Conflicts with diarrheal pathogens trigger rapid evolution of an intestinal signaling axis

Clayton M. Carey; Sarah E. Apple; Zoe A. Hilbert; Michael S. Kay; Nels C. Elde

To determine whether GC-C evolved under selective pressure to modulate interactions with enterotoxins, we first collected the sequences of GC-C orthologs from 19 primate genomes for phylogenetic analysis (Table S1 ). Using statistical models that compare ratios of nonsynonymous to synonymous mutations (dN/dS) across species 5,6 , we find strong evidence of recurrent positive selection in GC-C (Table S2 ). Similar patterns of abundant nonsynonymous substitutions are widely reported for genes encoding dedicated immune functions that are targeted by pathogen factors 7 . This suggests that GC-C may similarly be subject to recurrent selection based on temporary advantages conferred by mitigating pathogen interactions. Unlike dedicated immune proteins, however, GC-C is functionally constrained by its physiological function of regulating water secretion through interactions with endogenous ligands. Indeed, the GC-C signaling axis is found in all vertebrates 8 , a pattern associated with functional conservation. To better understand the role of toxin interactions in GC-C evolution, we compared sequence evolution in the paralogous natriuretic peptide receptors (NPR1-2), which share similar overall structure and function with GC-C. NPR1 and NPR2 are not known to interact with pathogen-derived toxins 9 , and in contrast to GC-C, bear signatures of purifying selection with highly conserved sequences across primates (Tables S1,S2). Rapid evolution of GC-C is therefore associated with its unique interaction with pathogens.