|First of six chromosomes sequenced in Dictyostelium discoideum|
By Kate Dalke
July 19, 2002
Scientists have sequenced and analyzed the first and largest chromosome of the slime mold Dictyostelium discoideum. The quirky amoeba has been studied extensively for its peculiar life cycle that includes both single-cell and multi-cellular stages. The genetics behind this unusual life cycle could provide clues about the evolution of multi-cellular life forms.
The sequence of chromosome two is the first step in a project to sequence the entire organism by an international consortium of researchers. Highly repetitive segments of the genome, as well as the extremely high content of A and T base pairs, have made sequencing Dictyostelium a challenge. For this reason, researchers are contributing data to complete each of the six chromosomes one by one.
Some of the roughly 2,800 genes found on chromosome two are similar to genes found in human disease, including cancer, hereditary deafness, and immunodeficiency. Many of the motility genes in Dictyostelium mirror those found in disease-fighting white blood cells in humans.
"When we compared Dictyostelium to other sequenced organisms, we found it had more homology to animals than to either plants or fungi," says Angelika A. Noegel, who led the sequencing at the University of Cologne, Germany.
'Dicty,' as scientists call it, lives in soil as a single-cell creature for most of its life. If food becomes scarce, it has a unique survival strategy. It converges with other single-cell amoebas to create a multi-cellular organism that will mature and release spores for reproduction.
"Dicty sits at the interface between single and multi-cellular organisms," says molecular biologist Rex Chisholm of Northwestern University in Chicago, Illinois. "From an evolutionary point that's an interesting place to be."
Chisholm is spearheading a new public database for organizing genetic information about the organism. Chromosome two will contribute some of the first data included in Dictybase, whose design is based on other model organism databases like mouse and yeast.
With its 8.1 million base pairs, chromosome two represents about 25 percent of the Dictyostelium genome. Based on the surprisingly large number of genes in chromosome two, scientists predict the entire genome contains about 11,000 genessimilar to the number found in the fruit fly, according to the paper in Nature.
Noegel says the research centers expect to have data for the five remaining chromosomes sometime next year.
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