Tiny water flea, many genes

A water flea about the size of the equal sign on a keyboard has more genes than any other creature analyzed so far, say scientists who suggest its sophisticated genome could one day double as a highly sensitive and inexpensive environmental monitoring tool.

A water flea about the size of the equal sign on a keyboard has more genes than any other creature analyzed so far, say scientists, who suggest its sophisticated genome could one day double as a highly sensitive and inexpensive environmental monitoring tool.

Daphnia pulex, the water flea, with a brood of genetically identical future offspring. The water flea's genome was recently sequenced by a consortium of international scientists. ((Paul D.N. Hebert, University of Guelph))

The tiny freshwater flea Daphnia pulex has nearly 31,000 genes, compared to our 23,000. The finding is part of a larger report published Thursday in the peer-reviewed journal Science by members of the Daphnia Genomics Consortium, an international network of 450 investigators who have been working on the project for nearly 10 years.

It turns out that while more than one-third of Daphnia 's genes have never been seen before, many of them hold the key to its uncanny ability to adapt to nasty changes in its freshwater habitats around the world, says project leader John Colbourne, director of Indiana University's Center for Genomics and Bioinformatics.

Scientists have known for some time that the water flea reacts to chemical signals from predators, growing features such as protective tail spines, helmets and neck teeth. 

"Basically, they are building armour against predatory fish," Colbourne said.

But this new research shows many of these previously unknown genes also allow the water flea to adapt to wide ranges in acidity, toxins, oxygen concentrations, food quality and temperatures by "tuning-up" or "tuning-down."

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Because water fleas reproduce by cloning themselves, it's possible to put identical creatures in two different types of freshwater environments — one that is contaminated with a specific toxin, and one that isn't. By figuring out the difference in the way its genes are expressed (tuned up or tuned down) in the contaminated water, scientists can then use that information to pinpoint specific toxins.

Colbourne likens the water flea to the canaries that were once used by coal miners as an early warning system of toxic gas buildup, which would kill the bird before the miners. But now that scientists have sequenced the water flea's genome, they can detect toxins by measuring how the water flea's genes react before they reach deadly levels, and so intervene sooner.

That's especially important given that of the roughly 80,000 man-made chemicals found in water supplies around the world, only seven to 10 per cent have ever been tested for their potential toxic properties, Colbourne says. "And another 2,000 are brought to market every year," he added.

"We need to find some method to catch up with the science of determining what's good and what's bad in our environment."

Colbourne suggests the humble water flea could very well be the answer.

Paul Hebert, director of the Biodiversity Institute of Ontario at the University of Guelph, says the research represents a very important first step in more sensitive environmental monitoring.

"As one begins to get a handle on the genes involved, one can move from no longer asking when an organism goes belly up." says Hebert, who was not involved in the Science paper. "Rather, [we can] start to see impacts on the genes themselves — turning genes on, turning genes off."

In fact, the water flea is one of the few tiny animals on the planet that can make hemoglobin, a protein in the blood that carries oxygen. This has important implications for monitoring lakes for low-oxygen levels brought on by agricultural runoff. Low oxygen levels are deadly for many aquatic animals.

But when oxygen levels plummet, the water flea is able to 'tune up' the genes that allow it to make hemoglobin, which is an early signal of declining oxygen, Hebert says.

"We don't need to wait for the canary to die. … I like the idea of walking down to the lake, grabbing a few fleas and asking, 'So how induced are your hemoglobin genes?'"

In addition, because the water flea shares many of its genes with humans, it can be used as a stand-in when investigating the impact of various toxins in fresh water, the researchers say. 

"This puts us in a position to begin integrating studies of environmental quality with research of human diseases," said co-author Joseph Shaw, an Indiana University biologist.

While scientists have already sequenced the genes of arthropods such as the honeybee and the fruit fly, this is the first time a crustacean's genome has been sequenced.