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Plant chloroplasts evolved more than once.

A disingenuous alga, aptly named Cryptomonas, picked up the ability to harvest solar energy by gobbling one of its photosynthesizing distant cousins, researchers report in the March 14 Nature.

The finding supports the idea that early complex cells gained new components with specialized functions by kiddnapping one another - not just by engulfing simplear cells. It also calls into question the family tree outlining the heredity of several hard-to-classify algae.

Scientists have long suspected that cellular organelles, such as energy-generating mitochondria and sugar-producing chloroplasts, were once independent organisms. A century ago, microscopists pointed out that organelles of complex cells look much like simpler free-living microbes. In the early 1960s, researchers showed that mitochondria and chloroplasts, which lie outside the cell nucleus, contain their own genetic material.

To explain these findings, scientists devised the serial endosymbiosis theory, which essentially holds that primitive microbes evolved into more complex ones by swallowing other microbes and putting them to work as organelles. But most researchers believed that each type of organism swallowed only organisms less advanced than itself and that a single organelle takeover sufficed over the evolutionary long run.

Now, a team led by Susan E. Douglas of the Canadian Institute of Marine Biosciences in Halifax, Nova Scotia, has demonstrated that Crptomonas - already advanced enough to have a nucleus -must have gained its chloroplast by gobbling another cell with a nucleus. Within Crytomonas, the researchers identified bits of genetic synthesizers, or ribosomes, that closely resemble the ribosomes of red algae, an equally advanced organism. They found the ribbosome fragments in a nucleus-like structure known as the nucleomorph.

Douglas surmises that the nucleomorph, which exists independently of the nucleus, is a vestige of a red alga coopted by Cryptomonas in the distant past. "This is the first definitive proof," she says, that cells with nuclei conquired other cells with nuclei.

"This is the type of evidence people will believe," comments algae researcher Sally Gibbs of McGill University in Montreal. Gibbs says the work confirms on a molecular level her own observations of the similarity between Cryptomonas' nucleomorph and red algae. "But this will get it out in the public and in the textbooks," she adds.

In a commentary accompanying the research report, David Penny and Charles J. O'Kelly of Massey University in Palmerston North, New Zealand, suggest the new finding will overturn the conventional notion that all chloroplasts are descendants to one evolutionary cellswallowing event. "These bits of molecular evidence support predictions . . that chloroplasts have arisen many times," they write.

Penny and O'Kelly add that Douglas' report will force biologists to reevaluate the relationships among various algae, fungi and green plants. "The simplistic first stage of the search for the single universal evolutionary tree is coming to an end," they assert.
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Author:Ezzell, Carol
Publication:Science News
Date:Mar 16, 1991
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