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Comparative Study
, 281 (1781), 20133120

The Origins of Adipose Fins: An Analysis of Homoplasy and the Serial Homology of Vertebrate Appendages

Affiliations
Comparative Study

The Origins of Adipose Fins: An Analysis of Homoplasy and the Serial Homology of Vertebrate Appendages

Thomas A Stewart et al. Proc Biol Sci.

Abstract

Adipose fins are appendages found on the dorsal midline between the dorsal and caudal fins in more than 6000 living species of teleost fishes. It has been consistently argued that adipose fins evolved once and have been lost repeatedly across teleosts owing to limited function. Here, we demonstrate that adipose fins originated repeatedly by using phylogenetic and anatomical evidence. This suggests that adipose fins are adaptive, although their function remains undetermined. To test for generalities in the evolution of form in de novo vertebrate fins, we studied the skeletal anatomy of adipose fins across 620 species belonging to 186 genera and 55 families. Adipose fins have repeatedly evolved endoskeletal plates, anterior dermal spines and fin rays. The repeated evolution of fin rays in adipose fins suggests that these fins can evolve new tissue types and increased structural complexity by expressing fin-associated developmental modules in these new territories. Patterns of skeletal elaboration differ between the various occurrences of adipose fins and challenge prevailing hypotheses for vertebrate fin origin. Adipose fins represent a powerful and, thus far, barely studied model for exploring the evolution of vertebrate limbs and the roles of adaptation and generative biases in morphological evolution.

Keywords: adaptation; complexity; constraint; convergence.

Figures

Figure 1.
Figure 1.
Adipose fins have evolved repeatedly. A summary of results from the Mk2 model of ancestral-state reconstruction run over 200 trees is plotted on a representative tree. Branch colour indicates the state that was recovered as optimal; black, red and blue indicate absent, present and neither, respectively. A particular character state is considered optimal when the log-likelihoods differed by 2 or more, the state with the lower likelihood being rejected. Thick lines indicate that 100% of the trees returned a particular state as optimal. Thin lines indicate that a fraction of the trees recovered that state as optimal. In these cases, the percentage of trees recovering that particular optimal solution is listed adjacent to the node, and a box-and-whisker plot of proportional probabilities, where 1 indicates presence and 0 absence, is shown below. Nodes boxed in grey varied across the distribution of trees. For those nodes, recovered topologies and their corresponding frequencies are shown at the left-hand side with the reconstructed ancestral states for those topologies. Nodes that varied but did not affect reconstructions (e.g. internal relations of the Gymnotiformes) are not indicated here.
Figure 2.
Figure 2.
The evolution of adipose fin skeleton is highly homoplastic. Distribution of adipose fin skeleton is mapped onto the phylogeny of Near et al. [34], which has been appended with the phylogeny of Siluriformes, modified from Sullivan et al. [39]. Adipose fin origination events recovered by ancestral-state reconstruction are shown on the phylogeny. Asterisks indicate additional groups that may have evolved adipose fins independently. Groups lacking adipose fins are distinguished with grey font. Cypriniformes is marked in grey, despite the observation of an adipose fin in Paracobitis rhadinaeus, because the rest of the 4000+ species in this clade lack adipose fins. Cartilage discs, anterior dermal spines, and fin rays have evolved repeatedly in adipose fins. (a) Example of adipose fin endoskeleton, Diaphus garmani, FMNH 64632. (b–d) Photographs of X-rays for representative species with anterior dermal spines. (b) Corydoras sychri, SU 51295; (c) Sisor torosus, CAS 96629; (d) Phractura clauseni, MNHN 1960-0148. (e–k) Photographs of X-rays for representative species with adipose fin rays; line drawings of whole bodies are in the grey panel. (e) Colossoma macropomum, FMNH 56826. (f) Clarotes bidorsalis, MNHN 1938-0139. (g) Phractocephalus hemioliopterus, FMNH 58032. (h) The posterior-most and smallest fin rays from panel (g) at a higher magnification. (i–k) Mochokus niloticus CT scans. lad, lepidotrichia of the adipose fin; ran, radials of the anal fin; lan, lepidotrichia of the anal fin. (k) A portion of the seventh adipose fin ray viewed anteriorly showing segmented and bilaterally paired hemitrichia. Photos in panels (b,d) reproduced with permission from the California Academy of Science. Photos in panels (c) and (f) reproduced with permission from the Le Muséum National d'Histoire Naturelle.

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