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Eukaryotic Cell doi:10.1128/EC.00138-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Unusually low levels of genetic variation among Giardia lamblia isolates

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

^* To whom correspondence should be addressed. Email: hge{at}georgetown.edu.

	Abstract

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 in G. lamblia is insufficiently understood, but it 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. Intra-isolate genetic variation has never been estimated in G. lamblia, and previous studies on inter-isolate genetic variation have included a limited sample of loci. Here we report a population genetics study of intra- and inter-isolate 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, yet allow identification of isolate specific markers. Low genetic diversity was discovered at both coding and noncoding regions, with an overall bias towards synonymous substitutions. Surprisingly, we find 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 regulation of gene expression, as well as the mode of reproduction in the parasite.

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