Unwrapping insect development
Changes to the sac that surrounds an embryo are crucial for the insect to form correctly.
Early in development a protective fluid-filled sac forms around an embryo. In humans, this sac bursts during birth, but the sac surrounding insect embryos ruptures long before these animals begin to emerge from their eggs. This early rupture is important for insects to develop normally: if an insect embryo’s sac remains intact too long, the animal’s back will not close properly.
The sac that surrounds insect embryos has two layers: an inner layer called the amnion, and a tough outer layer called the serosa. However, it has been difficult to study what happens to the amnion as the insect embryo develops because it is hard to distinguish it from the serosa. Now, Maarten Hilbrant and colleagues have used genetically engineered red flour beetles in which the cells of the amnion produce a fluorescent protein that can be viewed under a microscope. This allowed the amnion to be observed in living specimens during beetle embryo development, and revealed that the amnion attaches and detaches from the serosa more than once as the embryo develops. Furthermore, the amnion and serosa remain as distinct tissues as they withdraw from the embryo.
Hilbrant and colleagues also found that the cells in part of the amnion near the head of the beetle embryo have a special shape before the sac ruptures. This region of the amnion breaks apart first, and the serosa breaks open a few minutes afterwards. Once the two layers have broken, they pull back from the embryo like a pillowcase being turned inside out as it is removed from a pillow. In normal beetles, this process is quite rapid and squeezes the embryo’s abdomen. But in beetles genetically manipulated to lack a serosa the process is slower because the amnion is not strong enough by itself to squeeze the embryo.
Overall, the experiments show that the amnion starts the rupture of the red flour beetle embryo’s protective sac and the serosa drives the process of the sac being peeled back. Further research will now investigate the mechanics behind the two tissues’ roles and whether the amnion and serosa display similar behaviors in other related insect species.
To find out more
Read the eLife research paper on which this eLife digest is based: “The beetle amnion and serosa functionally interact as apposed epithelia” (January 29, 2016).
