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The dorsal, anal and caudal fins of vertebrates are proposed to have originated by the partitioning and transformation of the continuous median fin fold that is plesiomorphic to chordates. Evaluating this hypothesis has been challenging, because it is unclear how the median fin fold relates to the adult median fins of vertebrates. To understand how new median fins originate, here we study the development and diversity of adipose fins. Phylogenetic mapping shows that in all lineages except Characoidei (Characiformes) adipose fins develop from a domain of the larval median fin fold. To inform how the larva’s median fin fold contributes to the adipose fin, we study Corydoras aeneus (Siluriformes). As the fin fold reduces around the prospective site of the adipose fin, a fin spine develops in the fold, growing both proximally and distally, and sensory innervation, which appears to originate from the recurrent ramus of the facial nerve and from dorsal rami of the spinal cord, develops in the adipose fin membrane. Collectively, these data show how a plesiomorphic median fin fold can serve as scaffolding for the evolution and development of novel, individuated median fins, consistent with the median fin fold hypothesis.
Introduction
Fins have evolved repeatedly in vertebrates and, thus, provide a powerful system for studying how new body parts originate. Chordates are plesiomorphically characterized by a median fin fold (MFF), a midline structure comprised of dorsal and ventral portions that meet posteriorly to form a protocercal tail The extinct chordates Haikuichthys and Haikuella exhibit this condition, with the ventral portion of the MFF interrupted by the anus. The extant cephalchordate amphioxus also has a MFF, which passes to the right of the anus uninterrupted7. Spatially differentiated, individuated median fins evolved later, in craniates. These new fins are hypothesized to have originated by the partitioning of the MFF into multiple fin modules. Specifically, the dorsal, anal and caudal fins are predicted to have evolved from the MFF by its reduction in some positions and its retention in other. This ‘median fin-fold hypothesis’ is related to the ‘lateral fin-fold hypothesis’ of paired pectoral and pelvic fin origin, which itself posits that paired continuous fins along the flank were subdivided to create the pectoral and pelvic fins. Although the lateral fin-fold hypothesis has largely been abandoned in favor of a scenario where pectoral fins evolved first and pelvic fins evolved secondarily, the MFF hypothesis remains influential.
In many fishes, ontogeny appears to recapitulate the phylogenetic transformational scenario predicted by the MFF hypothesis. For example in zebrafish, Danio rerio (Cyprinidae), a larval median fin fold (LMFF) encompasses the trunk early in development16. The LMFF develops as the somites are forming; specification and outgrowth proceeds in a caudal-to-rostral direction, driven by Fgf signaling. The LMFF is composed of an epithelial bilayer medial to which are actinotrichia (tapered collagenous rods organized approximately parallel to the fin’s
Here, to inform hypotheses of (1) phylogenetic transformation from MFFs into individuated fins and (2) ontogenetic transformation of the LMFF into adult fins, we study the diversity and development of adipose fins. These appendages have evolved repeatedly within teleosts and are positioned on the dorsal midline between the dorsal and caudal fins. Adipose fins have been studied as models of how form and function evolves in vertebrate appendages and might also inform how development evolves to generate novel appendages. Descriptions of adipose fin morphogenesis are scattered throughout the literature—in taxonomies of larval fishes, staging papers for select taxa, and a study of early development of these fins35. We aggregate the data on adipose fin development from the literature and analyze them in a phylogenetic context. Additionally, we characterize adipose fin development in the South American armored catfish Corydoras aeneus (Gill 1858) (Siluriformes, Callicithyidae), focusing on the development of the adipose fin skeleton and sensory anatomy. Collectively, these data reveal that adipose fins can evolve and develop by retention and elaboration of a domain of the LMFF. We discuss how these data inform hypotheses of median fin origin in early vertebrates.
