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By studying corals, jellyfish, and sea anemones, scientists show that this diversity is partly explained by the physical properties of tissues, such as their ability to contract, stretch, or resist deformation. These characteristics also allow predicting the morphology of these marine animals. Published in the journal Cell, this work paves the way for a better understanding of the evolution of forms in living beings.
The starlet sea anemone, alongside corals and jellyfish, belongs to the cnidarian phylum.
© Aissam Ikmi
The forms of living beings are remarkably diverse. To explain this, research has mostly focused on genetics. While its role is central in development, it is not enough to explain how tissues fold, stretch, and reorganize to form a specific organism. This process, called morphogenesis, is notably studied in the team of Aissam Ikmi, group leader at EMBL Heidelberg and co-author of the study.
"Comparing genomes allows identifying genetic differences linked to the diversity of forms, but it does not allow predicting the final form of an organism. We need to understand how cells act collectively to generate mechanical forces," explains the researcher.
By highlighting the importance of the links between genes, mechanical forces, and morphology, this work opens new perspectives for the study of evolution.
The collaboration with the group of Guillaume Salbreux, full professor in the Department of Genetics and Evolution in the Section of Biology of the Faculty of Science at UNIGE, a specialist in theoretical physics and co-author of the study, allowed approaching the question from the angle of mechanobiology—that is, the role of physical forces in biological processes. Thus, scientists study how the diversity of forms emerges at the tissue scale, where cells interact and generate mechanical stresses.
Forces on tissues shape morphology
To test this idea, the team studied cnidarians—a group including corals, jellyfish, and sea anemones—known for the variety of their forms despite a relatively simple organization. By combining experimental observations and theoretical modeling, they identified three key physical parameters of tissues, which explain two major characteristics of morphology: elongation (the degree of body stretching) and polarity (the asymmetry between different body parts).
By adjusting these parameters in their model, the scientists were able to reproduce and predict different cnidarian forms observed in nature. Each combination of parameters defines a "mechanotype," that is, the combination of physical characteristics specific to each species. "This is the level at which molecular changes become predictive of form," emphasizes Aissam Ikmi. "We believe evolution acts on these mechanotypes to generate new morphologies."
Inspired by the theories of D'Arcy Thompson, scientists combined theoretical and experimental approaches to establish "mechanotypes" as the physical links between genes and body forms. The image shows cross-sections of Nematostella larvae (left) and Aiptasia larvae (right), with the sliders below representing the mechanical modules that combine to give rise to an organism's mechanotype.
© Daniela Velasco/EMBL.
The team then tested this hypothesis experimentally on the starlet sea anemone Nematostella. By modifying certain mechanical parameters through genetic interventions, they managed to transform the shape of the larvae. Initially elongated individuals thus adopted a more spherical morphology. "These experiments allow us to understand how the mechanical properties of a species determine its form," emphasizes Nicolas Cuny, a postdoctoral researcher in Guillaume Salbreux's group and co-first author of the study.
"Beyond its immediate results, the study confirms the relevance of an interdisciplinary approach combining biology, physics, and mathematics. By highlighting the importance of the links between genes, mechanical forces, and morphology, it opens new perspectives for the study of evolution," concludes Guillaume Salbreux.