Researchers at the University of Geneva (UNIGE), Switzerland, have studied the developing embryo of the 'frilled dragon' lizard and revealed that physical forces, rather than a genetic program, generate characteristic folds of its spectacular collar.
The frilled dragon displays a distinctive large erectile ruff. Ordinarily, this lizard keeps the frill folded back against its body, but can spread it as a spectacular display to scare off predators.
At present, scientists at the University of Geneva, and the SIB Swiss Institute of Bioinformatics detail their results in the journal eLife that an ancestral embryonic gill of the dragon embryo turns into a neck pocket that expands and folds to form the frill.
Then, the investigators demonstrate that this robust folding pattern emerges from mechanical forces during the homogeneous growth of the frill skin due to the tensions resulting from its attachment to the neck and head.
In the movie, Jurassic Park, though Dennis Nedry, the computer programmer, attempts to smuggle dinosaur embryos off the island, he was attacked and killed by a mid-sized dinosaur that erects a frightening frill. This fictional dinosaur is inspired by a real animal known as the 'frilled dragon,' that lives today in northern Australia and southern New Guinea.
Chlamydosaurus kingii, as these lizards are referred to, have a large disc of skin that sits around their head and neck. This frill is usually folded back against the body but can spread spectacularly to scare off predators and competitors. Folding of the left and right sides of the frill occurs at three pre-formed ridges. However, it remains unclear which genetic structure evolved to become the dragon's frill, and how the ridges in the frill form during development.
A professor at the Department of Genetics and Evolution of the UNIGE Faculty of Science, Michael Milinkovitch, led a multidisciplinary team and Group Leader at the SIB Swiss Institute of Bioinformatics, shows that the dragon's frill, as well as bones and cartilages that support it, develop from the branchial arches.
These are a series of bands of tissues in the embryo that evolved to become the gill supports in fish, and that now give rise to multiple structures in the ear and neck of land vertebrates. For some species, the second branchial arch will eventually fuse with the arches behind it. With the frilled dragon, however, this arch continues instead to expand, leading to the formation of the spectacular frill of the dragon. Michael Milinkovitch enthused that these changes in the development of gill arches highlight how evolution can "recycle" old structures into new forms playing different roles.
According to Swiss team that studied the formation of these ridges, they do not emerge from increased growth at the folding sites, but from physical forces, whereby the growth of the frill is constrained by its attachment to the neck. The impact makes the top layer to buckle and create the folds of the frill.
