The geometry of life

By | August 7, 2022

The New York Times reports:

In June, 100 fruit fly scientists gathered on the Greek island of Crete for their biennial meeting. Among them was Cassandra Extavour, a Canadian geneticist at Harvard University. Her lab works with fruit flies to study evolution and development — “evo devo.” Most often, such scientists choose as their “model organism” the species Drosophila melanogaster — a winged workhorse that has served as an insect collaborator on at least a few Nobel Prizes in physiology and medicine.

But Dr. Extavour is also known for cultivating alternative species as model organisms. She is especially keen on the cricket, particularly Gryllus bimaculatus, the two-spotted field cricket, even though it does not yet enjoy anything near the fruit fly’s following. (Some 250 principal investigators had applied to attend the meeting in Crete.)

“It’s crazy,” she said during a video interview from her hotel room, as she swatted away a beetle. “If we tried to have a meeting with all the heads of labs working on that cricket species, there might be five of us, or 10.”

Crickets have already been enlisted in studies on circadian clocks, limb regeneration, learning, memory; they have served as disease models and pharmaceutical factories. Veritable polymaths, crickets! They are also increasingly popular as food, chocolate-covered or not. From an evolutionary perspective, crickets offer more opportunities to learn about the last common insect ancestor; they hold more traits in common with other insects than fruit flies do. (Notably, insects make up more than 85 percent of animal species.)

Dr. Extavour’s research aims at the fundamentals: How do embryos work? And what might that reveal about how the first animal came to be? Every animal embryo follows a similar journey: One cell becomes many, then they arrange themselves in a layer at the egg’s surface, providing an early blueprint for all adult body parts. But how do embryo cells — cells that have the same genome but aren’t all doing the same thing with that information — know where to go and what to do?

“That’s the mystery for me,” Dr. Extavour said. “That’s always where I want to go.”

Seth Donoughe, a biologist and data scientist at the University of Chicago and an alumnus of Dr. Extavour’s lab, described embryology as the study of how a developing animal makes “the right parts at the right place at the right time.” In some new research featuring wondrous video of the cricket embryo — showing certain “right parts” (the cell nuclei) moving in three dimensions — Dr. Extavour, Dr. Donoughe and their colleagues found that good old-fashioned geometry plays a starring role.

Humans, frogs and many other widely studied animals start as a single cell that immediately divides again and again into separate cells. In crickets and most other insects, initially just the cell nucleus divides, forming many nuclei that travel throughout the shared cytoplasm and only later form cellular membranes of their own.

In 2019, Stefano Di Talia, a quantitative developmental biologist at Duke University, studied the movement of the nuclei in the fruit fly and showed that they are carried along by pulsing flows in the cytoplasm — a bit like leaves traveling on the eddies of a slow-moving stream.

But some other mechanism was at work in the cricket embryo. The researchers spent hours watching and analyzing the microscopic dance of nuclei: glowing nubs dividing and moving in a puzzling pattern, not altogether orderly, not quite random, at varying directions and speeds, neighboring nuclei more in sync than those farther away. The performance belied a choreography beyond mere physics or chemistry.

“The geometries that the nuclei come to assume are the result of their ability to sense and respond to the density of other nuclei near to them,” Dr. Extavour said. Dr. Di Talia was not involved in the new study but found it moving. “It’s a beautiful study of a beautiful system of great biological relevance,” he said. [Continue reading…]

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