Spiny and soft-rayed fin domains in acanthomorph fish are established through a BMP-- signaling network.

With over 18,000 species, the , or spiny-rayed fishes, form the largest and arguably most diverse radiation of vertebrates. One of the key novelties that contributed to their evolutionary success are the spiny rays in their fins that serve as a defense mechanism. We investigated the patterning mechanisms underlying the differentiation of median fin into discrete spiny and soft-rayed domains during the ontogeny of the direct-developing cichlid fish Distinct transcription factor signatures characterize these two fin domains, whereby mutually exclusive expression of with and marks the spine to soft-ray boundary. The soft-ray domain is established by BMP inhibition via , which synergizes in the posterior fin with secreted from a zone of polarizing activity. Modulation of BMP signaling by chemical inhibition or CRISPR/Cas9 knockout induces homeotic transformations of spines into soft rays and vice versa. The expression of spine and soft-ray genes in nonacanthomorph fins indicates that a combination of exaptation and posterior expansion of an ancestral developmental program for the anterior fin margin allowed the evolution of robustly individuated spiny and soft-rayed domains. We propose that a repeated exaptation of such pattern might underly the convergent evolution of anterior spiny-fin elements across fishes.