Caught in the act of triplication: TNF superfamily
In humans, the signaling mechanisms of the 19 paralogs of the tumor necrosis factor superfamily (TNFSF) and the 29 receptor paralogs of the tumor necrosis factor receptor superfamily (TNFRSF) are extensively characterized because of their therapeutic relevance. The functional expansion of TNFSF in vertebrates from a single ancestral gene through successive duplication events is also well established. However, apart from the first identification of a TNFSF homolog, Eiger (dmEiger), in Drosophila melanogaster in 2002, together with its receptor homologs Wengen (dmWgn) and Grindelwald (dmGrnd), this signaling system has remained largely unexplored in invertebrates. More recently, the implication of an Eiger homolog in Plasmodium resistance in malaria vectors has further highlighted the need for a systematic investigation of this pathway in lower invertebrates. Structural comparison of the dmEiger-dmGrnd complex with the canonical 3:3 ligand-receptor configuration observed in human TNFSF-TNFRSF signaling suggests either conservation of this signaling modality since before the bilaterian split or convergent evolution of a similar architecture in both branches. The recent explosion in high-quality proteomes spanning diverse phyla, together with advances in protein-complex prediction using AlphaFold-multimer, now enables large-scale exploration of ligand-receptor evolution across invertebrates. Here, we analyzed 148 near-complete proteomes spanning major invertebrate phyla and identified 290 TNFSF, 336 wengen (wgn), and 115 grindelwald (grnd) homologs, including homologs from lower invertebrates. Structural characterization of 140 selected complexes using AlphaFold and AlphaFold-multimer revealed several key findings: (i) TNFSF and TNFRSF homologs are present in majority of the phyla under invertebrates (ii) the canonical 3:3 ligand-receptor signaling configuration is conserved across invertebrates; (iii) orthologs of 25 out of the 26 genes implicated in TNF signaling pathways are present in lower invertebrates; and (iv) signaling through grnd-like receptors containing a single cysteine-rich domain with CXXCXXXC signature is the predominant signaling mode in invertebrates and becomes highly prevalent in Arthropoda. We also elaborate a hypothesize on the evolutionary trajectories toward a genetically parsimonious signaling by this complex system before functional expansion in vertebrates and species diversification in Arthropoda.