Differences in expression pattern and function between zebrafish hoxc13 orthologs: recruitment of Hoxc13b into an early embryonic role.

Vertebrate Hox genes are generally believed to initiate expression at the primitive streak or early neural plate stages. The timing and spatial restrictions of the Hox expression patterns during these stages correlate well with their demonstrated role in axial patterning. Here we demonstrate that one zebrafish hoxc13 ortholog, hoxc13a, has an expression pattern in the developing tail bud that is consistent with the gene playing a role in axial patterning. However, the second hoxc13 ortholog, hoxc13b, is maternally expressed and is detectable in every cell of early cleavage embryos through gastrulae. In addition, both transcript and protein are detectable at these stages. At 19 h post fertilization (hpf), hoxc13b expression is up-regulated in the tail bud, becoming restricted to the tail bud by 24 hpf. Importantly, by 24 hpf, hoxc13b morphants show a specific developmental delay, which can be rescued by co-injecting synthetic capped hoxc13a or hoxc13b message. These data suggest some functional divergence due to altered expression patterns of the two hoxc13 orthologs after duplication. Further characterization of the hoxc13b morphant delay reveals that it is biphasic in nature, with the first phase of the delay occurring before gastrulation, suggesting a new role for vertebrate Hox genes before their conserved role in axial patterning. The extent of the delay does not change through 20 hpf; however, an additional delay emerges at this time. Notably, this second phase of the delay correlates with hoxc13b expression pattern becoming restricted to the tail bud.