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The tomato wilt fungus is able to attack the plants thanks to genes on four extra chromosomes, University researchers have found.
A new way to swap DNA
Fungi surprise scientists with a new trick: swapping chromosomes, which can turn some into pathogens
By Deane Morrison
The news may sound innocuous: The fungus that makes your tomato plants wilt may have picked up that unsavory trait from another fungus by a novel means.
To the researchers, however, the discovery was a rare moment when an organism astounds you with an undreamed-of ability. In this case, fungi turned out to be passing whole chromosomes around like footballs, entirely outside the realm of reproduction.
No one knows how they do it. Chromosomes are bulky, and fungi just weren't supposed to be able to trade them. But the research team caught the fungus that causes tomato wilt harboring four extra chromosomes that seemed to come from a completely unrelated fungus. And they observed such a transfer in the laboratory.
Excess baggage
Why would a fungus pack the genes for infecting plants on a few extra chromosomes? Kistler says that in the normal life of the fungus, when it's feeding on dead matter in the soil and no plants are available, the genes for infecting plants aren't needed. With those genes bundled on a few extra chromosomes, the fungus can easily shed them—and the trouble of maintaining them—without detriment.
"It was surprising," says Corby Kistler, an adjunct University of Minnesota professor and a research geneticist for the USDA Agricultural Research Service. "This is the first time anyone has detected the transfer of pathogenicity carried on whole chromosomes between fungi."
The four traded chromosomes are "extras" that the fungus can live without. They are packed with genes that allow the fungus to infect plants or even people. Minus the extra chromosomes, the tomato wilt fungus is just a harmless soil variety.
The discovery means scientists can start working on a means to thwart the trading of chromosomes and the diseases that come with them, Kistler says. The work is published in the journal Nature.
Killer chromosomes
Some traded chromosomes may give the same fungus the ability to attack people with weakened immune systems, whom they reach via contaminated catheters.
"These seem to be specific strains specialized toward humans—and they have extra chromosomes," says Kistler. "Whatever causes [harm] to humans may be on these transferable chromosomes."
From friend to foe
The tomato wilt fungus spreads through farm implements and rain runoff, traveling mainly in water. Strains that infect plants get into the xylem tubes—a plant's water transport system—and gum them up.
Ironically, some of the noninfective strains of the fungus are beneficial to tomatoes. They ward off pathogenic fungi by blocking potential infection sites on the plants' roots and may even immunize the plants to infection by pathogens, Kistler says.
But that can change quickly if they pick up extra chromosomes. Then, says Kistler, "It's a friendly fungus gone bad."
Published in 2010