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MBE Advance Access originally published online on November 24, 2004
Molecular Biology and Evolution 2005 22(3):683-690; doi:10.1093/molbev/msi056
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Molecular Biology and Evolution vol. 22 no. 3 © Society for Molecular Biology and Evolution 2004; all rights reserved.

Research Article

Lack of Evidence for Horizontal Transfer of the lac Operon into Escherichia coli

Daniel M. Stoebel

Department of Ecology and Evolution, Stony Brook University

E-mail: dstoebel{at}life.bio.sunysb.edu.

The idea that Escherichia coli gained the lac operon via horizontal transfer, allowing it to invade a new niche and form a new species, has become a paradigmatic example of bacterial nonpathogenic adaptation and speciation catalyzed by horizontal transfer. Surprisingly, empirical evidence for this event is essentially nonexistent. To see whether horizontal transfer occurred, I compared a phylogeny of 14 Enterobacteriaceae based on two housekeeping genes to a phylogeny of a part of their lac operon. Although several species in this clade appear to have acquired some or all of the operon via horizontal transfer, there is no evidence of horizontal transfer into E. coli. It is not clear whether the horizontal transfer events for which there is evidence were adaptive because those species which have acquired the operon are not thought to live in high lactose environments. I propose that vertical transmission from the common ancestor of the Enterobacteriaceae, with subsequent loss of these genes in many species can explain much of the patchy distribution of lactose use in this clade. Finally, I argue that we need new, well-supported examples of horizontal transfer spurring niche expansion and speciation, particularly in nonpathogenic cases, before we can accept claims that horizontal transfer is a hallmark of bacterial adaptation.

Key Words: Adaptation • Escherichia coli • lateral gene transfer • Shimodaira-Hasegawa test • lac operon


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