San Francisco, CA, USA. Scientists have discovered tiny, freshwater flatworms that regenerate without a centrosome, a key cellular structure long considered essential for cell division.

Found in ponds and rivers around the world, the worm has intrigued scientists for its remarkable ability to regenerate, but the discovery adds a new wrinkle to biology.


Until now, every animal ever examined, from large mammals to the smallest insects, has centrosomes in their cells. "This is the first time we've found one that didn't," said Wallace Marshall, PhD, an associate professor in the Department of Biochemistry & Biophysics at the University of California, San Francisco (UCSF), who led the research. The fact that flatworms lack these centrosomes calls into question their purpose, Marshall added. "Clearly we have to rethink what centrosomes are actually doing," he said. The findings by researchers at UCSF and the Stowers Institute for Medical Research appear in the journal Science.



Photo courtesy of UCSF.

Wallace Marshall, PhD, is an associate professor in the Department of Biochemistry and Biophysics at the University of California, San Francisco (UCSF).The Necessity of Even Division

A central component of all multicellular life is the ability of cells to divide — and divide evenly. Before a cell divides, it has to assemble two exact copies of its DNA and then make sure that DNA sorts evenly into the two separate halves as they pinch off. Many health problems arise from cells losing this ability.

A hallmark of cancer, for instance, involves abnormalities in this division. Tumor cells often duplicate extra pieces of DNA. Certain forms of childhood mental retardation are also marked by abnormal divisions, which cause the loss of large pieces of DNA, leading to development problems in certain brain structures.

Centrosomes have been seen as animals' ultimate evolutionary fix for this problem. Plants and fungi don't have them, but animals have had centrosomes in their cells, as long as there have been animals. These structures were thought to play a central role in cell division — laying down track-like spindles onto which the cells sort their dividing DNA. Centrosomes were seen as so important to cell division that all animals were assumed to have them.

The discovery that at least one animal doesn't have them came quite unexpectedly. Interested in the basic mechanics of the centrosome, Marshall and UCSF postdoctoral researcher Juliette Azimzadeh, PhD, teamed up with Alejandro Sánchez Alvarado, PhD, a Howard Hughes Medical Institute and Stowers Institute investigator, who has worked with the flatworm Schmidtea mediterranea for several years.

Worm Regenerates Without Centrosomes

With a charming name that masks an otherwise humble appearance, this worm is a puddle wiggler, and at most just a few millimeters long. But its remarkable regenerative ability has made Schmidtea mediterranea a great scientific curiosity. When cut into tiny pieces, every piece will grow into a perfectly normal worm in a matter of days. Each offspring can then be segmented over and over again as well — it's how the worm reproduces.

The original intention of the study Azimzadeh, Marshall and Sánchez Alvarado devised was to see what happened to the worm when it lost its centrosome. Together they manipulated the flatworm to knock out genes needed to assemble these centrosomes. Without centrosomes the worms should have lost their ability to regenerate normally — or so they thought.

They were amazed to find that losing these structures didn't affect the worms' ability to regenerate at all. Then they looked more carefully at the worms and discovered that they never had these centrosomes in the first place. "It came as a surprise to all of us," said Alvarado.

What it means, he said, is that the evolutionary pressure that has maintained these structures in nearly all animals may have very little to do with cell division itself. "There may be another function for centrosomes that is still obscured," he said.

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