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Genetic Pickup: Did animals get brain genes from bacteria?

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10:32am, June 9, 2004
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Reviving a controversy about whether animals have acquired key genes from bacteria, a study suggests that microbes have provided genes that now play vital roles in brain-cell signaling and other forms of cell-to-cell communication. The genes implicated encode enzymes required for the metabolism and synthesis of crucial brain chemicals, including dopamine, serotonin, melatonin, and histamine.

"These enzymes did not evolve [in animals]. They were picked up, as a shortcut," contends David C. Klein of the National Institute of Child Health and Human Development in Bethesda, Md.

Klein's collaborators on the study, however, made a similar claim several years ago about the transfer of bacterial genes into human DNA. That assertion was strongly refuted, and the new work has quickly drawn pointed remarks from some of the same critics.

"I think the claims that they are making are scientifically questionable. I think it's actually somewhat irresponsible of them to keep publishing these papers without showing more skepticism," says Steven L. Salzberg of the Institute for Genomic Research in Rockville, Md.

The fiery debate centers on what all the scientists agree is a provocative notion, namely, that some genes didn't arise gradually through evolution but were picked up whole by early animals from the DNA of microbes. This possibility has gained interest since biologists found that bacteria, with surprising frequency, exchange genes with each other through a process called lateral gene transfer (SN: 7/22/00, p. 60: Pass the Genes, Please).

Klein entered this fray several years ago when he noticed that the gene for one of the enzymes used in the two-step synthesis of melatonin has been detected in the DNA of bacteria and animals but not in that of plants or many other multicellular organisms. He then joined forces with a group led by Eugene V. Koonin of the National Center for Biotechnology Information in Bethesda to tally which species share with bacteria the genes for other enzymes responsible for cell-signaling molecules.

In the July Trends in Genetics, they report that nearly a dozen of these genes are widespread in bacteria, present in animals, but missing from plants and other complex life forms. The genes are "all absent in large branches of the tree of life," says Klein.

He, Koonin, and their colleagues conclude that the most likely explanation for these gaps is that bacteria somehow introduced a few of their genes into some ancient animals' sperm or eggs, enabling future generations to inherit the genes. In this scenario, the genes would have bypassed plants and other forms of life.

The skeptics stress that there are many more-likely explanations for the genetic data. They point, for example, to the possibility of widespread loss of the studied genes from plants and other life forms. "We think that gene loss is still the simplest explanation," says Michael Stanhope of GlaxoSmithKline in Collegeville, Pa.

Also, biologists may have surveyed the DNA of too few creatures to find the genes that Klein and Koonin claim are absent from many branches of life. "We don't have enough genomes," says John Logsdon of University of Iowa in Iowa City. "Their interpretation is premature, given the data that are available."

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