DAHN, Randall D.*; DAVIS, Marcus C.; SHUBIN, Neil H.; University of Chicago: Conserved Shh function and regulation in gnathostome appendage patterning

The developmental mechanisms patterning the gnathostome appendage endoskeleton exhibit surprising molecular conservation considering the diverse spectrum of anatomical variation achieved. However, phylogenetic differences fundamental signaling pathways have been reported, rendering construction of a general appendage developmental bauplan problematic. One such gene is Sonic hedgehog (Shh), which controls the anteroposterior (A/P) polarity of developing osteichthyan appendages via asymmetric expression along the posterior fin/limb border. Two critical elements of Shh regulation are conserved across osteichthyes: (1) appendage-specific Shh expression is controlled by a DNA regulatory element located 1 Mb from the Shh locus; (2) treatment with retinoic acid (RA) results in ectopic anterior Shh expression, and results in mirror-image duplications of endoskeletal elements. Chondrichthyans have been reported to lack both Shh expression in developing appendages, and the appendage-specific DNA regulatory element, suggesting that Shh signaling is dispensible for normal fin development. We have re-examined the role of Shh in developing skate and shark appendages, and report that Shh is expressed along the posterior border of both paired and unpaired fins. We have identified appendage-specific DNA regulatory elements in the genomes of several chondrichthyan species, which exhibit striking conservation with tetrapod and actinopterygian orthologs. We further report RA treatment results in both ectopic Shh expression in the anterior fin border, and mirror-image endoskeletal duplications. Conserved Shh expression, regulation, and function in developing gnathostome fins/limbs confirms its fundamental role in appendage patterning, and has important implications for understanding the evolution of appendage pattern.