FROM WORM MUSCLE TO SPINAL DISCS : AN EVOLUTIONARY SURPRISE
Thoughts of the family
tree may not be uppermost in the mind of a person suffering from a slipped
disc, but those spinal discs provide a window into our evolutionary past. They
are remnants of the first vertebrate skeleton, whose origins now appear to be
older than had been assumed. Scientists at the European Molecular Biology
Laboratory (EMBL) in Heidelberg, Germany, have found that, unexpectedly, this
skeleton most likely evolved from a muscle.
The study, carried
out in collaboration with researchers at the Howard Hughes Medical Institute in
Janelia Farm, USA, is published today in Science.
Humans are part of a
group of animals called chordates, whose defining feature is a rod of cartilage
that runs lengthwise along the middle of their body, under their spinal chord.
This structure, called the notochord, was the first vertebrate skeleton. It is
present in human embryos, and is replaced with the backbone as we develop, with
the cartilage reduced to those tell-tale discs. Since starfish, sea urchins and
related animals have no such structure, scientists assumed the notochord had
emerged in a relatively recent ancestor, after our branch of the evolutionary
tree split away from the 'starfish branch'.
"People simply
haven't been looking beyond our direct relatives, but that means you could be
fooled, if the structure appeared earlier and that single group lost it,"
says Detlev Arendt from EMBL, who led the study. "And in fact, when we
looked at a broader range of animals, this is what we found."
Antonella Lauri and
Thibaut Brunet, both in Arendt's lab, identified the genetic signature of the
notochord -- the combination of genes that have to be turned on for a healthy
notochord to form. When they found that the larva of the marine worm
Platynereis has a group of cells with that same genetic signature, the
scientists teamed up with Philipp Keller's group at Janelia Farm to use
state-of-the-art microscopy to follow those cells as the larva developed. They
found that the cells form a muscle that runs along the animal's midline,
precisely where the notochord would be if the worm were a chordate. The
researchers named this muscle the axochord, as it runs along the animal's axis.
A combination of experimental work and combing through the scientific
literature revealed that most of the animal groups that sit between Platynereis
and chordates on the evolutionary tree also have a similar, muscle-based
structure in the same position.
The scientists
reason that such a structure probably first emerged in an ancient ancestor,
before all these different animal groups branched out on their separate
evolutionary paths. Such a scenario would also explain why the lancelet
amphioxus, a 'primitive' chordate, has a notochord with both cartilage and
muscle. Rather than having acquired the muscle independently, amphioxus could
be a living record of the transition from muscle-based midline to cartilaginous
notochord.
The shift from
muscle to cartilage could have come about because a stiffened central rod would
make swimming more efficient, the scientists postulate.
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