Winner for largest number of genes in any animal known so far … a water flea!

Of all the animals genome sequenced to date, the tiny water flea has the highest number of genes, more than mice, roundworms, yeast, bacteria … and humans.

A common water flea species, tiny and translucent Daphnia pulex, currently holds the record for highest number of genes among all animals that have been gene sequenced so far. It is also the first crustacean species to be gene sequenced. This tiny creature, just visible to the unaided eye, possesses about 31,000 genes!

The tiny water flea, Daphnia, has the most genes of any animal, some 31,000. Image Credit: Paul Hebert, University of Guelph

Humans, in comparison, have about 23,000 genes. These findings were reported in the Feb. 4 edition of the journal Science by an international team of scientists belonging to the Daphnia Genomics Consortium. Because Daphnia are highly adaptable to their environment, study of the Daphnia pulex genome could provide valuable insights into the ways that genes interact with the environment, these scientists say.

Daphnia are tiny aquatic crustaceans found in freshwater lakes, ponds, rivers, and streams. They have a voracious appetite for algae, and in turn, are an important food for fish.

When conditions are good, female Daphnia reproduce asexually, prolifically producing little cloned versions of themselves. As conditions grow harsher, such as at the onset of winter or drought, or when the food supply starts to diminish, male offspring are also produced. Female Daphnia mate with the males, producing fertilized eggs which remain dormant until conditions are suitable for hatching. This sexual reproduction of Daphnia may also enable a wider genetic variety in the offspring, thereby increasing the chances that some will survive under unpredictable conditions.

Daphnia, or water flea, with a clonal brood of offspring. Image Credit: Paul Hebert, University of Guelph

Daphnia pulex is interesting to study for several reasons. They are abundant, found in a variety of freshwater habitats, and they’re an important food source for animals higher in the food chain. These creatures also have a unique ability to alter their bodies in response to changes in their environment. For instance, Daphnia are able to adapt to survive in polluted water. When there are predators around, they’ve been known to change their shape – if there are many adult fish in their habitat, Daphnia are able to decrease in size, making themselves less visible to the larger predators. They do the opposite when there are younger fish around, increasing their size to be too much of a mouthful. Daphnia are even able to develop spines, helmets, and defensive neck-teeth to respond to predator threats.

Daphnia with mild defensive neckteeth; the teeth are used against predators. Image Credit: Paul Hebert, University of Guelph

The reason for their adaptability is related to their high number of genes. It appears that many of those genes are duplicate copies, with each copy having acquired a slightly different characteristic. And about one-third of those genes are completely new to science. The scientists have found that, in response to a change in its environment, Daphnia pulex is able to select appropriate genes to issue new instructions for building different proteins that are used to alter its body. This makes it an increasingly important “model organism” in a new field of research, environmental genomics, which is devoted to better understanding how genes interact with the environment. In a press release from the National Science Foundation, Dr. James Klaunig of Indiana University, Bloomington, said:

Genome research on the responses of animals to stress has important implications for assessing environmental risks to humans. Daphnia is an exquisite aquatic sensor, a potential high-tech and modern version of the mineshaft canary.

So, how does the gene tally for Daphnia pulex, around 31,000 genes, stack up against other organisms? As mentioned earlier, humans have about 23,000 genes – close in number to mice, roundworms, and a small flowering plant called Arabidopsis thaliana. Fruit flies have about 14,000 genes, yeast has about 6,000, and E. coli bacteria about 3,200.

With its record-breaking number of about 31,000 genes, highest of all organisms that have been gene sequenced so far, the remarkably adaptable Daphnia pulex water flea will be instrumental in helping scientists understand how genes interact with their surroundings.

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