Knotty evolutionary tree in plant world.

Knotty evolutionary tree in plant world

A tiny bacterium living in Dutch ponds is now the center of a debate concerning how higher plants first acquired the ability to harvest energy from the sun. In the Jan. 26 NATURE, two groups of researchers report conflicting evolutionary tales for this organism, called Prochlorothrix, which is actually a form of blue-green algae.

For decades biologists have worked with the idea that more than a billion years ago, a primordial marriage of convenience granted green plants the gift of photosynthesis. Green plant cells contain organelles called chloroplasts that hold all the machinery for photosynthesis, and scientists think chloroplasts evolved from free-living bacteria that were swallowed by more complex cells, which could not photosynthesize on their own.

When first identified in 1985, Prochlorothrix intrigued biologists because it has both chlorophyll a and chlorophyll b -- a combination of photosynthetic pigments that closely resembles the pigments inside chloroplasts. In contrast, almost all other blue-green algae (now known as cyanobacteria) lack chlorophyll b. The pigment similarity between Prochlorothrix and green plants led some researchers to speculate that this organism might be a free-living close relative of the chloroplast.

Two research teams have now completed the first genetic comparisons of Prochlorothrix, other cyanobacteria and chloroplasts from several kinds of plants. While one comparison places Prochlorothrix closer to chloroplasts than the cyanobacteria are, another study sits on the opposite side of the evolutionary fence.

To compare the organisms, Clifford W. Morden and Susan S. Golden of Texas A&M University in College Station focused on a protein that figures in photosynthesis. They found that in both chloroplasts and Prochlorothrix, the gene coding for this protein lacks seven amino acids that are included in the genes of other cyanobacteria. They conclude that Prochlorothrix is part of a lineage including chloroplasts that branched away from other cyanobacteria.

In the other study, Sean Turner and colleagues at Indiana University in Bloomington as well at the University of Amsterdam based their comparisons on genetic sequences for a segment of ribosomal RNA. Their results indicate that Prochlorothrix is no more closely related to chloroplasts than other cyanobacteria are. As one explanation for the pigment similarity, the researchers suggest that both chloroplasts and Prochlorothrix could have separately evolved chlorophyll b.

Microbiologist John Waterbury at the Woods Hole (Mass.) Oceanographic Institution says the debate cannot be resolved at this stage. However, he notes that the amino acid study suffers because the researchers used a relatively short sequence for comparison. "Probably when all else shakes out, it will fall into the $(RNA$) interpretation," he says. Researchers recently identified a marine organism similar to Prochlorothrix (SN: 7/30/88, p. 68), and Waterbury says it will be interesting to see where this new organism fits into the evolutionary tangle.