From: Michael M. Butler (butler@comp-lib.org)
Date: Fri Feb 08 2002 - 17:06:03 MST
http://ucsdnews.ucsd.edu/newsrel/science/mchox.htm has the nifty images.
Graphic and image of Artemia=A0
Credit: Matthew Ronshaugen, UCSD
FIRST GENETIC EVIDENCE UNCOVERED OF HOW MAJOR CHANGES
IN BODY SHAPES OCCURRED DURING EARLY ANIMAL EVOLUTION
http://ucsdnews.ucsd.edu/newsrel/science/mchox.htm
Biologists at the University of California, San Diego have uncovered the
first genetic evidence that explains how large-scale alterations to body
plans were accomplished during the early evolution of animals.
In an advance online publication February 6 by Nature of a paper scheduled
to appear in Nature, the scientists show how mutations in regulatory genes
that guide the embryonic development of crustaceans and fruit flies allowed
aquatic crustacean-like arthropods, with limbs on every segment of their
bodies, to evolve 400 million years ago into a radically different body
plan: the terrestrial six-legged insects.
The achievement is a landmark in evolutionary biology, not only because it
shows how new animal body plans could arise from a simple genetic mutation,
but because it effectively answers a major criticism creationists had long
leveled against evolution=8Bthe absence of a genetic mechanism that could
permit animals to introduce radical new body designs.
=B3The problem for a long time has been over this issue of macroevolution,=B2
says William McGinnis, a professor in UCSD=B9s Division of Biology who headed
the study. =B3How can evolution possibly introduce big changes into an
animal=B9s body shape and still generate a living animal? Creationists have
argued that any big jump would result in a dead animal that wouldn=B9t be abl=
e
to perpetuate itself. And until now, no one=B9s been able to demonstrate how
you could do that at the genetic level with specific instructions in the
genome.=B2
The UCSD team, which included Matthew Ronshaugen and Nadine McGinnis, showe=
d
in its experiments that this could be accomplished with relatively simple
mutations in a class of regulatory genes, known as Hox, that act as master
switches by turning on and off other genes during embryonic development.
Using laboratory fruit flies and a crustacean known as Artemia, or brine
shrimp, the scientists showed how modifications in the Hox gene Ubx=8Bwhich
suppresses 100 percent of the limb development in the thoracic region of
fruit flies, but only 15 percent in Artemia=8Bwould have allowed the
crustacean-like ancestors of Artemia, with limbs on every segment, to lose
their hind legs and diverge 400 million years ago into the six-legged
insects.
=B3This kind of gene is one that turns on and off lots of other genes in orde=
r
to make complex structures,=B2 says Ronshaugen, a graduate student working in
William McGinnis=B9 laboratory and the first author of the paper. =B3What we=B9ve
done is to show that this change alters the way it turns on and off other
genes. That=B9s due to the change in the way the protein produced by this gen=
e
functions.=B2
=B3The change in the mutated protein allows it to turn off other genes,=B2 says
William McGinnis, who discovered with two other scientists in 1983 that the
same Hox genes in fruit flies that control the placement of the head, thora=
x
and abdomen during development are a generalized feature of all animals,
including humans. =B3Before the evolution of insects, the Ubx protein didn't
turn off genes required for leg formation. And during the early evolution o=
f
insects, this gene and the protein it encoded changed so that they now
turned off those genes required to make legs, essentially removing those
legs from what would be the abdomen in insects.=B2
The UCSD team=B9s demonstration of how a mutation in the Ubx gene and changes
in the corresponding Ubx protein can lead to such a major change in body
design undercuts a primary argument creationists have used against the
theory of evolution in debates and biology textbooks. Their specific
objection to the idea of macroevolutionary change in animals is summed up i=
n
a disclaimer that the Oklahoma State Textbook Committee voted in November,
1999 to include in that state=B9s biology textbooks:
=B3The word evolution may refer to many types of change. Evolution describes
changes that occur within a species. (White moths, for example, may evolve
into gray moths). This process is microevolution, which can be observed and
described as fact. Evolution may also refer to the change of one living
thing into another, such as reptiles and birds. This process, called
macroevolution, has never been observed and should be considered a theory.=B2
=B3The creationists=B9 argument rests in part on the fact that animals have two
sets of chromosomes and that in order to get big changes, you=B9d need to
mutate the same genes in both sets of chromosomes,=B2 explains McGinnis. =B3It=B9=
s
incredibly unlikely that you would get mutations in the same gene in two
chromosomes in a single organism. But in our particular case, the kind of
mutation that=B9s in this gene is a so-called dominant mutation, so you only
need to mutate one of the chromosomes to get a big change in body plan.=B2
The discovery of this general mechanism for producing major leaps in
evolutionary change has other implications for scientists. It may provide
biologists with insights into the roles of other regulatory genes involved
in more evolutionarily recent changes in body designs. In addition, the
discovery in the UCSD study, which was financed by the National Institute o=
f
Child Health and Human Development, of how this particular Hox gene
regulates limb development also may have an application in improving the
understanding human disease and genetic deformities.
=B3If you compare this gene to many other related genes, you can see that the=
y
share certain regions in their sequences, which suggests that their functio=
n
might be regulated like this gene,=B2 says Ronshaugen. =B3This may establish
how, not only this gene, but relatives of this gene in many, many different
organisms actually work. A lot of these genes are involved in the
development of cancers and many different genetic abnormalities, such as
syndactyly and polydactyly, and they may explain how some of these
conditions came to be.=B2=A0
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reserved.
Official web page of the University of California, San Diego
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