The freshwater flatworm Schmidtea mediterranea lives in southern Europe and Northern Africa is the first animal ever discovered without a crucial structure inside its cells known as the centrosome. Image: UCSF/J.Azimzadeh |
A tiny, freshwater flatworm found in ponds and rivers around
the world that has long intrigued scientists for its remarkable ability to
regenerate has now added a new wrinkle to biology.
Reporting in Science,
researchers at the University of California, San Francisco (UCSF) and the Stowers
Institute for Medical Research in Kansas
City, Mo., have
discovered that the worm lacks a key cellular structure called a centrosome,
which scientists have considered essential for cell division.
Every animal ever examined, from the mightiest mammals to
the lowliest insects, has these 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 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 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 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 just a few millimeters long at most. 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 Sánchez 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.
