Phylogenetic evidence for the modular evolution of metazoan signalling pathways

Abstract

Communication among cells was paramount to the evolutionary increase in cell type diversity and, ultimately, the origin of large body size. Across the diversity of Metazoa, there are only few conserved cell signalling pathways known to orchestrate the complex cell and tissue interactions regulating development; thus, modification to these few pathways has been responsible for generating diversity during the evolution of animals. Here, we summarize evidence for the origin and putative function of the intracellular, membrane-bound and secreted components of seven metazoan cell signalling pathways with a special focus on early branching metazoans (ctenophores, poriferans, placozoans and cnidarians) and basal unikonts (amoebozoans, fungi, filastereans and choanoflagellates). We highlight the modular incorporation of intra- and extracellular components in each signalling pathway and suggest that increases in the complexity of the extracellular matrix may have further promoted the modulation of cell signalling during metazoan evolution. Most importantly, this updated view of metazoan signalling pathways highlights the need for explicit study of canonical signalling pathway components in taxa that do not operate a complete signalling pathway. Studies like these are critical for developing a deeper understanding of the evolution of cell signalling.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Keywords: cell signalling; complexity; ctenophore; evolution; metazoa.

Figure 1.

Schematic of the major components of the seven conserved metazoan signalling pathways: TGF-β (red), canonical Wnt (orange), nuclear receptor (yellow), Notch/Delta (green), Hedgehog (blue), RTK/growth factor (violet) and JAK/STAT (grey), and the components that comprise the basement membrane and adhesome complex (black). The origin of the canonical/functional pathway and the origin of secreted/extracellular and intracellular inhibitors are indicated on the cladogram. Abbreviations for taxa represented by each lineage are indicated in the glossary. The evolutionary origin of the canonical/functional version of each pathway is indicated at the node where conserved function has been identified experimentally.