Our lab studies the limbs of tetrapods.
Tetrapods are animals that include amphibians, reptiles, birds and mammals. They
are great organisms to study limbs because most have four of them. Despite
these animals having the shared characteristic of four limbs, they do have a
large variety in the shapes and functions of those limbs. For example, the wing
of the bat is used for flying while the wing of the emu is useless. Further
adaptions of the limb give us the flippers of whales and dolphins, the hooves
of horses, and even our arms and legs.
The use of model organisms is key to
understanding how all of these differences arose in evolution. For the study of
limbs, we need animals that readily provide embryos. Since limbs are usually
made when the animal is an embryo, we need an embryo to watch this process. One
such animal is the chicken, where most of our knowledge of tetrapod limb
development has been gained by using their embryos. Those studies using the
chicken gave rise to the question of when during development do animals make
their limbs? We know chickens make them early, but is that the case for every
tetrapod?
This question of when the limb develops has
led us to work on two very different model systems. The first is the emu, the
large flightless birds of Australia. These birds develop in a very similar
method as chickens with the exception that their wings are slightly delayed.
Believe it or not, emus make pretty good models to ask this question, their
eggs are available from farms and techniques that work in chick embryos can be
done in emus! The downside is that even though the emu egg is huge, the embryo
is no bigger than that of the chick. This means doing an experiment on
something the size of a watch’s minute hand in the middle of a softball-sized sea
of egg yolk. By using emus, we are able determine some of the evolutionary
changes that result in both the delay in limb formation and its tiny size.
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| Xenopus laevis |
We’re following up this question of when
organisms make their limbs by studying frogs. We all know that frogs begin as
tadpoles and then metamorphose into little frogs, but we’re not sure how they
make their arms and legs during this process. To answer this question, we use a
frog from South Africa –Xenopus laevis–
that has an amazing ability to lay eggs at any time of the year. Additionally, their eggs are quite big as
embryos go, which allow us to do small surgeries as well as manipulate their
genes. This way we can figure out how these animals build a limb when they are
no longer embryos!
By studying frogs and emus, we get a more
complete understanding of all the different ways an animal can make a limb. We
can see how evolution has tinkered with those limb-making genes, which also has
implications for humans. Our limbs are
thought to be similar to those of chickens and other tetrapods, so this work
can open new areas possible treatments for human limb defects and deformities.
This article was written by:
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| John holding an Ostrich egg! |
John J. Young, PhD
Assistant Professor
Department of Biology
Simmons University
Twitter: @lab_frog
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