Fins and Limbs: Emergence of Morphological Differences

Paired appendages (fins and limbs) are regarded as distinct morphologies by classification of skeletal patterns. On the basis of sequential orientation and articulation of tetrapod limb bones, we can understand that stylopodial/zeugopodial skeletal elements are present in an extinct and extant basal sarcopterygian (coelacanth and lungfish) fin and that only nonhomologous radial bones exist in a zebrafish fin. From these phylogenetic views, morphological differences between fins and limbs and paleontological discoveries of limb-like fins of basal sarcopterygians emphasize both the homologous skeletal elements and tetrapodomorph evolution. During embryogenesis, on the other hand, initial fin development requires apical ectodermal ridge (AER) signals as does tetrapod limb development, and then the AER itself starts to transform into a fin-specific structure, the apical fold (AF). HoxD genes are involved in fish fin development as in tetrapod limb development, but the resultant skeletal patterns of fins are very different from those of limbs as a result of differences in regulation of genes such as HoxD. From these developmental aspects, we can understand that both fins and limbs develop by common mechanisms, including fibroblast growth factors (FGFs) from the AER and Hox genes, and that alteration of basic mechanisms by heterochronic/heterometric change in expression of AER/AF signals and Hox gives rise to morphological differences among paired appendages. In this chapter, we describe homology and difference in several research fields (genome commonality/difference, developmental commonality/difference, and anatomical or paleontological correspondence/difference) and especially explain a scenario of fin-to-limb evolution.