Algal origins of a protozoan part.

In the microbiological world, small cells engulfed by big cells have sometimes evolved into essential cell parts. That endosymbiotic scenario was the likely origin of the energy-generating organelles known as mitochondria in human cells. Similarly, the chlorophyll-containing chloroplasts of plant cells were once free-living photosynthesizers. Algal cells, too, have engulfed and incorporated other algae (SN: 3/16/91, p. 164). Now comes another variation on that cell-swallowing theme: an organelle in protozoans that seems to have come from an algal cell.

The organelle contains DNA that resembles the genetic material from green algae chloroplasts, report Sabine Kohler and David S. Roos of the University of Pennsylvania in Philadelphia and their colleagues in the March 7 Science. In addition, four cell membranes surround the DNA like nested Russian dolls. These membranes are probably remnants of the series of evolutionary gulps that landed the DNA inside an alga that eventually ended up in the single-celled protozoans.

"It's a wonderful finding," says Lynn Margulis, a biologist at the University of Massachusetts at Amherst and a key proponent of endosymbiotic theory.

Besides bolstering the theory that endosymbiotic events were common in evolutionary history, the finding may have medicinal value: The protozoans belong to the group Apicomplexa, which includes the organisms responsible for malaria and toxoplasmosis. The ex-algal organelle may provide a unique DNA target for drug development.