Do fish have fingers? (continued)

After a reminder of the important concepts developed in the fifth lesson, the expression of Hox genes during the development of the pedunculated fin of the Australian lungfish Neoceratodus forsteri is described on the basis of two studies with somewhat different conclusions. Next, a scheme for the appearance of the autopod based on one of these two studies is discussed. Then, the fine regulation of the Hoxd gene during chiridial limb development is described at the level of chromatin structure and proximal and distal enhancers. The presence of this particular organization of regulatory landscapes is not observed in the lancelet (amphioxus), a cephalochordate animal, but is visible in the zebrafish. However, functional analysis of these teleost regulatory landscapes in mice reveals no capacity for expression in the most distal part of the limbs, suggesting that while the regulatory structures required for Hox gene deployment are already present in zebrafish, the potential distal enhancers are not. Consequently, at this level of super-deep homology, bony fish do not appear to have digits or structures that could be assimilated to them.