[Paleopsych] Science Daily: The Constitution Of A New Model Army: Genome Basis Of Working Together For A Common Good

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Fri Jun 10 23:47:48 UTC 2005

The Constitution Of A New Model Army: Genome Basis Of Working Together
For A Common Good

Democratic elections are the times when the actions of the majority form
the basis for the future of the whole; individual citizens gather
together to take part in deciding how they all will live. On Thursday 5
May 2005, the biological constitution of a remarkable organism that
votes with its feet was published in Nature by a team of scientists from
the UK, USA, Germany, Japan and France.

Dictyostelium discoideum, known as Dicty to researchers, spends most of
its time living alone in the soil as a single-celled amoeba. However, in
a food shortage the individual cells 'talk' to each other, aggregate and
then develop into a multicellular organism that produces spores, the
only survivors of the time of hunger. This unique and seemingly simple
development has helped biologists understand how, for example, cells in
the human immune system communicate and how that process can go wrong in
disease. It is even being used to try to understand the treatment of
bipolar disorder.

Paul Dear, a lead scientist from the MRC Laboratory of Molecular
Biology, Cambridge, UK, said, "Were it not for its tiny size and
unwieldly name, Dictyostelium would be familiar to us all as one of the
more bizarre forms of life on Earth. It represents a branch of life that
we now know parted company from animals shortly after plants and animals
went their separate ways. Its DNA sequence, now an open book for
researchers worldwide, shares genes with plants, animals and fungi, and
promises to shed light on many fundamental aspects of biology."

As a single cell, Dicty is ideal for mutation - the first choice from
the geneticists' toolbox - and many of its biochemical processes have
been modified, revealing the underlying chemistry of life. But its
development from single cell to true multicellular organism makes it
more valuable as a model, allowing biologists to define the ways in
which cells in complex organisms such as ourselves talk to each other.
Cell communication is an absolute requirement for multicellular life.

The genome sequence consists of 34 million base-pairs - letters of
genetic code - that contain the instructions for 12,500 proteins - about
half as many as the human genome and more than twice as many as simple
yeasts. Among these are genes involved in complex processes
characteristic of multicellular life - communication, adhesion, movement
- that cannot be modelled in simpler species.

However, Dicty is not a pared down human or a complex fungus. It shows a
unique combination of conserved and lost functions that span the
kingdoms of bacteria, plants and animals in a compact genome.

"The genome of Dictyostelium discoideum is one of the most distinctive
analysed so far, reflecting the intriguing biology of this organism,"
commented Marie-Adele Rajandream, leader of the sequencing component at
the Wellcome Trust Sanger Institute. "The chromosomes have an unusual
structure, the genome is nearly 80% A and T residues, whereas our genome
is 40% As and Ts, and it makes special use of ribosomal genes that is
unique to known biology."

More than one-tenth of the genome is composed of simple repeated
sequences, 2-6 bases repeated over and over again. Expansion of the
number of copies of simple repetitive sequences are characteristic of
some human diseases, such as myotonic dystrophy and Huntington's
disease, and understanding why and how these repeats are tolerated in
Dicty will help to understand human disease development.

A search for versions of genes in Dicty that resembled genes involved in
human disease found that between 10 and 20% are conserved. These include
nine cancer genes as well as genes similar to those implicated in human
Parkinson's, Tay-Sachs and cystic fibrosis.

For several decades, Dictyostelium has been studied in laboratories
around the world as a perfect test-bed in which to study processes such
as how cells move and how individual cells specialize and coordinate to
form complex organisms. The solitary amoeba shares many features with
our own cells, particularly those which patrol our bodies and engulf
harmful bacteria.

Just as important have been the insights into developmental biology -
how all complex life forms develop from a single cell to a multicellular
organism in which cells share duties and differentiate into specialized
functions. When Dicty is starved, each cell sends a pulse of a chemical
called cyclic AMP and, gradually, these pulses become synchronized and
the cells 'swim' in waves towards the highest concentration of cAMP. The
result is an aggregate of cells, which form a mobile 'slug' and then a
delicate spore- bearing fruiting body. Many of the molecules identified
in Dicty that underlie the processes of differentiation in other

Model organisms play an essential role in biology and medicine: our
understanding of human diseases has, in many cases, been derived from
studies in other organisms. From yeasts came genes involved in cell
division and cancer in humans; from worms came genes involved in
development, cell death and cancer; and from Dicty have come new details
of cell communication and cell movement.

The genome sequence has been used to identify 'novel' proteins that will
fill gaps in pathways in Dicty biology. The sequence will enrich our
understanding of this remarkable organism and our understanding of our
own biology.

Editor's Note: The original news release can be found here.

This story has been adapted from a news release issued by Wellcome Trust
Sanger Institute.

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