Genetic Alchemy: Turning Lobsters into Fruit Flies Astrobiology Magazine:
http://www.astrobio.net/exclusive/82/genetic-alchemy-turning-lobsters-into-fruit-flies
Genetic Alchemy: Turning Lobsters into Fruit Flies
How were body plans able to undergo large-scale changes during the course of evolution? For instance, how did something that
looked like a centipede evolve into something as different as a fruit fly? This is a question that has long concerned biologists that
study evolutionary history. Genetic mutations that would dramatically alter body structures could potentially kill an organism
before it even had a chance to live.
Biologists at the University of California, San Diego, now have genetic evidence that explains how such drastic alterations to
body plans were able to occur during the early evolution of animals.
In a paper published in the Feb. 21st issue of Nature, Matthew Ronshaugen, Nadine McGinnis, and William McGinnis
describe how a simple genetic mutation caused aquatic arthropods, with limbs on every segment of their bodies, to evolve 400
million years ago into the radically different body plans of terrestrial six-legged insects.
The UCSD team says a mutation occurred in a class of regulatory genes known as homeotic, or "Hox" genes. Hox genes act as
master switches, turning on and off other genes during embryonic development. Hox genes determine the fate of each cell in the
embryo, ensuring that organs and other body parts form properly in the developing animal.
A particular Hox gene called Ultrabithorax, or "Ubx," regulates limb formation. In insects, Ubx suppresses development of wings
and legs in the abdomen, ensuring that only the thorax develops these structures. If the Ubx gene is removed from modern
insects such as fruit flies, they develop legs sprouting from every abdominal segment.
The insect Ubx gene can also suppress 100 percent of leg development when it is inappropriately activated in the thorax. The
UCSD team found that a Ubx protein derived from a multi-limbed crustacean (Artemia, or brine shrimp) could only suppress 15
percent of the limb development when activated in the thoracic region of fruit flies. But
when the scientists made a few mutations in the crustacean version of the Ubx protein
to make it more similar to insect Ubx, the protein was able to suppress all limb
development.
Image of Artemia.
Credit: Matthew Ronshaugen,
UCSD
"This kind of gene is one that turns on and off lots of other genes in order to make
complex structures," says Ronshaugen. "What we've done is to show that this change
alters the way it turns on and off other genes."
The difference in Ubx protein limb suppression between fruit flies and Artemia reflects
the development of the gene over time. The ancestor of these two organisms had
limbs on every body segment. Over the course of evolution, the Ubx protein
developed to turn off the genes needed for leg formation. The fossil record shows
examples of various arthropod body plans that evolved because of the development
of this gene for example, early insects tended to have more sets of wings and legs
than are seen in modern insects.
"What is new in this study is that a change in a regulatory gene in this case the Ubx
gene - can have dramatic morphological consequences," says Andrew Knoll, a
paleobiologist with Harvard University who was not involved in the study. "We've long
known that mutations in regulatory networks drive morphological evolution, but most
discussion focused on timing of gene expression. Here, it is a structural change that
alters the regulatory pattern."
Hox genes are found in all animals, including humans. Hox genes seem to have
developed fairly early in the history of life on Earth, and may therefore provide vital
clues about the course of evolution.
Using laboratory fruit
flies and a crustacean
known as Artemia, or
brine shrimp, the
Scientists had already suspected that Hox mutations could cause dramatic changes in
body plans. One argument against this possibility was that, since each chromosome in a
scientists showed how
pair has its own Hox genes, you would need to get the same Hox mutation on both
chromosomes in order to see changes in body structure.
modifications in the Hox
gene Ubx which
"It's incredibly unlikely that you would get mutations in the same gene in two chromosomes
in a single organism," says McGinnis. "But in our particular case, the kind of mutations that
suppresses 100 percent
occur in this gene are so-called dominant mutations, so you only need to mutate one of the
chromosomes to get a big change in body plan."
of the limb development
in the thoracic region of
"All morphological change appears to be based on genetic events of a type somewhat like
DOUBLE WINGS: An
this one," adds Knoll. "Thus, we are slowly learning the genetic bases for the changes of
fruit flies, while its
morphological changes seen in the fossil record."
ultrabithorax mutant fly has a crustacean counterpart
total duplication of the body from Artemia only
segment that carries wings.
represses 15% would
Credit: The Scientist
McGinnis and his team now will test to see if an insect version of Ubx will repress limbs
have allowed the
when expressed during embryonic development in crustaceans. They are conducting this
test in collaboration with Bill Browne and Nipam Patel at the University of Chicago, on the amphipod crustacean Parhyale hawaiensis
crustacean-like
ancestors of Artemia,
They also plan to clone Ubx and AbdA genes from other crustaceans, as well as other types of multi-limbed arthropods like millipedes
and centipedes. They will test to see if these genes also can repress limbs in fruit fly embryos. This test may help determine whether a
with limbs on every
mutation of a particular amino acid in the proteins is responsible for the limb repression function.
segment, to lose their
hind legs and diverge
400 million years ago
into the six-legged
insects.
Credit: Matthew
Ronshaugen, UCSD
"Land plants contain Hox homologs, so it seems that the ancestral Hox-like gene existed in the last common ancestor of plants and
animals, which was a unicellular organism that lived at least 1200 million years ago," says Knoll. "The oldest fossils whose morphology
implies a set of Hox genes controlling anterior-posterior patterning are 555 million year old animal traces from northern Russia."
What Next?
Brine shrimp are multi-limbed.
Let us review some Biology 11: The Phylum Arthropoda have several subphyla and classes. Some arthropod classes
have multi-segmented / multi-limbed adults, often with two pairs of legs per segment.
Examples include Centipedes and Millipedes. Crustacea are also Arthopods; but as
adults they have two or three body sections. Many Crustacea (like brine-shrimp) are
still multi-limbed as adults. The Arthropod Class Insecta also have larvae with many
segments (like everything mentioned so far). However, insect adults have bodies
divided into three parts: head, thorax and abdomen with three pairs of legs and usually
one (maybe two) pairs of wings attached ONLY to the thorax. Check out the diagrams in
the article.
Do brine-shrimp have insect-like ancestors, or do insects have brine-shrimp-like
ancestors? Explain - What evidence do we have?
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Hox” is actually a nickname for ________________________________________________________________________
Describe what “Hox”genes do: ________________________________________________________________________
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“Ubx” is actually a nickname for _______________________________________________________________________
Describe what happens when “Ubx”gene expression is removed in insects: ____________________________________
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Is “Ubx”gene expression turned on or off in fly thoraxes? Explain ___________________________________________
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Transgenic flies were constructed. What happens when the brine shrimp version of the “Ubx”gene expression is
inappropriately expressed in fly thoraxes?
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Remember that flies have segmented larvae. Why would an ultrabithorax mutant fly (shown in the article) have a total
duplication of the body segment that carries wings?
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This insect fossil found in Kentucky (1980) was an insect wing from the Pennsylvanian rocks of
the Eastern Kentucky Coal Field. It came from an animal that had a wing span of about 4
inches, looked somewhat like a dragonfly, and could not fold up its wings. This fossil has 3 sets
of wings (the first set is smaller). Speculate what an even earlier ancestor may have looked
like look like.
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It would appear that timing of Ubx expression during development is important. What are the next series of
experiments the scientists want to attempt? _____________________________________________________________
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This is a great article. But you cannot always believe everything you read! The well known homeotic genes in plants
(MADS-box genes) are NOT homologous to Hox genes in animals. Plants and animals do NOT share the same homeotic
genes, and this suggests that homeotic genes arose INDEPENDENTLY in the early evolution of animals and plants.
When did Hox genes first appear in animal evolution?
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When did segmentation first appear in animal evolution?
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Why would the separate evolution of Hox-like genes in plants make sense? Explain (hint – vascular tissue in leaves, stems & roots?)
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Should humans be considered a segmented animal? Explain. _______________________________________________
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