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June 24, 1999
Molecular Biologists Prune Branches from the Animal Family Tree
Earthworms are kin to lobsters and flatworms
are cousins of roundworms. These kinds of relationships have been drawn over
the years by zoologists who painstakingly constructed evolutionary trees
using animal morphology, or comparisons of form and structure. Morphology
was, until recently, the best information available for such
classifications. But new molecular evidencegleaned directly from DNA, the
master blueprint of lifeis pruning the old evolutionary tree.
"Basically, we're redrawing the tree," says Jennifer Grenier, a Howard
Hughes Medical Institute (HHMI) associate at the University of
Wisconsin-Madison.
Grenier and HHMI investigator Sean Carroll, also at Wisconsin, were part of an international research team that performed the latest tree trimming.
Their work substantiates earlier genetic investigations suggesting that the
vast majority of animals, from oysters to humans, belong to one of three
primary evolutionary lines, rather than the multiple branches suggested by
morphological studies. The researchers reported their findings in the June
24, 1999, issue of the journal
Nature
.
Grenier and Carroll had been exploring the genetic regulation of animal
development when they realized that the genes they were studying could shed
light on animal evolution. Members of the
Hox
gene family ensure that
organs and other systems form properly and in the appropriate places in the
developing animal. For example, a
Hox
gene called
Ubx
allows
only one pair of wings to arise in a fruit fly instead of the two pairs
found in a dragonfly. Mutations in
Hox
genes cause severe
abnormalities.
Because
Hox
genes are ubiquitous and well conservedhaving changed
very slowly throughout animal evolutionthey provide powerful clues about
the evolutionary history of an organism. Closely related animals share a
similar assortment of
Hox
genes, while more distant relations have
fewer in common. By decoding the
Hox
gene sequences of ancient groups
of animals, Grenier and Carroll hoped to infer not only who was related to
whom, but also which developmental genes were present in the shared
ancestor.
The project quickly became an international collaboration. "Three labs
independently started wondering about these questions," says Grenier. She
and Carroll decoded the
Hox
genes of a priapulid, a little-known
marine worm of enigmatic evolutionary heritage, which looks like a small,
squishy feather duster. Researchers at the University of Paris studied the
Hox
genes of a brachiopod, an ancient marine animal that looks like a
clam. Scientists at Cambridge University worked on a polychaete, a marine
relative of earthworms and leeches.
By comparing the assortments of
Hox
genes found in these three
species to those found in previously studied animalsincluding mice, fruit
flies, leeches, and sea urchinsthe collaborating researchers confirmed the
division of the animal kingdom into three primary evolutionary lines.
On the new, gene-based tree, animals with backbones are on the same
branch as starfish and their relatives-a longstanding classification based
on similarities in development. But the new tree reorganizes almost all of
the remaining groups of animals. Animals that molt, such as crustaceans,
insects, roundworms, and priapulids, now sit together on a second branch. A
third branch holds brachiopods, earthworms, polychaetes, mollusks, and
flatworms, each of which either has a feathery feeding structure or a
special larval stage.
So far, the gene-based tree includes only animals with bilateral
symmetry, or mirror-image left and right sides. The data on jellyfish and
sponges-organisms which are radially symmetric, like a tire-are incomplete
and therefore difficult to interpret, Grenier says.
The new work not only supports the idea of a three-limbed animal tree,
explains Carroll, "but it also addresses the question of how sophisticated
life was some 600 million years ago." The findings indicate that ancestors
of these groups had more
Hox
genes than previously believed. "This
potential to control development implies that the common ancestor may well
have been a fairly sophisticated animal," Carroll says.
The reconstruction of the evolutionary tree has far-reaching
implications, the researchers observe. For example, investigators may need
to reexamine their choices of model species for basic research.
"Understanding animal relationships is at the foundation of the assumptions
we make when we do experiments on any animal," says Carroll.
The new understanding should also prompt changes in the way students are
taught, Grenier says. "We're hoping this will have an impact on
undergraduate classes," she says, "if only to emphasize that understanding
evolutionary relationships is a continuing process."
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