How mechanotypes help explain animal body shape

Animals have such varied body shapes that nature can seem to be making them up as it goes along. A jellyfish looks nothing like a coral, while a sea anemone resembles neither, despite all three being close relatives within the phylum Cnidaria.

Biologists have traditionally turned to genetics to account for these differences. A new study, however, suggests that genes explain only part of the picture.

According to the researchers, the missing link is physics, particularly the mechanical properties of tissues as they grow and change shape.

Scientists at EMBL and the University of Geneva propose that differences in a small number of mechanical tissue features can explain the diversity of shapes across species.

They call the resulting signature a species’ mechanotype: a mechanical “profile” that helps predict the form an organism will develop.

Physical laws and animal bodies

The approach revisits an idea that has been around for a long time. More than a century ago, Scottish biologist and mathematician D’Arcy Thompson argued that physical laws have a fundamental role in shaping living bodies.

Rather than replacing genetics, the new study seeks to connect it with a more predictive account of how tissues bend, stretch and produce force as they develop.

“Comparing genomes can reveal genetic differences linked to shape diversity, but genes cannot tell us how morphogenesis unfolds,” said Aissam Ikmi, group leader at EMBL Heidelberg and one of the senior authors.

“Even with a genome in hand, we still cannot yet predict the final shape of an organism.”

Why genes alone do not explain shape

Species that are genetically similar can nevertheless develop very different body plans. Morphogenesis – the process through which an embryo or larva develops into a shaped organism – is more than a genetic script: it is also a physical process.

Cells develop in contact with one another, pulling on their neighbours, forming sheets and tubes, and collectively creating tension and pressure.

These forces can cause tissues to fold, lengthen or narrow, with mechanical behaviour capable of transforming the resulting body shape.

Ikmi and his colleagues suspected that evolution might produce different body forms not just by changing genes, but also by changing how cells act together as a tissue, the level at which forces arise.

“What matters is how cells work together as a tissue to generate forces and mechanical constraints,” Ikmi said.

“If this is where morphogenesis operates, it may also be where shape diversity emerges across evolution.”

Cnidarians as a test case

Cnidarians provide an ideal way to investigate these questions: their body plans are comparatively simple, yet extraordinarily varied. They also pass through life stages involving dramatic changes in shape.

The researchers examined six cnidarian species: two corals, two anemones and two hydrozoans. Rather than describing each species in detail, they wanted to establish whether a few mechanical rules could account for major differences in shape.

Answering that question required more than conventional developmental biology. The team also needed theoretical physics and mathematical modelling, tools suited to investigating whether complex patterns can be reduced to a few governing parameters.

Collaborators at the University of Geneva supplied that expertise. They included Guillaume Salbreux and his postdoctoral researcher Nicolas Cuny, as well as Richard Bailleul, a former EMBL postdoctoral researcher with a background in mathematics.

“An important idea in physics is that when described on the right scale, emergent features of complex systems can be understood through models involving only a few key parameters,” Salbreux said.

The three mechanical modules of cnidarian shape

By combining experimental observations with theoretical modelling, the researchers identified three key “mechanical modules”. Different combinations of these modules could explain two major features of animal shape.

The first is elongation: how extended or compact an animal is along its main body axis. The second is polarity, or whether the top end, where the mouth sits, is wider or narrower than the base.

This oral–aboral difference in shape is central to the visual distinctions between cnidarian species.

Changing the values of the mechanical modules, rather like adjusting dials on a control panel, allowed the model to predict different combinations of elongation and polarity. The researchers call each species’ distinctive combination of settings its mechanotype.

“Mechanical changes ultimately arise from molecular changes, but the mechanotype is where that information becomes predictive of form,” Ikmi said.

“We believe evolution acts on these modules to generate new forms.”

The study therefore makes a subtle change in emphasis. Rather than saying “this gene causes this shape,” it proposes that genes matter because they alter mechanical behaviours, which in turn make shape predictable.

Changing animal body shapes

The next question was whether the approach could do more than describe shapes: could it predict what would happen if the system were disturbed?

To find out, the team carried out “reshaping” experiments on the sea anemone Nematostella, whose larvae are normally elongated with a narrow oral end.

When the researchers introduced genetic changes affecting one mechanical module, the larvae became round rather than elongated.

Changing polarity proved more difficult. To make the asymmetry between the oral end and the base resemble that of another species, Aiptasia, the scientists had to perturb several modules at the same time.

The result suggests that a single dominant mechanical lever may control some aspects of form, whereas others depend on more complicated combinations.

Taken together, the experiments showed that mechanotypes are more than labels: at least in these model organisms, they can be used to predict shape quantitatively and even manipulate it.

“I think the main achievement of this study is to demonstrate the relevance of addressing morphological evolution from the perspective of mesoscopic physical principles, an idea that was proposed more than a century ago by D’Arcy Thompson and which our study now puts into practice,” Cuny said.

Work across multiple disciplines

Collaboration is a major part of the paper’s message. The experimental researchers contributed extensive expertise in cnidarian development and morphogenesis.

The theorists supplied the modelling tools required to distil complex biological variation into a small set of meaningful parameters.

“Collaboration between theorists and experimental biologists are ideal when sharing an enthusiasm for the question asked,” Salbreux said.

“This was the case here: both teams were interested in the broad question of variation of shape.”

Ikmi likewise stressed that this was not a “theory team plus biology team” arrangement. It was a genuinely shared project in which the modelling informed the experiments and the experiments informed the modelling.

Future research directions

The researchers plan to extend their framework beyond the larval stage to the polyp stage of cnidarian life cycles, as well as include more species. That matters because a biological “principle” must hold up when examined across a broader range of cases.

If it does, mechanotypes could offer a different way to understand morphological evolution: not as an endless catalogue of genetic details, but as changes to a small set of tissue-level mechanical rules that evolution can adjust to generate new forms.

In this sense, the approach bridges two worlds. Genetics supplies the raw instructions, while physics helps explain how those instructions become a recognisable body.