Abstract
Egg activation is the essential process in which mature oocytes gain the competency to proceed into embryonic development. Many events of egg activation are conserved, including an initial rise of intracellular calcium. In some species, such as echinoderms and mammals, changes in the actin cytoskeleton occur around the time of fertilization and egg activation. However, the interplay between calcium and actin during egg activation remains unclear. Here, we use imaging, genetics, pharmacological treatment, and physical manipulation to elucidate the relationship between calcium and actin in living Drosophila eggs. We show that, before egg activation, actin is smoothly distributed between ridges in the cortex of the dehydrated mature oocytes. At the onset of egg activation, we observe actin spreading out as the egg swells though the intake of fluid. We show that a relaxed actin cytoskeleton is required for the intracellular rise of calcium to initiate and propagate. Once the swelling is complete and the calcium wave is traversing the egg, it leads to a reorganization of actin in a wavelike manner. After the calcium wave, the actin cytoskeleton has an even distribution of foci at the cortex. Together, our data show that calcium resets the actin cytoskeleton at egg activation, a model that we propose to be likely conserved in other species.
Highlights
Production of female gametes is an important part of the faithful passage of genetic information from parents to offspring
We showed that filamentous actin (F‐actin) disperses and become more dynamic during Drosophila egg activation and that this dynamic actin cytoskeletal network is required for a normal calcium wave to occur during ex vivo egg activation (5e)
The calcium wave mediates a reorganization of F‐actin in a wavelike manner
Summary
Production of female gametes is an important part of the faithful passage of genetic information from parents to offspring. When a mature oocyte is ovulated and moves from the ovary to the lateral oviduct, it is thought to experience pressure and fluid uptake This results in its swelling and a transient wave of increased cytoplasmic calcium that initiates from oocyte poles, predominantly the posterior, due to calcium influx through Trpm channels (Horner & Wolfner, 2008; Hu & Wolfner, 2019; Kaneuchi et al, 2015). Since our model links calcium, a ubiquitous secondary messenger, to actin, a key cytoskeletal component, we expect that the interactions established in this study will be conserved in other species
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