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[email protected] https://eprints.whiterose.ac.uk/ RESEARCH ARTICLE Modularity, criticality, and evolvability of a developmental gene regulatory network Berta Verd1,2,3,4†*, Nicholas AM Monk5, Johannes Jaeger1,2,3,5,6,7,8,9‡* 1EMBL/CRG Systems Biology Research Unit, Centre for Genomic Regulation (CRG), The Barcelona Institute of Science and Technology, Barcelona, Spain; 2Universitat Pompeu Fabra (UPF), Barcelona, Spain; 3Konrad Lorenz Institute for Evolution and Cognition Research (KLI), Klosterneuburg, Austria; 4Department of Genetics, University of Cambridge, Cambridge, United Kingdom; 5School of Mathematics and Statistics, University of Sheffield, Sheffield, United States; 6Wissenschaftskolleg zu Berlin, Berlin, Germany; 7Center for Systems Biology Dresden (CSBD), Dresden, Germany; 8Complexity Science Hub (CSH), Vienna, Austria; 9Centre de Recherches Interdisciplinaires (CRI), Paris, France Abstract The existence of discrete phenotypic traits suggests that the complex regulatory processes which produce them are functionally modular. These processes are usually represented *For correspondence: by networks. Only modular networks can be partitioned into intelligible subcircuits able to evolve [email protected] (BV); relatively independently. Traditionally, functional modularity is approximated by detection of [email protected] (JJ) modularity in network structure. However, the correlation between structure and function is loose. Many regulatory networks exhibit modular behaviour without structural modularity. Here we Present address: †Department partition an experimentally tractable regulatory network—the gap gene system of dipteran of Genetics, University of Cambridge, Cambridge, United insects—using an alternative approach. We show that this system, although not structurally Kingdom; ‡Department of modular, is composed of dynamical modules driving different aspects of whole-network behaviour. Molecular Evolution and All these subcircuits share the same regulatory structure, but differ in components and sensitivity Development, University of to regulatory interactions. Some subcircuits are in a state of criticality, while others are not, which Vienna, Vienna, Austria explains the observed differential evolvability of the various expression features in the system. DOI: https://doi.org/10.7554/eLife.42832.001 Competing interests: The authors declare that no competing interests exist. Funding: See page 22 Introduction Received: 14 January 2019 Systems biology aims to understand the function and evolution of complex regulatory networks. This Accepted: 05 June 2019 requires some sort of hierarchical decomposition of these networks into manageable and intelligible Published: 06 June 2019 subsystems, whose properties and behaviour can be analysed and understood in relative isolation (Simon, 1962; Riedl, 1975; Lewontin, 1978; Bonner, 1988; Raff, 1996; West-Eberhard, 2003; Reviewing editor: Lee Altenberg, The KLI Institute, Schlosser and Wagner, 2004; Callebaut et al., 2005). If each subsystem possesses a clearly delim- United States ited and discernible function, the network can be subdivided into functional modules (Raff, 1996; von Dassow and Munro, 1999; Hartwell et al., 1999; Wagner et al., 2007; Mireles and Conrad, Copyright Verd et al. This 2018). In the Introduction of our paper, we provide a careful argument showing that the most com- article is distributed under the mon approach to identify functional modules has severe limitations, and propose an alternative terms of the Creative Commons Attribution License, which method, which we then use to dissect and analyse a specific pattern-forming network, the gap gene permits unrestricted use and system of the vinegar fly, Drosophila melanogaster. redistribution provided that the The most common strategy to identify functional modules is to partition the graph representing a original author and source are network into simple motifs (Shen-Orr et al., 2002; Alon, 2007) or subcircuits (also called subnet- credited. works or communities; (Girvan and Newman, 2002; Oliveri and Davidson, 2004; Babu et al., Verd et al. eLife 2019;8:e42832. DOI: https://doi.org/10.7554/eLife.42832 1 of 40 Research article Computational and Systems Biology Developmental Biology 2004; Levine and Davidson, 2005; Newman, 2006; Davidson and Erwin, 2006; Oliveri and Davidson, 2007; Erwin and Davidson, 2009; Davidson, 2010). Network motifs are small subgraphs that are identified through their statistical enrichment (Alon, 2007; Alon, 2006), while subcircuits are characterised by a high connection density among their component nodes contrasting with sparse connections to the outside (Girvan and Newman, 2002; Radicchi et al., 2004; New- man, 2006; Wagner et al., 2007; Fortunato, 2010). In both cases, subsystems are defined in terms of the regulatory structure or network topology: they are structural modules. This approach presup- poses a strong connection between functional and structural modularity (see, for example, Lim et al., 2013). Strictly interpreted, structural modules are mutually exclusive: they are disjoint subgraphs of a complex regulatory network that do not share nodes between each other (Girvan and Newman, 2002; Radicchi et al., 2004; Palla et al., 2005). And yet, such modules can never be fully isolated: their context within the larger network influences behaviour and function. The structural approach therefore relies on the assumption that context-dependence is weak, and structural modularity is generally pronounced enough, to preserve the salient properties and behaviour of a motif or subcir- cuit in its native network context. Structural modularity is widely regarded as a necessary condition for the evolvability of complex networks. ‘Evolvability,’ in the general sense of the term, is defined as the ability to evolve (Daw- kins, 1989; Wagner and Altenberg, 1996; Hendrikse et al., 2007; Pigliucci, 2008). More specifi- cally, evolvability refers to the capacity of an evolving system to generate or facilitate adaptive change (Wagner and Altenberg, 1996; Pigliucci, 2008). Structural modularity can boost this capac- ity in several ways. Entire modules can be co-opted into new pathways during evolution, generating innovative change (Raff, 1996; von Dassow and Munro, 1999; True and Carroll, 2002; Davidson and Erwin, 2006; Erwin and Davidson, 2009; Monteiro and Podlaha, 2009; Wag- ner, 2011). Furthermore, each module can vary relatively independently, and it has been argued that this accounts for the individuality, origin, and homology of morphological characters as well as their trait-specific variational properties (Wagner and Altenberg, 1996; Wagner et al., 2007; Wag- ner, 2014). Finally, structural modularity allows for a fine-tuned response to specific selective pres- sures by minimizing off-target pleiotropic effects (Wagner and Altenberg, 1996; Pavlicev et al., 2008; Wagner and Zhang, 2011). The identification and analysis of structural modules has been very successful in many cases. For example, it has been used to understand the regulatory principles of segment determination in Dro- sophila (von Dassow et al., 2000; Ingolia, 2004), the origin and evolution of butterfly wing spots (Carroll et al., 1994; Brakefield et al., 1996; Keys et al., 1999; Beldade et al., 2002; Monteiro et al., 2003; Monteiro et al., 2006) and beetle horns (Moczek, 2006), and the mecha- nism and evolution of larval skeleton formation in sea urchins and sea stars (Hinman et al., 2003; Hinman and Davidson, 2007; Oliveri et al., 2008; Gao and Davidson, 2008). Other examples abound in the literature (see Raff, 1996; Schlosser and Wagner, 2004; Callebaut et al., 2005; Peter and Davidson, 2015 for comprehensive reviews). In spite of its usefulness, structural modularity has a number of serious limitations. Some model- ling studies suggest that it is not necessary for evolvability (see, for example, Crombach and Hoge- weg, 2008). Furthermore, it is notoriously difficult to identify structural modules and delimit their boundaries with any precision. One reason for this may be that the definition of (sub)system bound- aries is fundamentally context- and problem-dependent (see, for example, Chu et al., 2003; Chu, 2011). More to the point, even the simplest subcircuits tend to exhibit a rich dynamic reper- toire comprising a range of different behaviours depending on context (boundary conditions), quan- titative strength of parameter values (determining genetic interactions as well as production and decay