Tinkering with Fins

Last year, I wrote about the adipose fin found in some fish, which researchers had considered a vestigal organ until a team from the University of Chicago showed that it has, in fact, repeatedly evolved in several lineages. Thomas Stewart, lead author of the study, has continued studying the adipose fin. While its function is still a mystery, Stewart's investigation of the skeletal development of the adipose fin can teach us something about how developmental processes evolve.

Stewart studied the development of the adipose fin in a single fish species, the redtail catfish Phractocephalus hemioliopterus. Using a combination of histology and X-ray imaging, he mapped the bony rays in the fin and tracked how they develop. He found that bony rays develop differently in the adipose fin than in other fins. In redtail catfish, rays normally grow from the base of a fin to its tip and appear in a specific order within the fin. In the adipose fin, the rays begin developing at the tip and appear randomly. In addition, rays in adipose fins can grow in both directions, towards the tip or the base, while rays in other fins always grow from the base to the tip of the fin. Finally, the adipose fin develops its rays in adult fish, while other fins acquire rays while the fish are still larvae.

Basically, Stewart's data show that ray development in the adipose fin is much more variable than in other fins; in fact, he describes its skeleton as "the most variable of any vertebrate appendage." The variability certainly raises questions about the function of the adipose fin (at least in redtail catfish), but it's also an intriguing example of how new morphologies evolve.

Stewart argues that the variation reflects an absence of canalisation during adipose fin development. Adipose fins are serial homologues that have appeared by translocation of a developmental program to a new location. Even if it has a function, the adipose fin is free from the constraints that evolved around the developmental program to ensure the correct output elsewhere in the body. As a result, the developmental process may be more sensitive to different influences, leading ray development to be more variable. It's a neat idea that nicely captures an important interaction between evolution and development and the eternal tinkering that is evolution. Personally, I think the best adjective to describe ray development in adipose fins is 'protean', with its evocation of the ever-changing Proteus and its primordial resonance.

Ref
Stewart, T. The origin of a new fin skeleton through tinkering. Biology Letters 11(7):20150415. doi: 10.1098/rsbl.2015.0415

Image credits
The Redtail Catfish image is by photographer Mark Sabaj Pérez and is made available by the iXingu Project.