Molecular systematics of the parasitic protozoan Giardia intestinalis

Abstract

The long-standing controversy regarding whether Giardia intestinalis is a single 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, veterinary, and environmental management implications. In the past decade, highly 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 analysis of G. intestinalis that were representative of all known major genetic groups and compare them with other Giardia species, viz. G. ardeae, G. muris, and G. microti. Segments from four "housekeeping" genes (specifying glutamate 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 electrophoretic 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 outgroups supported the monophyly of G. intestinalis and also showed that this species includes genotypes that represent at least seven deeply rooted lineages, herein designated assemblages A-G. Inclusion of G. microti in the analysis of 18S rRNA sequence data demonstrated the monophyly of Giardia with the same median body morphology but did not support the monophyly of G. intestinalis, instead placing G. microti within G. intestinalis. The findings support the hypothesis that G. intestinalis is a species complex and suggest that G. microti is a member of this complex.

Genotyping of Giardia in Dutch patients and animals: A phylogenetic analysis of human and animal isolates

“…In addition, an 18S rDNA and gdh-specific DNA sequence analysis was performed. Clearly two genotype groups could be recognised, similar to the two assemblages A and B described by Homan et al (1998) and Monis et al (1999) ( Table 3). In total, 34 samples were identified as Giardia Assemblage A, 64 as Assemblage B and two samples remained negative.…”

“…The Dutch goat and sheep samples clustered together with the calf isolate (AY178740), Assemblage E and far distant from the Dutch dog samples (AY827498 and AY827499), which clustered together with the Australian dog (U60986), Assemblage D. The Dutch roe deer gdh sequence clustered together with the human isolates of Assemblage A, however, not in the same subcluster as the human Assemblage A isolates analysed. In addition, some animal gdh DNA sequences derived from GenBank clustered together with human Assemblage B. Subgenotyping of our human Assemblage B-derived gdh sequences showed that those with GenBank accession number AY826193 were located in the cluster of Australian human isolates of Assemblage B group III (AY178756/AF069059) (Monis et al, 1999), those with accession number AY826191 clustered in Assemblage B group IV containing the Australian human isolates (AY178738/AY178739, AY178754, L40508), the Canadian and Czech dog isolates (AY178750/AY178749) and the Czech chinchilla isolates (AY178751). Our human isolates with accession numbers AY826192, AY826197 belonging to Assemblage B clustered together in the IV-like group together with the marmosets AY178752, AY178753.…”

“…Despite morphological uniformity, considerable biotypic and genetic diversity exists within the G. duodenalis species (Monis et al, 1996 andThompson et al, 2000). The species includes several 'assemblages' or genotypes, A-G, that can be discriminated on the basis of host selection and genomic mutations (Monis et al, 1999). Although several genes encoding proteins involved in meiosis are present in Giardia (Ramesh et al, 2005), direct evidence for sexual recombination has not yet been shown.…”

“…Although several genes encoding proteins involved in meiosis are present in Giardia (Ramesh et al, 2005), direct evidence for sexual recombination has not yet been shown. Phylogenetic multilocus analysis using 18S rRNA, glutamate dehydrogenase (Gdh), elongation factor 1 alpha (EF-1 ) and triose phosphate isomerase (Tpi) gene based molecular methods have been used on representatives of each major genetic group to study the relations among assemblages from different hosts (Monis et al, 1999, van Keulen et al, 2002and Caccio et al, 2005. Zoonotic transmission of G. duodenalis is still under debate and despite increasing knowledge of the molecular identification of Giardia from different host species; the zoonotic potential of G. duodenalis is not clear (Monis and Thompson, 2003, Thompson, 2004and Hunter and Thompson, 2005.…”

Constitutive aneuploidy and genomic instability in the single‐celled eukaryote Giardia intestinalis

“…1995; Monis et al. 1999). A very low frequency of recombination between assemblages led to the suggestion of their genetic isolation and mutual speciation (Xu et al.…”