The evolutionary mechanism that gave rise to both paired limbs in humans and paired fins in rays, sharks, and skates can likely be traced back to the transformation of gill arches in early fish, according to new research from the Marine Biological Laboratory (MBL) in Massachusetts.
Writing in this week’s edition of the journal Development, MBL scientist Andrew Gillis and his colleagues present genetic evidence that the mechanisms used to pattern the gill-arch appendages known as branchial rays are strikingly similar to those responsible for paired fins and limbs.
Gillis, who is also a research fellow at the University of Cambridge, and his co-authors studied the embryos of the little skate, focusing their efforts on the “Sonic hedgehog” gene (yes, named after the video game). This particular gene produces a signaling protein in tetrapods which establishes the anteroposterior axis of their limb buds and maintains proliferative expansion of limb endoskeletal progenitors.
The protein’s function is well known in mammals, the study authors said in a statement, and the new research found that it plays a similar role in branchial ray development: it sets up the axis of development, and eventually helps maintain the growth of the limb skeleton of the skates.
Credit: J. Andrew Gillis
Is there a complex evolutionary relationship between the species?
As part of their study, the researchers used loss-of-function, label-retention, and fate-mapping methods to show that Sonic hedgehog gene secretions from a signaling center in developing gill arches helped establishes gill anteroposterior polarity and maintained the proliferative expansion of branchial ray endoskeletal progenitor cells, much like they do in mammalian limbs.
The research highlighted similarities in the genetic mechanisms of branchial rays and paired limbs and fins. “The shared role of Sonic hedgehog in patterning branchial rays and limbs may be due to a deep evolutionary relationship between the two,” Gillis explained, “or it may simply be that two unrelated appendages independently use the same gene for the same function.”
Additional research should help discover which is the case by comparing the function of other kinds of genes during the development of each type of appendage, he added. The scientists will continue their work with skates at the MBL this summer, with the hopes that they can find out which scenario is most likely.
No matter what he and his colleagues find, Gillis is convinced that branchial rays “will figure prominently in the story of the evolutionary origin of vertebrate animal appendages, either by shedding light on the evolutionary antecedent of paired fins/limbs, or by teaching us about the genetic mechanisms that animals can use to invent new appendages.”
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Image credit: Thinkstock
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