|Wigglesworthia wiggles into the world of sequenced genomes|
By Kate Dalke
September 13, 2002
Scientists have sequenced the genome of a bacterium that lives inside the gut of the blood-sucking tsetse fly. The fly transmits a parasite causing the deadly African sleeping sickness. The bacterium and fly live in symbiosis, and scientists hope that studying the bacterium's genome will help in developing better controls for the spread of the disease.
Wigglesworthia glossinidia is on par with some of the smallest genomes sequenced. The bacterium has co-evolved with its insect host over millions of years; this co-evolution has allowed the bacterium to streamline its genome, eliminating genes found in its host.
"The Wigglesworthia genome has shrunk," says Serap Aksoy of Yale University School of Medicine in New Haven, Connecticut, who led the sequencing project and first named the organism. Wigglesworthia bears the name of British entomologist Sir Vincent Brian Wigglesworth, who described the organism beautifully, says Aksoy.
Researchers compared Wigglesworthia to the well-studied intracellular Buchnera bacterium, which lives symbiotically in aphids. Although there are similarities between the two bacteria, the genomes are actually quite different.
Unlike Buchnera, Wigglesworthia's genome still contains remnants of a free-living organism, such as genes for motility. Although scientists have never seen Wigglesworthia swim, they found genes that synthesize flagellawhip-like cellular propellers.
The flagella may help Wigglesworthia travel from adult tsetse flies to larvae, says Aksoy. The female tsetse fly fertilizes its young in its uterus, and the bacteria are then transferred through the mother's milk. One theory for flagella is that they may aid bacteria in locomotion to or invasion of the larval cells.
The Wigglesworthia genome also contains over 60 genes involved in the synthesis of vitaminsnutrients that the tsetse fly relies on for its fertility. This finding confirms previous studies that have indicated that the fly depends on the bacteria to provide these vitamins not found in its restricted diet of blood. Without the bacteria (and vitamins), the tsetse fly is sterile.
This sterility may someday be used to prevent the spread of disease. By removing the bacteria from tsetse flies, scientists would stop the development of offspringreducing fly populations and disease transmission. Field studies are already underway testing the efficacy of sterility in tsetse flies.
The tsetse fly is the carrier of African trypanosomes, the parasite that causes the deadly sleeping sickness. Aksoy says she plans to investigate the "big picture" interactions between the parasite, fly and symbiotic bacteriaincluding whether the bacteria supply essential nutrients not only to the tsetse fly, but also to the parasite.
"Wigglesworthia has a fascinating biology," says Aksoy, "but stopping transmission of disease is always the most important background to our work."
Aksoy maintains one of only a handful of tsetse fly colonies in the world. She breeds five different species of tsetse flies, with about 5,000 breeding adults at any one time. Members of her lab feed the hungry mouths a diet of bovine blood, which the flies suck up through a specialized membrane, not from live animals.
The research team does not work with flies that have parasites that infect humans, but rather a sibling species that causes disease in animals. The fly-holding cages are "well-designed" so bites are "a rare event" in the laboratory. Nevertheless, lab coats are a must, and the scientists leave their smock in the insectary at the end of each day.
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