Genome Shattered to Smithereens

A strange, two-tailed marine alga hosts an even stranger set of chromosomes, researchers report in today's Nature. Inside the alga's chloroplasts, or photosynthetic fuel cells, each gene occupies its own tiny circular chromosome.

Chloroplasts have an odd evolutionary history: They started out as cyanobacteria--organisms that use photosynthesis to make energy--but were engulfed by the ancestors of modern plants and algae to become cell organelles. Since being pressed into service by plants, chloroplasts have lost many of their ancestors' genes. Most have retained 100 to 200 genes along one chromosome and rely on their host's DNA for some essential proteins. But when biologist Tom Cavalier-Smith of the University of British Columbia in Vancouver and his colleagues sequenced the genome of chloroplasts from the dinoflagellate Heterocapsa triquetra, they found only 10 genes, each contained in its own miniature chromosome.

The researchers found no H. triquetra chloroplast genes coding for transfer RNA, ribosomal proteins, and RNA polymerase--essential players in most other chloroplasts. Instead, most of the 10 genes encode proteins needed for photosynthesis. Each is on a short, 2000 to 3000 base-pair stretch of DNA, whose ends appear to connect to form loops.

"There's no other example in any chloroplast of this phenomenon," says Cavalier-Smith, who has no idea why the rest of the chloroplast's genome vanished. "It's probably a freak of evolution," he says.

A chloroplast with so few genes "is rather shocking," says molecular evolutionist Jeffrey Palmer of the University of Indiana, Bloomington. One explanation, he says, may be that coordinating many mini-chromosomes during cell division would be a difficult task for a cell. Perhaps mistakes during cell division, he says, accelerated the loss of the chloroplast's genes.