Contributions to Zoology, 84 (4) – 2015Darko D. Cotoras; Miguel L. Allende: Was the tail bud the ancestral centre where the fin developmental program evolved in chordates?

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Based on the current state of knowledge, is very difficult to assess whether the gill arch or a fin fold gave rise to the extremities (Fig. 1). Both hypotheses – the Archy­ptergygium and the Fin Fold Hypothesis – are partially supported by gene expression data. On the other hand, the hypothesis of the ventralization of the zones of competence could be broadly grouped with the Fin Fold Hypothesis, because in both cases the original genetic mechanism ia present in a ribbon-like fin present in the outer body of the organism. Here, as an alternative hypothesis (Fig. 1, left side), we presented evidence that similar genes are involved in the formation of the tail and the limb. Part of this evidence is the common role of RA in the duplication of limbs and tails on different lineages of vertebrates. Moreover, other genetic elements related with extremities and tail development are present in non-vertebrate chordates. This suggests that the genetic tool kit involved in tail development could have been co-opted by the extremities (Schubert et al., 2000). This observation is congruent with the hypothesis of Axis paramorphism (Minelli, 2000, 2003). We think that further studies including gene expression analysis of the tail bud and gill arches in amphioxus and the larval tail of Ciona will help to confirm or disprove this idea.