Of Migrations and Variations

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Science  19 Dec 2008:
Vol. 322, Issue 5909, pp. 1759
DOI: 10.1126/science.322.5909.1759a

Once it was thought that Mycobacterium tuberculosis was genetically uniform. However, global surveys of clinical samples have shown that like other human-specific pathogens, it has a marked biogeography. Hershberg et al. have compared 90 genes in over 100 strains of the tuberculosis bacterium and established that the geographic variation has arisen as a consequence of human migrations over the millennia—first by land out of Africa 50,000 years ago and then by sea back to Africa over the past few centuries—and subsequent genetic drift rather than immune selection. In the apparent absence of purifying selection, many of the mutations are retained and result in nonsynonymous changes in amino acids, which are likely to have functional effects. It is not clear how M. tuberculosis tolerates the potentially deleterious consequences of genetic drift, but this cryptic variation needs to be taken into account in vaccine and drug design. — CA

PLoS Biol. 6, e311 (2008).

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