Endosymbiosis: Lynn Margulis (2 of 2)

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Mitochondria are thought to have descended from close relatives of typhus-causing bacteria.

The Genetic Evidence
In the 1970s scientists developed new tools and methods for comparing genes from different species. Two teams of microbiologists—one headed by Carl Woese, and the other by W. Ford Doolittle at Dalhousie University in Nova Scotia—studied the genes inside chloroplasts of some species of algae. They found that the chloroplast genes bore little resemblance to the genes in the algae’s nuclei. Chloroplast DNA, it turns out, was cyanobacterial DNA. The DNA in mitochondria, meanwhile, resembles that within a group of bacteria that includes the type of bacteria that causes typhus (see photos, right). Margulis has maintained that earlier symbioses helped to build nucleated cells. For example, spiral-shaped bacteria called spirochetes were incorporated into all organisms that divide by mitosis. Tails on cells such as sperm eventually resulted. Most researchers remain skeptical about this claim.

Phylogenetic analyses based on genetic sequences support the endosymbiosis hypothesis.

It has become clear that symbiotic events have had a profound impact on the organization and complexity of many forms of life. Algae have swallowed up bacterial partners, and have themselves been included within other single cells. Nucleated cells are more like tightly knit communities than single individuals. Evolution is more flexible than was once believed.

• Mitochondria image courtesy of the CDC, Public Health Image Library.
• Typhus-causing bacteria (Rickettsia) image © David H. Walker and Vsevolod Popov, authors. Licensed for use, ASM MicrobeLibrary.