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Presenting Author(s) Renata Freitas
Abstract Title Hox genes in the median fins of sharks and the evolution of fin development.
Full author List Renata Freitas and Martin J. Cohn
Text of abstract

Most fishes develop two different types of appendages; the paired fins in the abdominal region, and the unpaired median fins (dorsal, caudal and anal) that form along the dorsal and ventral midlines. These two classes of fins have different embryonic origins, yet they give rise to remarkably similar structures. Median fins appear in the fossil record of early vertebrates prior to the origin of the paired fins, suggesting that the genetic program for fin development may have originated in the midline and later been co-opted during the evolution of lateral paired appendages. Although development of the paired limbs and fins has been intensively investigated, surprisingly little is known about the molecular mechanisms involved in median fin development. Understanding the genetic program for median fin development is crucial to uncovering the mechanisms responsible for the origin of paired fins. Sharks possess both median and paired fins, and occupy a key phylogenetic position as the most basal living gnathostome. As such, inclusion of sharks in comparative developmental studies provides a unique opportunity to identify the mechanisms that govern the development of primitive fin morphologies. We have carried out a detailed analysis of paired and unpaired fin development in the catshark Scyliorhinus canicula. We show that the individual dorsal, anal and caudal fins develop within a continuous median finfold, and the outgrowth of this finfold is maintained by an elongated apical ectodermal ridge that expresses Fgf8 and Dlx genes. Interestingly, we find that HoxA and HoxD genes are differentially expressed within the embryonic median finfold, prior to its subdivision into dorsal, anal and caudal fins, and the expression boundaries mark the positions of the future median fins. Each median fin then takes on the classic nested, collinear pattern of Hox gene expression along the anterioposterior axis. Together, these findings suggest that the developmental program for fin development first evolved in midline appendages and was co-opted by lateral plate mesoderm for paired fin development.

Which session is your work most relevant to: Limb evolution