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Primitive Archezoa Dethroned
26 August 1997 9:00 pm
CHAFFEY'S LOCKS, ONTARIO--Since ancient fossils are scarce, scientists trying to understand the early evolution of life turn to single-celled organisms that appear very primitive. Now it turns out that two organisms thought to record early events in the evolution of the cell may not be good models after all. The findings, announced this week here at the evolutionary biology meeting of the Canadian Institute for Advanced Research, might also help medical researchers better treat certain infections.
A key division among types of life is based on the presence of DNA-bearing organelles called mitochondria. Bacteria lack these energy-producing structures, which are found only in eukaryotic cells--cells with nuclei. For more than two decades, many scientists have believed that mitochondria originated as bacteria that took up residence inside a primitive eukaryotic cell. Some scientists speculated that groups of single-celled organisms, the so-called archezoa, were descendants of those first eukaryotes, which lacked mitochondria.
But in recent years phylogenetic evidence has mounted that more and more of the archezoa once contained mitochondria, then somehow lost them. Now Andrew Roger, a postdoctoral fellow at the Marine Biological Laboratory in Woods Hole, Massachusetts, reports that Giardia lamblia--a common parasite that scientists regard as one of the last three models for early eukaryotes--also had a mitochondrion at one point. He and his colleagues found that the parasite has a gene for a protein called chaperonin 60. This gene is thought to be a good tracer for mitochondria, because it is known to have moved from mitochondria to the nucleus in other organisms. The Giardia gene, says Roger, "looks like it came from the precursor organelle," which the parasite later lost.
Another model organism was struck down by Martin Embley, microbiologist from the Natural History Museum in London. He found that organisms called Microsporidia, which cause potentially deadly infections in immune-compromised people, contain heat shock protein 70, which closely resembles a mitochondrial-derived protein in other organisms. The good news, Embley says, is that if Microsporidia did once harbor mitochrondria, it may actually be a fungus and may be susceptible to anti-fungal drugs in patients.
"We've lost our model organism," laments Tom Cavalier-Smith of the University of British Columbia, who helped develop the archezoa hypothesis. "We're not going to be able to deduce what [these organisms] looked like before the mitochrondion came in--or at least it will be more complicated to do so." The last holdout--a symbiont in the hindgut of termites--is also in doubt. If mitochondria had already been imported before the Giardia struck out on its own, Roger says, these organelles must be far more ancient than scientists had thought.