New Findings: How Do Scorpions Make Their Tails?
A new study led by scientists at the American Museum of Natural History
reveals the genetic blueprint behind the patterning of scorpion tails.
Scientists have long been puzzled by the development of scorpion
tails—which in addition to venom-producing glands also have
light-sensing capabilities—because there weren't enough known genes to
code for their many segments.
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But the new research, which was published today in Proceedings of The Royal Society B, reveals that scorpions have more "body-planning" genes than previously thought, potentially solving the scorpion tail mystery.
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"Scorpions
have six segment-types in the back-end of their body, almost double the
number seen in their closest relatives. They also are the only
arthropods to have a group of segments exclusively dedicated to prey
capture and defense," says Prashant Sharma, a postdoctoral researcher in
the Museum'sDivision of Invertebrate Zoology and lead author of the paper. "The question is how to pattern this kind of complexity."
Along with Ward Wheeler,
a curator in the Museum's Division of Invertebrate Zoology, and
colleagues at Harvard University, Sharma focused on a group of genes
known as the Hox family, which encode the body plan in numerous
organisms from worms to humans. By acting in different combinations,
these genes control whether a given portion of the embryo will develop
mouthparts, wings, or gills, for example.
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Hox
genes are expressed, from head to tail, in the same order as they
appear in the genetic code. The system works by "staggering" the
expression of the gene family. The first gene in the Hox family will be
expressed starting in the head. Subsequent genes, however, begin to be
expressed one section of the embryo at a time, causing each section to
have a unique genetic cocktail: If , say, a given section has genes X
and Y, for example, it may produce legs, while if it had genes X, Y, and
Z, it would make lungs. The staggering of these Hox genes allows many
different segment types to develop. Manipulation of a single Hox gene
can turn what would be a fly's antenna into a leg, or even be used to
create a 10-legged spider.
Arachnids—the
group of arthropods that includes scorpions, spiders, and
daddy-long-legs—are presumed to have 10 Hox genes. In non-scorpion
arthropods, six of the 10 Hox genes have been shown to aid in the
patterning of the front part of the organism that includes the legs and
feeding appendages. This leaves only four to control the back end. Four
genes, however, are not enough to pattern the scorpion's tail.
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"If
the previous model were true, we couldn't actually make a scorpion,"
Sharma said. "We would need either more genes or a different model."
The
researchers used the Arizona bark scorpion to investigate Hox gene
makeup. By taking tissue samples of scorpion embryos and determining the
genes that were being expressed at a given developmental stage, they
discovered that 19 different Hox genes are active during development,
instead of the typical 10.
"But
just because scorpions have a lot of genes, it doesn't mean those genes
have anything to do with body patterning," Sharma said.
The
group began the process of testing whether the genes are all actually
involved in shaping the scorpion's tail. First, the researchers bathed
embryonic tissue samples with probes that change color if a certain gene
was being expressed. Their results uphold the model: the appearance of
each gene in the family is staggered and coincides with a shift in
segment identity. While further mutative experiments would be required
to definitively prove the connection between genetic code and body form,
it seems that the scorpion's extra genes do in fact pattern its tail.
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