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Eukaryotic Cell, August 2007, p. 1421-1430, Vol. 6, No. 8
1535-9778/07/$08.00+0     doi:10.1128/EC.00138-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Unusually Low Levels of Genetic Variation among Giardia lamblia Isolates ^,

Smilja Teodorovic, John M. Braverman, and Heidi G. Elmendorf^*

Biology Department, 406 Reiss Bldg., 37th and O Sts. NW, Georgetown University, Washington, DC 20057

Received 23 April 2007/ Accepted 29 May 2007

Giardia lamblia, an intestinal pathogen of mammals, including humans, is a significant cause of diarrheal disease around the world. Additionally, the parasite is found on a lineage which separated early from the main branch in eukaryotic evolution. The extent of genetic diversity among G. lamblia isolates is insufficiently understood, but this knowledge is a prerequisite to better understand the role of parasite variation in disease etiology and to examine the evolution of mechanisms of genetic exchange among eukaryotes. Intraisolate genetic variation in G. lamblia has never been estimated, and previous studies on interisolate genetic variation have included a limited sample of loci. Here we report a population genetics study of intra- and interisolate genetic diversity based on six coding and four noncoding regions from nine G. lamblia isolates. Our results indicate exceedingly low levels of genetic variation in two out of three G. lamblia groups that infect humans; this variation is sufficient to allow identification of isolate-specific markers. Low genetic diversity at both coding and noncoding regions, with an overall bias towards synonymous substitutions, was discovered. Surprisingly, we found a dichotomous haplotype structure in the third, more variable G. lamblia group, represented by a haplotype shared with one of the homogenous groups and an additional group-specific haplotype. We propose that the distinct patterns of genetic-variation distribution among lineages are a consequence of the presence of genetic exchange. More broadly, our findings have implications for the regulation of gene expression, as well as the mode of reproduction in the parasite.

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* Corresponding author. Mailing address: Biology Department, 406 Reiss Bldg., 37th and O Sts. NW, Georgetown University, Washington, DC 20057. Phone: (202) 687-9883. Fax: (202) 687-5662. E-mail: hge{at}georgetown.edu

Published ahead of print on 8 June 2007.

Supplemental material for this article may be found at http://ec.asm.org/.

Present address: 2538 Virginia St., Berkeley, CA 94709-1109.

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Eukaryotic Cell, August 2007, p. 1421-1430, Vol. 6, No. 8
1535-9778/07/$08.00+0     doi:10.1128/EC.00138-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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