Adipose fin development and its relation to the evolutionary origins of median fins

Abstract

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.

Developmental Independence of Median Fins From the larval Fin Fold Revises Their Evolutionary Origin

“…1). Furthermore, although previous studies supposed that LMFF reduction is caused by apoptosis along the inter-n areas of the LMFF 15,23,25 , our data suggest that cell death did not play a role in LMFF reduction (Fig. 2).…”

“…The separated median ns in modern sh evolved from the continuous MFF in the basal chordate, resembling the developmental process derived from the LMFF 6,8, 12,13,15 . Based on this resemblance, the MFF hypothesis argues that reduction of the n fold is a key evolutionary process of the segregation among independent median ns in vertebrate phylogeny from the view of recapitulation theory 6,8,14-1615 .…”

“…They had an MFF, though previous studies lacked enough evidence to con rm the presence of endoskeletal elements [44][45][46] . Thus, it was naturally assumed that animals in the lineage from basal chordates to early vertebrates continuously possessed MFFs, and that sh LMFFs are homologous to these MFFs 15,46,47 . Addition to this premise, the MFF hypothesis presumes that median n development occurred by a reduction of the inter-n region based on morphological observations of ontogeny in living sh 2,6,8,12−14 .…”

“…As described above, these median ns develop through dynamic morphological changes in the LMFF during their ontogeny 3,[9][10][11]15,[17][18][19] , but their developmental processes are still poorly understood at the cellular and molecular levels. For instance, previous studies have supposed that the reduction of the LMFF is provided by apoptotic cell death 15,23 , and cell death at the LMFF has been reported in some studies [23][24][25] . However, cell death at the LMFF has been examined only during the embryonic stage when the median n primordia have not yet appeared.…”

“…The median ns in teleosts are composed of proximal cartilaginous skeleton (pterygiophores) and distal dermal skeleton ( n rays) 17,20−22 . As described above, these median ns develop through dynamic morphological changes in the LMFF during their ontogeny 3,[9][10][11]15,[17][18][19] , but their developmental processes are still poorly understood at the cellular and molecular levels. For instance, previous studies have supposed that the reduction of the LMFF is provided by apoptotic cell death 15,23 , and cell death at the LMFF has been reported in some studies [23][24][25] .…”