Drift, selection and convergence in the evolution of a nonribosomal peptide

Nonribosomal peptides play important roles in microbial ecology and medicine, for example as toxins and antibiotics such as penicillin 1 . Like ordinary proteins, nonribosomal peptides are subject to evolutionary change, which is driven by rampant recombinations of their synthetases 2 . This makes it difficult to study the evolutionary histories of nonribosomal peptides, because recombination is a serious challenge for phylogenetic inference 3 . Here we use recombination-aware phylogenetics and ancestral sequence reconstruction to retrace the evolution of GameXPeptides, nonribosomal peptides entomopathogenic bacteria use to kill their insect prey 4 . By untangling the complex histories of their synthetases, we discover dozens of new GameXPeptides and show striking patterns of structural convergence. In analogy to classic statistical test for natural selection acting on nucleotide mutations, we develop a similar test for whether selection acts to fix recombinations. We provide statistical and experimental evidence of both neutral genetic drift and natural selection in the diversification of GameXPeptides. GameXPeptides variants rapidly evolved highly insect specific toxicities, suggesting that their diversification may be driven by an evolutionary arms race. Our work suggests that small, fast evolving alteration to nonribosomal peptides can overcome host resistance on short timescales, with important implications for the evolution of resistance evading antibiotics.