The role of posterior HoxA genes in the evolution of novel fins

Batoids exhibit unique body plans, with derived fin morphologies such as the anteriorly expanded pectoral fins that fuse to the head, or distally extended anterior pelvic fin lobes used for a modified swimming technique utilized by skates (Rajidea). The little skate (Leucoraja erinacea), exhibits both of these unique fin morphologies. These fin modifications are not present in a typical shark body plan, and little is known regarding the mechanisms underlying their development. A recent study identified a novel apical ectodermal ridge (AER) associated with the development of the anterior pectoral fin in the little skate, but the role of the posterior FIoxA genes was not featured during skate fin development. We present the first evidence for FIoxA expression (HoxA 11 and HoxA 13) in novel AER domains associated with the development of these novel fin morphologies in a representative batoid, L. erinacea. We found HoxA 13 expression associated with the recently described novel AER in the anterior pectoral fin, and HoxA 11 expression in a novel AER domain in the anterior pelvic fin that we describe here. Further, we found that HoxA 11 expression is associated with the developing fin rays in paired fins. Finally, we found evidence for the "distal phase" FIoxA expression in the developing claspers, suggesting a more complex Hox code than previously described during specification of these modified fin domains.