By exploiting the cell-cycle-dependent proteolysis of two ubiquitination oscillators, human Cdt1 and geminin, which are the direct substrates of SCF ^Skp2 and APC ^Cdh1 complexes, respectively, Fucci technique labels mammalian cell nuclei in G ₁ and S/G ₂ /M phases with different colors. Transgenic mice expressing these G ₁ and S/G ₂ /M markers offer a powerful means to investigate the coordination of the cell cycle with morphogenetic processes. We attempted to introduce these markers into zebrafish embryos to take advantage of their favorable optical properties. However, although the fundamental mechanisms for cell-cycle control appear to be well conserved among species, the G ₁ marker based on the SCF ^Skp2 -mediated degradation of human Cdt1 did not work in fish cells, probably because the marker was not ubiquitinated properly by a fish E3 ligase complex. We describe here the generation of a Fucci derivative using zebrafish homologs of Cdt1 and geminin, which provides sweeping views of cell proliferation in whole fish embryos. Remarkably, we discovered two anterior-to-posterior waves of cell-cycle transitions, G ₁ /S and M/G ₁ , in the differentiating notochord. Our study demonstrates the effectiveness of using the Cul4 ^Ddb1 -mediated Cdt1 degradation pathway common to all metazoans for the development of a G ₁ marker that works in the nonmammalian animal model.

In vertebrates, the total number of vertebrae is precisely defined. Vertebrae derive from embryonic somites that are continuously produced posteriorly from the presomitic mesoderm (PSM) during body formation. We show that in the chicken embryo, activation of posterior Hox genes (paralogs 9–13) in the tail-bud correlates with the slowing down of axis elongation. Our data indicate that a subset of progressively more posterior Hox genes, which are collinearly activated in vertebral precursors, repress Wnt activity with increasing strength. This leads to a graded repression of the Brachyury/T transcription factor, reducing mesoderm ingression and slowing down the elongation process. Due to the continuation of somite formation, this mechanism leads to the progressive reduction of PSM size. This ultimately brings the retinoic acid (RA)-producing segmented region in close vicinity to the tail bud, potentially accounting for the termination of segmentation and axis elongation.

The skeletal muscles and axial skeleton of vertebrates derive from the embryonic paraxial mesoderm. In amniotes, paraxial mesoderm is formed bilaterally to the nerve cord as a result of primitive streak and tail-bud regression during body axis formation. In chick and mouse embryos, paraxial mesoderm was proposed to derive from a population of resident cells located in the regressing primitive streak and tail bud. In contrast, in lower vertebrates, paraxial mesoderm is formed as a result of the continuation of ingression movements of gastrulation. Here, we reinvestigate paraxial mesoderm formation in the chicken embryo and demonstrate that these two modes are concomitantly at work to set up the paraxial mesoderm. Although the medial part of somites derives from stem cells resident in the primitive streak/tail bud, the lateral part derives from continuous ingression of epiblastic material. Our fate mapping further shows that the paraxial mesoderm territory in the epiblast is regionalized along the anteroposterior axis as in lower vertebrates. These observations suggest that the mechanisms responsible for paraxial mesoderm formation are largely conserved across vertebrates.