The diversity of life is not just a matter of genetics ????

A UNIGE study reveals how mechanical constraints, linked to tissue growth, help generate the diversity of biological structures.

How can we explain the morphological diversity of species? If genetics is the answer that spontaneously comes to mind, it is not the only explanation. By combining observations of embryonic development, techniques of microscopy advances and computer modeling, a multidisciplinary team from theUniversity of Geneva (UNIGE) demonstrates that the development of crocodile head scales results from a process linked to the mechanical of growing tissues, rather than molecular genetics. The diversity of these scales, observed in different species of crocodilians, therefore comes from the evolution of mechanical parameters. These results offer unprecedented insight into the physical forces involved in the development and evolution of the diversity of living forms. They could be applied to other complex biological systems. These works can be read in the journal Nature.

The origin of the diversity and morphological complexity of living beings remains one of the greatest mysteries of science. To elucidate this, scientists study various biological species. The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution of the Faculty of Sciences of UNIGE, studies the development and evolution of the integumentary appendages of vertebrates – that is to say the feathers, hairs and scales – to understand the fundamental mechanisms responsible for this diversity. It is generally considered that the embryonic development of these appendages is dictated by genetic processes involving interactions between numerous molecules resulting from gene expression.

Like a “crack” that spreads

However, previous analyzes of the development of crocodile embryos have enabled the Geneva laboratory to show that, unlike those of the body, the scales covering the snout and jaws come from a process reminiscent of the propagation of cracks within the body. A material undergoing mechanical stress. However, the exact nature of this physical process remained unknown.

The UNIGE team has solved this mystery thanks to new highly multidisciplinary work. She first observed the appearance of the scales during the development of the Nile crocodile embryo, which lasts about 90 days. While at 48^th day, the skin covering the jaws and muzzle is still smooth, skin folds appear from 51^th day then spread and interconnect to form polygonal scales of two types: wide and elongated on the top of the snout, smaller and irregular on the sides of the jaws.

Michel Milinkovitch’s group wanted to know if differences in growth speed between the epidermis, the dermis and the underlying skull bones could explain the appearance of folds, and therefore scales. To achieve this, he developed a technique of injecting a hormone into the crocodile egg that activates the growth and stiffening of the epidermis – the growth factor EGF (for Epidermial Growth Factor). He then discovered that theactivation growth and increased rigidity of the superficial layer of the skin lead to a spectacular modification of theorganisation skin folds.

“We observe that the skin first folds abnormally and forms a labyrinthine network resembling the folds of the brain, but ends up forming much smaller scales like in caimans,” explain Gabriel Santos-Durán and Rory Cooper, postdoctoral researchers in the laboratory of Michel Milinkovitch and co-authors of the study. These observations show that variation in the speed of growth and stiffening of skin layers is a simple evolutionary mechanism, capable of generating a great diversity of scale shapes among different crocodilian species.

A 3D model of jaw development

The scientists then used advanced microscopy techniques, known as “fluorescence light sheet”, to quantify the growth speed and the variation in thickness of the different tissues (epidermis, dermis, bone tissue) everywhere on the head of the embryo, but also the organization of the fibers of collagen in the dermis. The Geneva team used this data to build a model computer science three-dimensional (3D) allowing the growth speed and stiffness of the tissues to be varied.

“By exploring these different parameters, we can generate the different shapes of scales corresponding to the Nile crocodiles treated and not treated with EGF, but also the spectacled caiman or the American alligator. These computer simulations demonstrate that the mechanics of tissues makes it easy to explain the diversity of shapes of certain anatomical structures in different species, without involving molecular genetic factors”, concludes Ebrahim Jahanbakhsh, engineer computer scientist in the laboratory of Michel Milinkovitch and co-author of the study.