Humans like to think of themselves as residing at the top of the evolutionary chain, but the tiny water flea beats them out by at least one measure — the size of its genome.
The creature, which lives in freshwater ponds around the world, requires nearly 31,000 genes to complete its genetic blueprint, almost a third more than humans and 50% more than the common housefly, researchers reported last week in the journal Science.
That gives Daphnia pulex, only 1 millimeter long, more genes than any creature whose genome has been decoded to date, according to geneticist John Colbourne of Indiana University, who led the team that performed the sequencing.
One reason it has so many genes, he said, is that it is creating copies of existing genes and retaining them at a very high rate, about three times faster than any other invertebrate and about 30% faster than humans.
For example, humans have four light-sensing proteins called opsins. Water fleas have 46. That may be because they need to use light to explore their aquatic environment, said marine biologist Todd Oakley of UC Santa Barbara, who was involved in the study.
Colbourne speculated that the proliferation of genes allows the tiny creatures to survive in increasingly polluted waters.