Magazine article Natural History

The Beast with Five Genomes

Magazine article Natural History

The Beast with Five Genomes

Article excerpt

Inside a termite's gut lives Mixotricha paradoxa, a microscopic organism comprising hundreds of thousands of smaller life-forms. M. paradoxa is an extreme example of how all plants and animals-including ourselves-have evolved to contain multitudes. By Lynn Margulis and Dorion Sagan

she hullabaloo over mapping the human genome-the sum of all the genes in an individual-might lead one to think that each species has only a single genome and that the genetic makeup of individual organisms is discrete and unitary. Such is far from the case. Paraphrasing Walt Whitman, we multicellular beings contain multitudes. All animals' cells have at least two interacting genomes. One is the DNA in the cell nucleus; this is the genome that has recently been "mapped." The other is that of the DNA in the mitochondria-the cell's multiple oxygen-breathing organelles that are inherited only through the maternal line. For more than a century, some scientists have known that every organism is in fact a multiple being, but until recently these unorthodox researchers were ignored.

In most of the animals we think we know best (mammals, reptiles, insects), the genomes that determine limbs, eyes, and nervous systems, for example, are very similar to our own. These animals, like us, are doubly genomic. Even some unicellular beings that do not have eyes, limbs, or nervous systems-such as amoebas and paramecia-contain both nuclear and mitochondrial genomes. Plants and algae have these double genomes as well, plus a third genome, of symbiotic origin. During their evolutionary history, they ingested (but did not digest) photosynthetic blue-green bacteria. Therefore, all visible photosynthetic organisms have at least three genomes. But many organisms--such as the protists that inhabit termites-contain within them up to five or more genomes.

The great nineteenth-century naturalist Joseph Leidy, one of the founders of the Academy of Natural Sciences in Philadelphia, was the first to take a close-up look at the contents of a termite's gut. "In watching the Termites from time to time wandering along their passages beneath stones," he wrote, "I have often wondered as to what might be the exact nature of their food." What he saw under his microscope amazed him. If the termite's intestine is ruptured by the experimenter, he wrote, "myriads of the living occupants escape, reminding one of the turning out of a multitude of persons from the door of a crowded meeting-house." Leidy immediately realized that what he knew as "white ants" were actually composed of dozens of different kinds of tiny life-forms, including bacteria and what we now call protists. (Protists are microbes with nuclei; more complex than bacteria, the group includes amoebas, slime molds, and algae.) We now recognize that the immense and motley crew that Leidy observed within a termite is in no way a gratuitous add-on or a pathological infection. Rather, it is a necessary part of the termite's digestive system and is organized as a particular tissue: an aggregate working mechanism that turns the refractory compounds lignin and cellulose (the main constituents of wood) into food. This composite fabric, or living consortium, has evolved in the nearly oxygen-free closed system of the termite's abdomen for probably 100 million years; without the living, wood-degrading factories that have become their digestive systems, these termites starve.

The pioneering biologist Konstantin S. Merezhkovsky first argued in 1909 that the little green dots (chloroplasts) in plant cells, which synthesize sugars in the presence of sunlight, evolved from symbionts of foreign origin. He proposed that "symbiogenesis"-a term he coined for the merger of different kinds of life-forms into new species-- was a major creative force in the production of new kinds of organisms. A Russian anatomist, Andrey S. Famintsyn, and an American biologist, Ivan E. Wallin, worked independently during the early decades of the twentieth century on similar hypotheses. …

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