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Abstract

Developmental patterning shows remarkable robustness in the face of changing environmental conditions. One particular challenge faced by externally fertilized embryos is how to maintain proper growth and patterning despite temperature variation. In order to address the mechanism behind temperature-invariant patterning, I study somitogenesis in Japanese medaka (Oryzias latipes), which has been shown to tolerate a wide range of temperatures. The periodic formation of somites from the presomitic mesoderm (PSM) in vertebrates is under the control of a molecular “clock”, consisting of oscillatory target genes in the Notch, Wnt and FGF signaling pathways. While it is clear that these periodic signals are involved in regulating the timing of somitogenesis, how oscillations encode information, and how this is coordinated in space is still a matter of ongoing research. To study somitogenesis in medaka, I generated endogenous knock-in reporters to visualize signaling activity in the Notch, Wnt and FGF pathways during somite formation. Importantly, an oscillating Notch signaling reporter, Her7-Venus, allows quantification of segmentation clock oscillations in medaka for the first time. Time-lapse imaging of Her7-Venus oscillations revealed coherent waves that follow a period gradient in the PSM, which is reminiscent of dynamics in higher vertebrates. Imaging of this reporter at different temperatures revealed that segmentation clock oscillations are globally faster at higher temperatures. Importantly, while period changes 2.2 fold, average somite size changes 1.15 fold between 23-35°C. A detailed analysis of the period gradient reveals that oscillations change their period differently in the posterior and anterior PSM, resulting in a constant period gradient amplitude. In addition, the phase gradient amplitude is temperature-invariant. These results provide the first quantitative insight into how underlying signaling dynamics respond to temperature changes and allow robust patterning during somitogenesis. Examining these findings in the context of existing models of somitogenesis could provide insight into how robustness is achieved in this complex system.