Gene cooption without duplication during the evolution of a male-pregnancy gene in pipefish

Comparative studies of developmental processes suggest that novel traits usually evolve through the cooption of preexisting genes and proteins, mainly via gene duplication and functional specialization of paralogs. However, an alternative hypothesis is that novel protein function can evolve without gene duplication, through changes in the spatiotemporal patterns of gene expression (e.g., via cis-regulatory elements), or functional modifications (e.g., addition of functional domains) of the proteins they encode, or both. Here we present an astacin metalloprotease, dubbed patristacin, which has been coopted without duplication, via alteration in the expression of a preexisting gene from the kidney and liver of bony fishes, for a novel role in the brood pouch of pregnant male pipefish. We examined the molecular evolution of patristacin and found conservation of astacin-specific motifs but also several positively selected amino acids that may represent functional modifications for male pregnancy. Overall, our results pinpoint a clear case in which gene cooption occurred without gene duplication during the genesis of an evolutionarily significant novel structure, the male brood pouch. These findings contribute to a growing understanding of morphological innovation, a critically important but poorly understood process in evolutionary biology.