Abstract
A major goal of evolutionary developmental biology (evo-devo) is to understand how multicellular body plans of increasing complexity have evolved, and how the corresponding developmental programs are genetically encoded. It has been repeatedly argued that key to the evolution of increased body plan complexity is the modularity of the underlying developmental gene regulatory networks (GRNs). This modularity is considered essential for network robustness and evolvability. In our opinion, these ideas, appealing as they may sound, have not been sufficiently tested. Here we use computer simulations to study the evolution of GRNs' underlying body plan patterning. We select for body plan segmentation and differentiation, as these are considered to be major innovations in metazoan evolution. To allow modular networks to evolve, we independently select for segmentation and differentiation. We study both the occurrence and relation of robustness, evolvability and modularity of evolved networks. Interestingly, we observed two distinct evolutionary strategies to evolve a segmented, differentiated body plan. In the first strategy, first segments and then differentiation domains evolve (SF strategy). In the second scenario segments and domains evolve simultaneously (SS strategy). We demonstrate that under indirect selection for robustness the SF strategy becomes dominant. In addition, as a byproduct of this larger robustness, the SF strategy is also more evolvable. Finally, using a combined functional and architectural approach, we determine network modularity. We find that while SS networks generate segments and domains in an integrated manner, SF networks use largely independent modules to produce segments and domains. Surprisingly, we find that widely used, purely architectural methods for determining network modularity completely fail to establish this higher modularity of SF networks. Finally, we observe that, as a free side effect of evolving segmentation and differentiation in combination, we obtained in-silico developmental mechanisms resembling mechanisms used in vertebrate development.
Highlights
A major goal of evolutionary developmental biology is to understand how multicellular body plans of increasing complexity have evolved, and how the underlying developmental programs are encoded in the genome and gene regulatory network (GRN)
It is frequently argued that developmental GRNs are typically modular, i.e. that different functions are performed by largely independent network parts [2,13,14,15,16], and that this is the key property responsible for both network robustness and evolvability
An important question in evolutionary developmental biology is how the complex organisms we see around us have evolved, and how this complexity is encoded in their DNA
Summary
A major goal of evolutionary developmental biology (evo-devo) is to understand how multicellular body plans of increasing complexity have evolved, and how the underlying developmental programs are encoded in the genome and gene regulatory network (GRN).Modern evo-devo research shows more and more that a shared developmental toolkit of signaling, adhesion and transcription factor genes are essential for the development of organisms ranging in body plan complexity from cniderians to arthropods and vertebrates [1,2,3]. The idea is that modularity reduces pleiotropy, allowing for the malfunctioning of or tinkering with network parts involved in one function without producing failure in other functions [2,13,14,15,16]. This reasoning sounds appealing and intuitively correct, little research has been done to explicitly test the roles and relationships of developmental network modularity, robustness and evolvability in the evolution of complex body plans. We argue that it is currently unclear how modular developmental networks are, how such modularity evolves, and how this modularity looks
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