Retinoic acid-dependent establishment of positional information in the hindbrain was conserved during vertebrate evolution.

Zebrafish hoxb1b is expressed during epiboly in the posterior neural plate, with its anterior boundary at the prospective r4 region providing a positional cue for hindbrain formation. A similar function and expression is known for Hoxa1 in mice, suggesting a shared regulatory mechanism for hindbrain patterning in vertebrate embryos. To understand the evolution of the regulatory mechanisms of key genes in patterning of the central nervous system, we examined how hoxb1b transcription is regulated in zebrafish embryos and compared the regulatory mechanisms between mammals and teleosts that have undergone an additional genome duplication. By promoter analysis, we found that the expression of the reporter gene recapitulated hoxb1b expression when driven in transgenic embryos by a combination of the upstream 8.0-kb DNA and downstream 4.6-kb DNA. Furthermore, reporter expression expanded anteriorly when transgenic embryos were exposed to retinoic acid (RA) or LiCl, or injected with fgf3/8 mRNA, implicating the flanking DNA examined here in the responsiveness of hoxb1b to posteriorizing signals. We further identified at least two functional RA responsive elements in the downstream DNA that were shown to be major regulators of early hoxb1b expression during gastrulation, while the upstream DNA, which harbors repetitive sequences with apparent similarity to the autoregulatory sequence of mouse Hoxb1, contributed only to later hoxb1b expression, during somitogenesis. Possible implications in vertebrate evolution are discussed based on these findings.

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