SCIENCE

A Water Flea Has More Genes Than You Do

February 4, 2011

Think you're something special? With your ability to speak and spend hours playing Farmville and dominate the entire planet? Well, think again, buddy. The tiny water flea (Daphnia pulex) has one up on you: it's got more genes.

At least 30,907 genes, say scientists who just completed sequencing the crustacean's genome. Humans have only about 23,000.

How did this creature, more closely related to a cockroach than to us, end up with so many genes? And why did scientists bother sequencing it?

Let's start with the second question: The common water flea is a keystone species in freshwater ecosystems. It eats algae, and fish eat the water fleas. Perhaps more importantly, though, water fleas are highly responsive to environmental stresses—producing exaggerated spines or helmets in the presence of some predators, for example—and are used by scientists assessing changes in freshwater ecosystems.

Now that researchers have the Daphnia genome sequenced, they're hoping that they can pump up the water flea's usefulness in evaluating environmental contaminants. "The costly challenge of evaluating conditions in the environment and of our water supplies may be overcome by Daphnia's potential use as a high-tech and modern version of the mineshaft canary," said project leader John Colbourne, of Indiana University. "Our initial studies revealed that Daphnia's genes are evolved to be fine-tuned to environmental changes."

The water flea's flexible responses to environmental change may be contributing to its large number of genes, says Colbourne. But the main reason it has so many genes is that those genes are multiplying within the genome at higher rates than in other species—three times the rate of other invertebrates and 30 percent higher than the rate in humans. All that copying and multiplying has given Daphnia not only a huge number of genes but also plenty of unique ones; a third are found in no other sequenced organism.