Complete canthi removal reveals that forces from the amnioserosa alone are sufficient to drive dorsal closure in Drosophila.
Bottom Line: Canthi maintain purse string curvature (necessary for their dorsalward forces), and zipping at the canthi shortens leading edges, ensuring a continuous epithelium at closure completion.Dissection of one or both canthi resulted in tissue recoil and flattening of each purse string.How the embryo coordinates multiple, large forces (each of which is orders of magnitude greater than the net force) during native closure and is also resilient to multiple perturbations are key extant questions.
Affiliation: Department of Biology, Duke University, Durham, NC 27708.Show MeSH
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Mentions: Here we examine the combined roles of zipping and purse string curvature during dorsal closure with two novel laser protocols that dissect one or both canthi from the rest of the dorsal opening (Figures 1, A and B, and 2, A and B). The time course of the shape changes of the amnioserosa cells and the geometry of the leading edges were quantified. One key advantage for both protocols is the acute removal of one or both canthi, such that normal zipping and seam formation did not occur. A second key advantage is that the native curvature of the purse strings was not maintained after canthi removal. The purse strings recoil/shorten and flatten after the dissection of a canthus, reducing κ (and likely T), thus significantly reducing Tκ, the dorsalward component of the force due to the purse string. Remarkably, even when segments of the apposing purse strings were parallel to each other (and the dorsal midline), progress of the leading edge toward the dorsal midline was maintained at native or nearly native (unperturbed) rates until the end stages of closure, when pronounced slowing occurred before the completion of closure. Thus closure can proceed in the absence of purse string curvature, which we attribute to an increase in the force contribution from the amnioserosa. Quantitative analysis of amnioserosa cell shape shows changes in the length-to-width ratio of cells in approximately half of the dorsal opening closest to the removed canthus. In contrast, analysis of amnioserosa cell oscillations showed little or no change after canthus removal. Together our experiments confirm the robust nature of the dorsal closure process and support the existence of emergent properties that respond to alterations in purse string/actomyosin cable geometry to ensure successful closure.
Affiliation: Department of Biology, Duke University, Durham, NC 27708.