Apr. 17, 2002 — A researcher studying the
last common link between invertebrate and vertebrate animals has found
a key genetic change that separates the spineless from the backboned.
Jeremy Gibson-Brown, Ph.D., assistant professor of biology at
Washington University in St. Louis, studies amphioxus, a small marine
worm, a primitive invertebrate species that is the closest living
invertebrate related to vertebrates like ourselves. Gibson-Brown has
found that a gene involved in the development of a body layer in
invertebrates duplicated within the vertebrate lineage after the
development of amphioxus.
However, in vertebrates, this gene, AmphiEomes/Tbr1, gave rise to two
genes, Eomesodermin and T-brain-1, involved in brain development. While
the vertebrate Eomesodermin gene has retained its original function in
forming the mesoderm, or "middle skin" layer,in all vertebrate studies
from fish, to amphibians to humans, the duplicate copy has lost that
function and instead has evolved a role in forebrain development.
"This shows us how ‘old’ genes can give birth to new ones, and how the
origins of novel developmental functions can be traced," said
Gibson-Brown, who will have his results published in a forthcoming
issue of the Journal of Experimental Zoology. His next step will be to
look for these genes in lampreys, primitive jawless fish similar to the
ancestors of later vertebrates.
"I want to see whether this gene duplication predated the separation of
jawless fish and vertebrates and whether the role in forebrain
development had yet been acquired."
Fruit flies, mice, worms and apes share an amazing amount of genetic
information with us humans and with each other. For instance, there is
only one-tenth of one percent genetic variation between a human and a
A field of research has arisen to address what kinds of genetic change
over time have occurred in different species to account for so many
physical differences despite such genetic similarity. It is called
"Evo-devo," as Gibson-Brown affectionately refers to this budding
discipline, combines the principles of traditional evolutionary and
developmental biology in examining the change in gene sequence and
regulation that over time lead to the development of new species and
eventually new body plans.
"We seek to unravel the history of the evolution of developmental
programs in animals," Gibson-Brown explained. Gibson-Brown is studying
the evolution of T-box genes, a group of genes that encode
transcription factors regulating gene expression in embryogenesis, or
the development of embryos.
Simply put, T-box genes control when and where a particular gene is
turned on (expressed) or turned off during the course of an animal’s
development. The vast diversity of body plans seen in animals alive
today — and those who have lived in the past — are due in part to
different expression patterns of these genes. T-box genes are present
both in vertebrates and invertebrates, and so offer valuable insight
into the emergence of new developmental programs, and hence new body
plans, during the course of evolution.
Amphioxus is a small marine worm, a primitive invertebrate species
whose last common ancestor with humans lived 600 million years ago.
Amphioxus is the closest living invertebrate relative to the
vertebrates, making it a very attractive target for Gibson-Brown’s
research. He is interested in how these T-box genes, present in
Amphioxus, humans and everything in between, have adapted their
function and expression patterns to yield such a vast array of body
plans, from worms to mice to humans.
"What I’ve been looking at is where and when these T-box genes are
expressed in the development of amphioxus in order to understand the
function of those genes in the last common ancestor of amphioxus and
humans," Gibson-Brown said.
He has just begun work with lampreys, a very primitive vertebrate and
one of the last species of jawless fish still alive today. Because
lamprey ancestors evolved relatively shortly after the divergence of
vertebrates from invertebrates they provide the next stepping stone in
the story of T-box gene evolution.
By comparing the expression of T-box genes in Amphioxus, lamprey and
mice, Gibson-Brown hopes to better understand the role that changes in
gene regulation have played in the evolution of T-box genes.
"I want to understanding the regulatory elements controlling the
expression of T-box genes in different species because the evolution of
new developmental functions by genes is primarily achieved by the
evolution of regulatory elements," he said.
Source : Washington University In St. Louis.