Hox genes and axis paramorphism
Another important insight about the origin of limbs is based on the observation that they are placed in the transition zones between different kinds of vertebrae: forelimb in the cervical-thoracic transition and hindlimbs in the dorso-sacral transition (Mabee et al., 2002). Coates and Cohn (1999) argue that these domains could have evolved in relationship to the regionalization of the gastrointestinal tract by Hox genes. Later Tanaka and Onimaru (2012) proposed a more comprehensive model where not only an anteroposterior patterning, related with Hox genes, is required for the origin of paired fins. They also included the need of dorsoventral differentiation, subdivision of somitic and splanchnic mesoderm and different initiation signals (as Tbx4/5).
Independently, Minelli (2000, 2003) pointed out the similarity between limbs and the main body axis due to the presence of sexually dimorphic traits on both structures. Another similitude corresponds to the fact that, all appendages develop from ‘buds’ devoid of endoderm. Examples from vertebrates include the tail and the paired fins, as well as the fleshy posterior dorsal and anal fins of Latimeria, which are considered to be median fins homeotically changed into a paired fin identity (Tabin and Laufer, 1993).
Which of all the appendages is the ancestral vertebrate appendage? The tail bud is present in all vertebrates, even before the origin of the extremities. Actually, the oldest fin known is the caudal fin of the Burgess Shale fossil Pikaia gracilens (Walcott, 1911) (Morris and Caron, 2012). Therefore, we argue that it is possible that much of the developmental pathway involved in the formation of this type of fins may have been co-opted later in evolution, for the development of paired appendages.