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CHAMPAIGN, Ill. — The planarian is
not as well known as other, more widely used subjects of scientific
study -- model creatures such as the fruit fly, nematode or mouse.
But University of Illinois
cell and developmental biology professor Phillip Newmark thinks
it should be. As it turns out, the tiny, seemingly cross-eyed
flatworm is an ideal subject for the study of germ cells, precursors
of eggs and sperm in all sexually reproducing species.
The planarian Newmark studies,
Schmidtea mediterranea, is a tiny creature with a lot of interesting
traits. Cut it in two (lengthwise or crosswise) and each piece will
regenerate a new planarian, complete with brains, guts and -- in
most cases -- gonads. Even when the planarian's brain is severed
from its body, it can regenerate all that is removed, including the
reproductive organs.
 In a new
study published ... [in April] in the Proceedings of the National
Academy of Sciences, Newmark and his colleagues at the U of I report
that planarians share some important characteristics with mammals
that may help scientists tease out the mechanisms by which germ
cells are formed and maintained. Newmark's team made a few
discoveries related to a gene, called nanos, which was previously
known to play a critical role in germ cell development in several
other model organisms. Unlike
fruit flies and nematodes, which show signs of germ cell initiation
in the earliest stages of their embryonic development, planarians do
not generally express nanos or produce germ cells until several days
after hatching. This delayed initiation of germ cell growth is
called inductive specification, and is common to mammals and a
number of other animals.
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 Graduate student Yuying Wang and
the other team members were able to show that nanos is essential for
inductive specification in planarians. Blocking nanos expression by
means of RNA interference immediately after the planarians hatched
prevented the emergence and development of germ cells. Blocking
nanos in mature adults caused their ovaries and testes to disappear.
And when the researchers blocked nanos expression in planarians that
had had their bodies and reproductive organs detached from their
brains, the planarians regenerated new bodies, but with no
reproductive cells. "This is
the first time that nanos gene function has been studied in a
nontraditional model organism," Newmark said. "This is important
because planarians, like mammals, seem to make their germ cells by
an inductive mechanism. So we're hoping that we can use the
molecular biological tools available for studying planarians to get
at the mechanisms that tell a cell: 'You're going to be a germ
cell.'"
 S. mediterranea also
has the ability to reproduce asexually: It clones itself by means of
fission. In looking at nanos in asexual individuals of this species,
the researchers made the surprising discovery that these asexual
individuals also express nanos and produce germ cells. Some other
mechanism, as yet unknown, prevents these germ cells from developing
into functional testes and ovaries.
"Having a simple organism that also
uses inductive signaling is going to help us tease apart the more
conserved mechanisms (of germ cell development and maintenance),"
Newmark said. "We hope that this information will also prove
informative for understanding these processes in higher organisms."
[Text copied from
University
of Illinois news release]
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