Molecular systematics of the parasitic protozoan Giardia intestinalis

The long-standing controversy regarding whether Giardia intestinalis is a s ingle species prevalent in both human and animal hosts or a species complex consisting of morphologically similar organisms that differ in host range and other biotypic characteristics is an issue with important medical, vete rinary, and environmental management implications. In the past decade, high ly distinct genotypes (some apparently confined to particular host groups) have been identified by genetic analysis of samples isolated from different host species. The aim of this study was to undertake a phylogenetic analys is of G. intestinalis that were representative of all known major genetic g roups and compare them with other Giardia species, viz. G. ardeae, G. muris , and G. microti. Segments from four "housekeeping" genes (specifying gluta mate dehydrogenase, triose phosphate isomerase, elongation factor 1 alpha, and 18S ribosomal RNA) were examined by analysis of 0.48-0.69-kb nucleotide sequences determined from DNA amplified in polymerase chain reactions from each locus. In addition, isolates were compared by allozymic analysis of e lectrophoretic data obtained for 21 enzymes representing 23 gene loci. The results obtained from these independent techniques and different loci were essentially congruous. Analyses using G. ardeae and/or G. muris as outgroup s supported the monophyly of G. intestinalis and also showed that this spec ies includes genotypes that represent at least seven deeply rooted lineages , herein designated assemblages A-G. Inclusion of G. microti in the analysi s of 18S rRNA sequence data demonstrated the monophyly of Giardia with the same median body morphology but did not support the monophyly of G. intesti nalis, instead placing G. microti within G. intestinalis. The findings supp ort the hypothesis that G. intestinalis is a species complex and suggest th at G. microti is a member of this complex.