Dose-Dependent Stability of the Segmentation Clock Under Thermal Fluctuations

Abstract

Embryonic development must remain robust against environmental perturbations to ensure a precise body plan. A striking example of developmental robustness is the segmentation of the body axis into a consistent number and size of vertebrae across different temperatures in the Japanese rice fish (Oryzias latipes, Medaka). Segmentation begins during embryogenesis, when the axis rhythmically divides into somites, a process governed by intracellular molecular oscillations of Notch signaling-related genes such as Her7. In this study, we examined the role of Her7 gene dosage in maintaining the robustness of axis segmentation under the natural circadian temperature cycles experienced by Medaka embryos in the wild. We raised Medaka embryos with varying genetic doses of Her7 under two temperature regimes: constant laboratory conditions (27 °C) or wild-like circadian cycling temperatures (21–27 °C, 24-hour period). We then assessed axis segmentation by quantifying vertebral number and morphology in hatchlings, somite patterning in embryos, and Her7 oscillatory dynamics during somite formation. Our results demonstrate that at least a single functional copy of Her7 is required to maintain normal segmentation in both temperature conditions. Embryos with reduced Her7 dosage (hypomorph/KO) exhibited vertebral defects – with fusions dominating in constant temperature and irregular shape dominating in cyclic temperature, indicating temperature-dependent modulation of the segmentation process. Notably, these defects were predominantly localized to the mid-body region (vertebrae 10–20), suggesting a defined developmental window during which Her7 gene dosage is particularly critical. In contrast, complete loss of Her7 function (KO/KO) resulted in severe segmentation failure across the entire axis. Importantly, we found that both the position and nature of vertebral effects can be traced back disrupted somite patterning and altered Her7 oscillations, highlighting their embryonic origin. Taken together, these results indicate a hierarchical and spatially restricted requirement for Her7 gene dosage, in which partial function supports robustness of segmentation within a defined axial window, whereas complete loss results in improper segmentation across the entire axis. To sum, our findings imply Her7 gene dosage as a key contributor of thermally robust axis segmentation. More broadly, it provides insights into how organisms navigate the interplay between genetic inheritance and environmental variability to ensure stable morphological outcomes.