Allelic Heterozygosity Within and Among Giardia Lamblia Genotype B Isolates

Abstract

Giardia lamblia is a binucleate intestinal pathogen belonging to an early diverging eukaryotic lineage. It is responsible for many cases of diarrheal disease worldwide, with two distinct Giardia genotypes, A and B, known to infect human hosts. The level of genetic diversity among genotype B isolates is not sufficiently understood. Thus, we attempt to analyze what is happening at a population genetics level to produce unique allelic sequence heterozygosity patterns. In this report, I analyzed an 869 bp region on the chromosome 3 locus and a 532 bp region on chromosome 5, by comparing multiple cloned PCR products from the reference genotype B isolate GS and 6 genotype B field isolates. We found a total of 44 and 19 SNP sites at the chromosome 3 and 5 loci, respectively. Each of the seven isolates had much higher numbers of haplotypes at the two chromosomal loci than could be explained by allelic sequence heterozygosity (ASH), and haplotype ratios did not suggest ASH as the reason for the diversity. We narrow down the reasons for allelic divergence, and implicate the potential accumulation of sequence variability in individual lineages. A comprehensive understanding of the genetic diversity within and among genotype B organisms will be important in evaluating parasite variation, the mechanisms for genetic exchange among early eukaryotes, and the taxonomic classification of Giardia genotypes.