Though present in more than 6,000 living species of fish, the adipose fin, a small appendage that lies between the dorsal fin and tail, has no clear function and is thought to be vestigial. However, a new study analyzing their origins finds that these fins arose repeatedly and independently in multiple species. In addition, adipose fins appear to have repeatedly and independently evolved a skeleton, offering a glimpse into how new tissue types and structural complexity evolve in vertebrate appendages.
Adipose fins therefore represent a unique example of convergent evolution and new model for exploring the evolution of vertebrate limbs and appendages, report scientists from the University of Chicago in the Proceedings of the Royal Society B on March 5.
"Vertebrates in general have conserved body plans, and new appendages, whether fins or limbs, evolve rarely," said senior author Michael Coates, PhD, chair of the Committee on Evolutionary Biology at the University of Chicago. "Here, we have a natural experiment re-run repeatedly, providing a superb new system in which to explore novelty and change."
Usually small and structurally simple, adipose fins tend to get attention only when they are clipped from farm-raised trout and salmon as a tag. Despite their presence in thousands of fish species, they have been dismissed as a remnant of a once-functional fin. This assumption puzzled Coates and co-authors, as they saw no evidence of deterioration in adipose fin structure or function in the fossil record.
To study the evolutionary origins of this fin, Coates and lead author Thomas Stewart, graduate student in organismal biology and anatomy at the University of Chicago, turned to a technique known as ancestral-state reconstruction. With co-author W. Leo Smith, PhD, from the Biodiversity Institute at the University of Kansas, they created an evolutionary tree describing the relationships between fish with and without adipos
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University of Chicago Medical Center