The mysterious dance of the cricket fetus

In June, 100 fruit fly scientists gathered on the Greek island of Crete for their biennial meeting. Among them was Cassandra Extevar, a Canadian geneticist at Harvard University. Her laboratory works with fruit flies to study growth and development – “evo devo.” Often, such scientists choose as their “model organism” species Drosophila melanogaster – a winged workhorse that has acted as an insect ally on at least some Nobel Prize in Physiology and Medicine.

But Dr. Extover is also known for developing alternative species as model organisms. He is particularly keen on cricket, especially Gryllus bimaculatus, the two-spotted field cricket, even though it does not yet enjoy anything near that of a fruit fly. (About 250 principal inventors applied to attend the gathering in Crete.)

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

Cricket already listed in studies on circadian clocks, organ regeneration, learning, memory; They have served as disease models and drug factories. True Polymath, Cricket! They are also becoming increasingly popular: Meal, is covered with chocolate or not. From an evolutionary perspective, crickets offer more opportunities to learn about a past common insect ancestor; They share more traits with other insects than fruit flies. (Notably, insects make up more than 85 percent of animal species).

Dr. Extover’s research focuses on the fundamentals: How do embryos work? And what does this reveal about how the first animal came to be? Each animal embryo follows a similar journey: A cell becomes many, then they arrange themselves in a single layer on the surface of the egg, providing the initial blueprint for all adult body parts. But how do embryonic cells – cells that have the same genome but not all doing the same thing with that information – know where to go and what to do?

“That’s the secret to me,” said Dr. Extover. “I’ve always wanted to go there.”

biologist and data scientist at the University of Chicago and Dr. Extover’s lab alumnus Seth Donoghe described embryology as the study of how a developing animal makes “the right parts in the right place at the right time.” In some new research featuring wondrous videos of cricket embryos – showing some of the “perfect parts” (cell nuclei) moving in three dimensions – Dr. Extover, Dr. Donoghe and his colleagues found that the good old-fashioned geometry plays a starring role.

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

In 2019, Stefano Di Talia, quantitative developmental biologist at Duke University, Studied the movement of nuclei in fruit fly and showed that they are carried by the pulsating current in the cytoplasm – much like leaves traveling along the edges of a slow-moving stream.

But some other mechanism was at work in the cricket embryo. The researchers spent hours observing and analyzing the nuclei’ subtle dance: the glowing nubs split and move in a strange pattern, not completely arranged, not at all random, at different directions and speeds, from neighboring nuclei. More in sync than far away. The performance relied on a choreography beyond mere physics or chemistry.

“The geometry that nuclei assume is the result of their ability to sense and react to the densities of other nuclei,” said Dr. Extover said. Dr. Di Talia was not involved in the new study, but he found it dynamic. “It’s a beautiful study of a beautiful system of great biological relevance,” he said.

Cricket researchers first took a classic approach: look closely and pay attention. “We just saw it,” Dr Extovar said.

They shot the video using a laser-light sheet microscope: the snapshot captured the dance of nuclei every 90 seconds during the embryo’s early eight hours of development, in which time 500 or so nuclei had accumulated in the cytoplasm. (Crickets hatch after about two weeks.)

Typically, organic material is translucent and difficult to see even with the most microscope. But Taro Nakamura, then a postdoc in the lab of Dr. Extavar, now a developmental biologist at the National Institute for Basic Biology in Okazaki, Japan, did the engineering. a particular strain of cricket with the nucleus glowing fluorescent green, As Dr. Nakamura pointed out, the results were “astonishing” when they recorded the development of the fetus.

That was the “jumping point” for the exploratory process, Dr. Donoghe said. He explained a comment sometimes attributed to science fiction author and biochemistry professor Isaac Asimov: “Often, you’re not saying ‘eureka’! When you search for something, you’re saying, ‘ Huh. That’s weird.'”

Initially biologists watched the video on loop, projected onto a conference-room screen – the cricket-equivalent of IMAX, noting that the embryo (long grain) is about a third the size of a grain of rice. They tried to detect patterns, but the data sets were overwhelming. He needed more quantitative knowledge.

Dr. Donoghe contacted Christopher Rycroft, now an applied mathematician at the University of Wisconsin-Madison, and showed him the dance nucleus. ‘very nice!’ Dr. Rycroft said. He had never seen anything like it before, but he recognized the potential of data-driven collaboration; He and Jordan Hoffman, then a doctoral student in Dr. Rycroft’s laboratory, joined the study.

In several screenings, the Mathematica-Bio team considered several questions: How many nuclei were there? When did they start dividing? In which direction were they going? where did they end up? Why were some moving around and others crawling?

Dr. Rycroft often works at the intersection of life and the physical sciences. (Last year, he published on the physics of paper break.) “Mathematics and physics have had great success in getting general laws that are widely applicable, and this approach could help biology as well,” he said; Dr. Extover has said the same thing.

The team spent a lot of time rotating the ideas on the white board, often drawing pictures. The problem reminded Dr. Rycroft of the Voronoi diagram, a geometric construction which divides a space into non-overlapping sub-regions – polygons, or Voronoi cells, each emerging from a single seed point. It is a multifaceted concept that applies to things as diverse as the growth patterns of galaxy clusters, wireless networks, and forest canopies. (The trunk of the tree is the seed point and the crown is voronoi cells, which closely intersect but do not encroach upon each other, a phenomenon known as crown shyness.)

In terms of crickets, the researchers counted the Voronoi cell around each nucleus and observed that the shape of the cell helped predict which direction the nucleus would move next. Originally, Dr. Donoghe said, “nuclei tended to move into a nearby open space.”

Geometry, he said, provides an abstract way of thinking about cellular mechanics. “For most of the history of cell biology, we could not directly measure or observe mechanical forces,” he said, although it was clear that “motors and squish and push” were in play. But researchers can observe higher-order geometric patterns produced by these cellular dynamics. “So, thinking about the difference in cells, the size of the cells, the shape of the cells – we know they come from mechanical constraints on a very fine scale,” Dr. Donoghe said.

To extract such geometric information from cricket videos, Dr. Donoghe and Dr. Hoffman tracked the nucleus step-by-step to measure location, speed and direction.

“It’s not a trivial process, and it involves a lot of forms of computer vision and machine-learning,” said Dr Hoffman, now an applied mathematician at DeepMind in London.

They manually verified the software’s results, clicking through 100,000 positions, linking the nucleus’s lineage through space and time. Dr. Hoffman found it tedious; Dr. Donoghe thought of it as playing a video game, “zooming in high speed through the tiny universe inside a single embryo, stitching together the travel threads of each nucleus.”

He then developed a computational model that tested and compared hypotheses that could explain the motion and position of the nucleus. Overall, they ruled out the cytoplasmic flux that Dr. Di Talia had observed in the fruit fly. He refuted random motion and the notion that nuclei physically separate each other.

Instead, they arrived at a plausible explanation by building on another known mechanism in fruit fly and roundworm embryos: miniature molecular motors in the cytoplasm that extend clusters of microtubules from each nucleus, not unlike the forest canopy.

The team proposed that a similar type of molecular force pulled the cricket’s nucleus into empty space. “Molecules may be microtubules, but we don’t know for sure,” Dr. Extavar said in an email. “We’ll have to do more experiments in the future to find out.”

This cricket odyssey would not be complete without a mention of Dr. Donoghee’s custom-built “embryo-contraction apparatus”, which he built to test various hypotheses. it repeated a old school technology but Dr. Was inspired by previous work with Extavar and others Egg size and shape,

This contraption allowed Dr. Donoghe to carry out the complex task of looping a human hair around a cricket egg – creating two spheres, one containing the parent nucleus, the other partially pin-of-the-seat. Annex takes place.

Then, the researchers looked at nuclear choreography again. In the core region, the nucleus slows down once the crowding density is reached. But when some nuclei penetrated through the tunnel into the constriction, they accelerated again, loose like horses in open pasture.

This was the strongest evidence that the movement of nuclei was controlled by geometry, Dr. Donoghe said, and “is not controlled by global chemical cues, or flow or all the other hypotheses that could possibly coordinate the behavior of the entire embryo.”

By the end of the study, the team had accumulated more than 40 terabytes of data on 10 hard drives and refined a computational, geometric model that was added to Cricket’s tool kit.

“We want to make cricket embryos more versatile to work with in the laboratory,” said Dr. Extover – that is, more useful in the study of even more aspects of biology.

The model can simulate the shape and size of any egg, which makes it useful as a “testing ground for other insect embryos,” Dr. Extover said. She noted that this would make it possible to compare diverse species and probe deeper into evolutionary history.

But the study’s biggest reward, all the researchers agreed, was the collaborative spirit.

“There is a place and time for specialized knowledge,” said Dr. Extover. “Just as often as scientists are in pursuit, we need to expose ourselves to people who are not as invested in a particular outcome as we are.”

The questions asked by mathematicians were “free from all forms of bias”, Dr. Extover said. “Those are the most exciting questions.”

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