Evolution of cis-regulatory modules for the head organizer gene in chordates: comparisons between and .

BACKGROUND

In cephalochordates (amphioxus), the notochord runs along the dorsal to the anterior tip of the body. In contrast, the vertebrate head is formed anterior to the notochord, as a result of head organizer formation in anterior mesoderm during early development. A key gene for the vertebrate head organizer, , is broadly expressed in the dorsal mesoderm of amphioxus gastrula. Amphioxus expression subsequently becomes restricted to the posterior notochord from the early neurula. This has prompted the hypothesis that a change in expression patterns of led to development of the vertebrate head during chordate evolution. However, molecular mechanisms of head organizer evolution involving have never been elucidated.

RESULTS

To address this question, we compared cis-regulatory modules of vertebrate organizer genes between amphioxus, , and frogs, and . Here we show conservation and diversification of gene regulatory mechanisms through cis-regulatory modules for , , and in and . Reporter analysis using embryos demonstrates that activation of by Nodal/FoxH1 signal through the 5' upstream region, that of by Nodal/FoxH1 signal through the first intron, and that of by Lim1 through the second intron, are conserved between amphioxus and . However, activation of by Lim1 and Otx through the 5' upstream region in are not conserved in amphioxus. Furthermore, the 5' region of amphioxus recapitulated the amphioxus-like posterior mesoderm expression of the reporter gene in transgenic embryos.

CONCLUSIONS

On the basis of this study, we propose a model, in which the gene acquired the cis-regulatory module bound with Lim1 and Otx at its 5' upstream region to be activated persistently in anterior mesoderm, in the vertebrate lineage. Because Gsc globally represses trunk (notochord) genes in the vertebrate head organizer, this cooption of in vertebrates appears to have resulted in inhibition of trunk genes and acquisition of the head organizer and its derivative prechordal plate.