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Protists push animals aside in rule revamp

Redefined kingdoms give centre stage to single-celled organisms.

Organisms whose cells have a nucleus — eukaryotes — have traditionally been separated into four ‘kingdoms’; now they have been reorganized into six. The authors of the revision hope that it will bring peace to a long-divided discipline, and raise awareness of the diversity of single-celled organisms.

Textbooks generally divide eukaryotes into plants, animals, fungi and protists. The protist kingdom mostly consists of single-celled organisms such as amoebae. Bacteria make up a fifth kingdom.

Since the late 1970s, data from electron microscopy and DNA sequences have indicated that the traditional groupings do not make sense. Protist classification was particularly troubled. But the evidence was not clear enough for a consensus on a new regime, and fierce disagreements became common.

“I've seen people throw things at each other,” says Sina Adl, a soil-organism specialist at Dalhousie University in Halifax, Canada, who coordinated the group of 28 protist experts that produced the new classification. It was commissioned by the International Society of Protistologists and is published in The Journal of Eukaryotic Microbiology (S. M. Adl et al. J. Eukaryot. Microbiol. 52, 399–451; 2005).

The experts have given protists a mighty four kingdoms out of six. Animals do not even get their own group — fungi and animals have been merged into Opisthokonta, and plants are called Archaeplastida.

As for the protists, amoebae and slime moulds form Amoebozoa, and various single-celled organisms are now Rhizaria. The remaining two groups — Chromalveolata and Excavata — are the most contentious. “There have almost been fist fights over the mention of these groups,” says Adl. “It has taken a lot of diplomacy to get people to sit down and talk.”

The tensions arise from conflicting DNA evidence. Some genes are different enough to suggest that the groups should be separate kingdoms, others are not. Adl's team decided that enough evidence had accumulated to declare them distinct. “A lot of people will be upset,” he admits. “But it needed doing — over the past 20 years the classification we had has fallen apart.”

One dissenter is evolutionary biologist Blair Hedges, based at Pennsylvania State University in University Park. He says genomics work by himself and others suggests that the Excavata should not be a separate group. “People have ignored the evidence and gone on gut feeling,” he told Nature.

Trypanosomes (purple), which cause sleeping sickness, were one of the hardest protists to classify. Credit: EYE OF SCIENCE/SPL

The two contentious groups include parasites that cause diseases such as malaria and sleeping sickness; Adl hopes his classification will aid drug development. “Placing these organisms in the wrong group is in part responsible for the fact that we do not have specific drugs for these diseases, because of wrong assumptions about their biochemistry,” he says.

But Michael Gaunt, who studies the Chagas' disease parasite at the London School of Hygiene and Tropical Medicine, is less convinced that the change will have dramatic practical effects. “It is important to understand the relationships between these organisms if we are to tackle these diseases,” he says. “But the lack of effective drugs is largely due to poor industrial interest.”


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The international society of protistologists

Journal of Eukaryotic Microbiology

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Simonite, T. Protists push animals aside in rule revamp. Nature 438, 8 (2005).

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