Reconstructing the deep phylogeny of the MAPK signaling network: functional specialization via multi-tier coevolutionary expansion

The mitogen-activated protein kinase (MAPK) signaling network is a three-tier cascade that regulates key cellular responses in eukaryotes. However, the evolutionary origins of its complex interactions and functional diversity remain poorly understood. Here, we conducted a comprehensive phylogenetic analysis of MAPK components across Eukarya to delineate divergences of non-human orthologs of human paralogs along the human evolutionary backbone. We identified two major pulses of coevolutionary expansion: one predating the divergence of fungi and animals, and another predating the origin of animals. Our reconstruction also infers a polyphyletic origin for the atypical MAPKs. Integrating functional literature across eukaryotic taxa with our reconstructed trees reveals that the two clades of MAP3K, Sterile-like (STE) and tyrosine kinase-like (TKL), had distinct evolutionary trajectories and influences on downstream pathway diversification. STEs that function as MAP3Ks are conserved across extant eukaryotes. Despite the absence of TKL MAP3Ks in many early diverging eukaryotes, the expansion of TKL MAP3Ks aligns phylogenetically and functionally with that of the downstream MAP2Ks and MAPKs. We thus propose that the MAPK network originated as a STE-regulated pathway, and that subsequent radiations of the TKLs drove the diversification of downstream components and top-down finetuning of pathway specificity. We thus provide an evolutionary framework for generating novel hypotheses on the functional diversity of this key signaling network, including potential insights into the evolution of animal multicellularity. Our study demonstrates that phylogenetics can offer new perspectives towards understanding complex cellular physiology.