Combining classic comparative approaches, including collecting species from the wild, and cutting-edge light-induced gene manipulation technology, researchers from Japan and Germany have discovered how developing fly embryos solve the fundamental problem of “tissue tectonic collision” when the rapidly expanding head and torso tissues crash into each other. Different species have evolved different solutions, one of which, the ‘cephalic furrow’, has long been a mystery to developmental biologists because it forms and disappears without leaving a trace.

For an animal to develop properly, two fundamental processes need to happen: cells need to be made and then they need to be assembled into functional tissues and organs. However, especially in the early stage of development, these processes often occur in a confined space, such as within a hard eggshell or the uterus. Given that developmental processes require mechanical forces, the embryo faces a fundamental engineering problem: how to prevent these active, essential forces from interfering with other parts of the embryo.

For the current study, published in Nature, the research team led by Yu-Chiun Wang of the RIKEN Center for Biosystems Dynamics Research in Japan and Steffen Lemke of the University of Hohenheim in Germany, examined fruit flies and their relatives, and found that different species evolved different solutions to this problem.

Bipasha Dey, a co-lead author and postdoctoral researcher in the Wang group, employed highly sophisticated genomic engineering and laser-based microscopic technologies to remove the cephalic furrow in the common fruit fly Drosophila with surgical precision. She found that the area between the head and trunk yields to the pressure coming from the expanding tissues, revealing that this evanescent structure is actually needed to prevent the head-on collision. Moreover, embryos without the cephalic furrow often developed abnormally with severe and potentially fatal defects several hours later—preventing tissue collision can be a matter of life and death.

When the researchers mapped the cephalic furrow to the evolutionary tree of flies, they saw that the group of flies that includes the fruit fly all bend and temporarily form the cephalic furrow to create a ‘sink’ that releases tissue pressure, “and yet, surprisingly, all those outside of it do not”, explains Girish Kale, a co-lead author and postdoctoral researcher in the Lemke group, who compiled the evidence with their ‘fly zoo’ in the lab and by capturing flies near the municipal compost. “These data confirmed what we realized by painstakingly combing through early 20th century entomology drawings—the cephalic furrow is an evolutionary innovation, just like feathers in birds”, explains Dr. Lemke. Similar findings were reported in a companion study, led by Pavel Tomancak at the Max Planck Institute of Molecular Cell Biology and Genetics in Germany and published also in Nature this week. Their work identified the genetic changes that led to this innovation and used a physical model to provide theoretical support for how the cephalic furrow can be a timely and optimally placed ‘sink’ to prevent tissue collision. The two groups worked closely during the studies.

How then do flies without the cephalic furrow prevent tissue collision? The researchers found that they change the angle of cell division when the head region expands, causing the cells to divide inward, or ‘out-of-plane’, with one daughter cell retained on the surface, while the other is pushed inside. Using a midge species called Chironomus, Verena Kaul, a PhD student and co-lead author in the Lemke group, obtained definitive evidence that this alternative workaround reduces head expansion and shortens the time it spends growing, effectively preventing the collision. To support this assertion, Dr. Dey used a genetic trick to turn the angle of Drosophila cell division by 90 degrees, in essence mimicking the midge’s workaround, and the embryos often did not need the cephalic furrow anymore—the two solutions are nearly interchangeable.

“This research shows us that evolution can find multiple solutions to the same problem”, explains Dr. Lemke. According to Dr. Wang, “The findings suggest that mechanical forces play a more important role in shaping evolutionary innovation than previously appreciated, and that embryos possess sophisticated mechanical stress management strategies to resolve internal conflicts during development.”