Tuesday, April 20 at 11:00am to 12:00pm
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Abstract: Animal embryos pass through a stage called the blastoderm, in which cells are arranged in a continuous layer at the periphery of the embryo. Despite the broad evolutionary conservation of this embryonic stage, the cellular behaviours that lead to blastoderm formation are poorly understood. In most insects, pre-blastoderm development begins as a syncytium, meaning that nuclei divide without cytokinesis, and move within the shared cytoplasm of the embryo. To form a blastoderm at the right time, with the proper number and spatial arrangement of cells, these syncytial nuclei must move from their scattered positions within the cytoplasm, forming a single layer at the cortex. In the fruit fly Drosophila melanogaster, the early nuclear movements are caused by pulses of cytoplasmic flows coupled to synchronous divisions. Here, we show that another insect, the cricket Gryllus bimaculatus, has an altogether different solution to the problem. We quantified nuclear dynamics during the period of syncytial cleavages and movements that lead to blastoderm formation in G. bimaculatus embryos and found that unlike the fruit fly, (1) cytoplasmic flows do not direct nuclear movement; and (2) division cycles, nuclear speeds, and directions of nuclear movement are not synchronized across the embryo, but instead are heterogeneous in space and time. We developed a novel geometric model that uses local nuclear density to determine division timing, speed of movement, and orientation of movement. This model accurately predicts nuclear behaviour in unperturbed cricket embryos and in embryos that were physically manipulated to contain regions of atypical nuclear densities. Going forward, we will use this model to make precise, falsifiable predictions about the dynamics of blastoderm formation in other insect species.
Host: Andy McMahon