.\) Then \(\mathcal{R}'' \subseteq s(\mathcal{R}'-\{r\})\) and so \(s(\mathcal{R}'-\{r\}) \neq \emptyset.\) This establishes Claim 2, as required, and completes the proof. \(\square\)

Proof of Corollary 1

The algorithm constructs the Hasse diagram from the top down, starting from the single node \(\mathcal{R}'.\) We apply Part 3(i) of Theorem 1 to list all the maximal proper subRAFs of \(\mathcal{R}',\) and then place edges from each of these to \(\mathcal{R}'\) (if \(\mathcal{R}'\) has no maximal proper subRAFs then \(\mathcal{R}'\) is irreducible and we leave the node as it is). Now we repeat this step recursively on these subRAFs, introducing edges as before, and also identifying any two (or more) nodes labeled by the same subRAF. We continue in this way until the network can be extended no further, in which case all the nodes with no children comprise the set of irrRAFs of \(\mathcal{R}'.\)

The resulting network N that we have constructed contains all the nodes of the Hasse diagram of the poset (i.e. it contains all the subRAFs of \(\mathcal{R}'\)); moreover, the edge set is a subset of the edges in the Hasse diagram. This last claim needs a short proof: if we have constructed an edge in N from \(\mathcal{R}_1\) to \(\mathcal{R}_2,\) where \(\mathcal{R}_1 \subset \mathcal{R}_2\) we need to show that there is no other path in N from \(\mathcal{R}_1\) to \(\mathcal{R}_2\) via a sequence of increasing subRAFs (which would make the edge \((\mathcal{R}_1, \mathcal{R}_2)\) redundant). Suppose there were such a second path, and let \((\mathcal{R}_3, \mathcal{R}_2)\) be the last edge on this path. Then, referring to Claim 1 (in the proof of Part 2 of Theorem 2), \(\mathcal{R}_1 = s(\mathcal{R}_2-\{r\})\) would be strictly contained in \(\mathcal{R}_3 = s(\mathcal{R}_2-\{r'\})\) for some reactions rr′ and this is forbidden in allowing \(\mathcal{R}_1\) to be selected as a maximal proper subRAF of \(\mathcal{R}_2.\)

Thus, each edge in N will be present as an edge in the Hasse diagram. Moreover, all edges in the Hasse diagram are present in N, for suppose that in the Hasse diagram there is an edge from \(\mathcal{R}_1\) to \(\mathcal{R}_2.\) where \(\mathcal{R}_1 \subset \mathcal{R}_2.\) Then \(\mathcal{R}_1\) must be a maximal subRAF of \(\mathcal{R}_2\) and so, by construction, the algorithm inserts an edge from \(\mathcal{R}_1\) to \(\mathcal{R}_2\) during the step at which the subRAF \(\mathcal{R}_2\) and its maximal subRAFs are considered.

In summary, we have verified that the algorithm described constructs exactly the Hasse diagram of subRAFs of \(\mathcal{R}' .\) \(\square\)

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Hordijk, W., Steel, M. & Kauffman, S. The Structure of Autocatalytic Sets: Evolvability, Enablement, and Emergence. Acta Biotheor 60, 379–392 (2012). https://doi.org/10.1007/s10441-012-9165-1

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Keywords

  • Origin of life
  • Autocatalytic sets
  • Evolvability
  • Emergence
  • Functional organization