Mathematics explains how giant ‘whirlpools’ form in developing egg cells

sc604 — Wed, 13 Jan 2021 16:35:52 +0000

Egg cells are among the largest cells in the animal kingdom. Unpropelled, a protein could take hours or even days to drift from one side of a forming egg cell to the other. Luckily, nature has developed a faster way: scientists have spotted cell-spanning whirlpools in the immature egg cells of animals such as mice, zebrafish and fruit flies. These vortices make cross-cell commutes take just a fraction of the time. But scientists didn’t know how these crucial flows formed.

Using mathematical modeling, researchers say they now have an answer. ̽��ֱ��gyres result from the collective behavior of rodlike molecular tubes called microtubules that extend inward from the cells’ membranes. Their results are reported in the journal Physical Review Letters.

“While much is not understood about the biological function of these flows, they distribute nutrients and other factors that organise the body plan and guide development,” said study co-lead author David Stein, a research scientist at the Flatiron Institute’s Center for Computational Biology (CCB) in New York City. And given how widely they have been observed, “they are probably even in humans.”

Scientists have studied cellular flows since the late 18th century, when Italian physicist Bonaventura Corti peered inside cells using his microscope. What he found were fluids in constant motion, however scientists didn’t understand the mechanisms driving these flows until the 20th century.

̽��ֱ��culprits, they found, are molecular motors that walk along the microtubules. Those motors haul large biological payloads such as lipids. Carrying the cargo through a cell’s relatively thick fluids is like dragging a beach ball through honey. As the payloads move through the fluid, the fluid moves too, creating a small current.

Sometimes those currents aren’t so small. In certain developmental stages of a common fruit fly’s egg cell, scientists spotted whirlpool-like currents that spanned the entire cell. In these cells, microtubules extend inward from the cell’s membrane like stalks of wheat. Molecular motors climbing these microtubules push downward on the microtubule as they ascend. That downward force bends the microtubule, redirecting the resulting flows.

Previous studies looked at this bending mechanism, but only for isolated microtubules. Those studies predicted that the microtubules would wave around in circles, but their behavior didn’t match the observations.

“ ̽��ֱ��mechanism of the swirling instability is disarmingly simple, and the agreement between our calculations and the experimental observations by various groups lends support to the idea that this is indeed the process at work in fruit fly egg cells,” said Professor Raymond Goldstein from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “Further experimental tests should be able to probe details of the transition between disordered and ordered flows, where there is still much to be understood.”

In the new study, the researchers added a key factor to their model: the influence of neighboring microtubules. That addition showed that the fluid flows generated by the payload-ferrying motors bend nearby microtubules in the same direction. With enough motors and a dense enough packing of microtubules, the authors found that all the microtubules eventually lean together like wheat stalks caught in a strong breeze. This collective alignment orients all the flows in the same direction, creating the cell-wide vortex seen in real fruit fly cells.

While grounded in reality, the new model is stripped down to the bare essentials to make clearer the conditions responsible for the swirling flows. ̽��ֱ��researchers are now working on versions that more realistically capture the physics behind the flows to understand better the role the currents play in biological processes.

Stein serves as the co-lead author of the new study along with Gabriele De Canio, a researcher at the ̽��ֱ�� of Cambridge. They co-authored the study with CCB director and New York ̽��ֱ�� professor Michael Shelley and ̽��ֱ�� of Cambridge professors Eric Lauga and Raymond Goldstein.

This work was supported by the US National Science Foundation, the Wellcome Trust, the European Research Council, the Engineering and Physical Sciences Research Council, and the Schlumberger Chair Fund.

Reference:
D.B. Stein, G. De Canio, E. Lauga, M.J. Shelley, and R.E. Goldstein, “Swirling Instability of the Microtubule Cytoskeleton”, Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.028103

̽��ֱ��swirling currents occur when the rodlike structures that extend inward from the cells’ membranes bend in tandem, like stalks of wheat caught in a strong breeze, according to a study from the ̽��ֱ�� of Cambridge and the Flatiron Institute.

̽��ֱ��mechanism of the swirling instability is disarmingly simple, and the agreement between our calculations and experimental observations supports the idea that this is indeed the process at work in fruit fly egg cells

Raymond Goldstein

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Biological arms races in birds

gm349 — Wed, 13 Apr 2011 09:56:24 +0000

Brood parasitic birds such as cuckoos lay eggs that mimic those of their hosts in an effort to trick them into accepting the alien egg and raising the cuckoo chick as one of their own.

New research from the ̽��ֱ�� of Cambridge has found that different bird species parasitised by the African cuckoo finch have evolved different advanced strategies to fight back.

One strategy is for every host female to lay a different type of egg, with egg colour and pattern varying greatly among nests. These egg 'signatures' make it harder for the cuckoo finch to lay accurate forgeries. Since the female cuckoo finch always lays the same type of egg throughout her lifetime, she cannot change the look of her egg to match those of different host individuals - thus her chances of laying a matching egg are exasperatingly small.

Dr Claire Spottiswoode, a Royal Society Dorothy Hodgkin Research Fellow from the ̽��ֱ�� of Cambridge’s Department of Zoology, said: “As the cuckoo finch has become more proficient at tricking its hosts with better mimicry, hosts have evolved more and more sophisticated ways to fight back. Our field experiments in Zambia show that this biological arms race has escalated in strikingly different ways in different species. Some host species – such as the tawny-flanked prinia – have evolved defences by shifting their own egg appearance away from that of their parasite. And we see evidence of this in the evolution of an amazing diversity of prinia egg colours and patterns.

“These variations seem to act like the complicated markings on a banknote: complex colours and patterns act to make host eggs more difficult to forge by the parasite, just as watermarks act to make banknotes more difficult to forge by counterfeiters.”

̽��ֱ��researchers also found that some cuckoo finch hosts use an alternative strategy: red-faced cisticolas lay only moderately variable eggs but are instead extremely discriminating in deciding whether an egg is their one of their own. Thanks to their excellent discrimination, these hosts can spot even a sophisticated mimic.

Dr Martin Stevens, a BBSRC David Phillips Research Fellow from the ̽��ֱ�� of Cambridge’s Department of Zoology, commented on this aspect of the findings: “Our experiments have shown that these different strategies are equally successful as defences against the cuckoo finch. Moreover, one species that has done a bit of both – the rattling cisticola – appears to have beaten the cuckoo finch with this dual strategy, since it is no longer parasitised. ̽��ֱ��arms race between the cuckoo finch and its host emphasises how interactions between species can be remarkably sophisticated especially in tropical regions such as Africa, giving us beautiful examples of evolution and adaptation.”

Their findings are reported today in the journal Proceedings of the Royal Society B.

New research reveals how biological arms races between cuckoos and host birds can escalate into a competition between the host evolving new, unique egg patterns (or ‘signatures’) and the parasite new forgeries.

As the cuckoo finch has become more proficient at tricking its hosts with better mimicry, hosts have evolved more and more sophisticated ways to fight back.

Dr Claire Spottiswoode, a Royal Society Dorothy Hodgkin Research Fellow from the ̽��ֱ�� of Cambridge’s Department of Zoology

Claire Spottiswoode

Image shows a variety of cuckoo finches each adapted to mimic a different host species or colour morph

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̽��ֱ�� of Cambridge - eggs

Mathematics explains how giant ‘whirlpools’ form in developing egg cells

Biological arms races in birds