Review and experimental evaluation of the embryonic development and evolutionary history of flipper development and...

Cetaceans are the only mammals to have evolved hyperphalangy, an increase in the number of phalanges beyond the mammalian plesiomorphic condition of three phalanges per digit. In this study, cetaceans were used as a novel model to review previous studies of mammalian hyperphalangy and contribute new experimental evidence as to the molecular origins of this phenotype in embryos of the pantropical spotted dolphin (Stenella attenuata). Results show embryos of dolphins, mice, and pigs share similar spatiotemporal patterns of signaling proteins known to shape limbs of mammals (e.g., FGF8, BMP2/4, WNT, GREM). However, fetal dolphins differ in that their interdigital tissues are retained, instead of undergoing apoptosis, and that multiple waves of interdigital signals likely contribute to the patterning of supernumerary joints and phalanges in adjacent digits. Integration of fossil and experimental evidence suggests that the presence of interdigital webbing within the fossils of semi-aquatic cetaceans, recovered from the Eocene Epoch (49Ma), was probably the result of BMP-antagonists counteracting interdigital apoptosis during embryonic limb development. Modifications to signals originating in these interdigital tissues likely contributed to the origin of an incipient form of hyperphalangy in obligatorily aquatic cetaceans about 35Ma. Finally, an extreme form of hyperphalangy, with six or more phalanges per digit, evolved independently in rorqual whales (Balaenopteridae) and delphinids, and was probably associated with a wave of signaling within the interdigital tissues.