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Abstract

The speed of embryonic development varies considerably across mammalian species. These differences in the tempo or duration of developmental processes are known to influence the final size and shape of organisms, serving as an important mechanism of evolutionary change. This thesis aims to understand how different mammals, despite using seemingly indistinguishable molecular toolkits, exhibit speciesspecific developmental rates. To this end, I have focused on investigating the timing of the vertebrate body axis segmentation. The rate at which body segments form is controlled by the segmentation clock, the oscillatory gene expression found in the pre-somitic mesoderm (PSM) cells. The period of the segmentation clock oscillations differs greatly across vertebrates. However, investigating these temporal differences has proved challenging due to the difficulties in obtaining and quantitatively comparing embryos from different animal species. To overcome these challenges, I have used pluripotent stem cells (PSCs) from various mammals, a "stem cell zoo", to develop in vitro models of the segmentation clock. By differentiating PSCs into PSM cells, I have been able to study the developmental tempo of six mammalian species under similar experimental conditions. These species include humans, mice, rabbits, cattle, rhinoceros, and marmosets, which span a wide range of body sizes and morphologies. Quantification of the segmentation clock oscillations revealed that their period scaled with the embryogenesis length rather than animal body weight. The biochemical kinetics of the core clock gene HES7 showed clear scaling with the species-specific segmentation clock period. However, cellular metabolic rates did not exhibit a similar correlation. Instead, genes involved in biochemical reactions displayed expression patterns that scaled with the segmentation clock period, providing evidence of the transcriptional regulation of developmental tempo. To further explore this transcriptomic signature, I established a pipeline for screening genetic modifications affecting the segmentation clock period in human cells. By combining novel fluorescent reporters of biochemical kinetics with gene expression perturbations, I isolated human cell clones with accelerated or decelerated segmentation clock periods. Characterization of these clones revealed specific genes capable of modulating the segmentation clock period. Overall, the stem cell zoo has uncovered general scaling laws governing developmental tempo at the cellular level. This research provides further insights into the mechanisms used by evolution to generate morphological diversity across species.