title: Dynamics of the slowing segmentation clock reveal alternating two-segment periodicity.
abstract: The formation of reiterated somites along the vertebrate body axis is controlled by the segmentation clock, a molecular oscillator expressed within presomitic mesoderm (PSM) cells. Although PSM cells oscillate autonomously, they coordinate with neighboring cells to generate a sweeping wave of cyclic gene expression through the PSM that has a periodicity equal to that of somite formation. The velocity of each wave slows as it moves anteriorly through the PSM, although the dynamics of clock slowing have not been well characterized. Here, we investigate segmentation clock dynamics in the anterior PSM in developing zebrafish embryos using an in vivo clock reporter, her1:her1-venus. The her1:her1-venus reporter has single-cell resolution, allowing us to follow segmentation clock oscillations in individual cells in real-time. By retrospectively tracking oscillations of future somite boundary cells, we find that clock reporter signal increases in anterior PSM cells and that the periodicity of reporter oscillations slows to about ∼1.5 times the periodicity in posterior PSM cells. This gradual slowing of the clock in the anterior PSM creates peaks of clock expression that are separated at a two-segment periodicity both spatially and temporally, a phenomenon we observe in single cells and in tissue-wide analyses. These results differ from previous predictions that clock oscillations stop or are stabilized in the anterior PSM. Instead, PSM cells oscillate until they incorporate into somites. Our findings suggest that the segmentation clock may signal somite formation using a phase gradient with a two-somite periodicity.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
authors.forename: Nathan P
authors.initials: NP
authors.lastname: Shih
authors.affiliation: Department of Physics, McGill University, Montréal, Canada H3A 2T8.
authors.forename: Paul
authors.initials: P
authors.lastname: François
authors.affiliation: UMR 5305 CNRS/UCBL, 7 passage du Vercors, Lyon 69367, Cedex 07, France.
authors.forename: Emilie A
authors.initials: EA
authors.lastname: Delaune
authors.affiliation: Departments of Molecular Genetics and Molecular and Cellular Biochemistry, The Ohio State University, Columbus, OH 43210, USA amacher.6@osu.edu.
authors.forename: Sharon L
authors.initials: SL
authors.lastname: Amacher
mesh_terms: D000818:Animals; D004625:Embryo, Nonmammalian / Q000378:metabolism*; D012189:Retrospective Studies; D019170:Somites / Q000378:metabolism; D015027:Zebrafish / Q000196:embryology* / Q000378:metabolism; D029961:Zebrafish Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D029961:Zebrafish Proteins
keywords: In vivo imaging; Oscillations; Segmentation clock; Somite; Two-segment periodicity; Zebrafish; her1