A species of protoctists (sometimes called G. intestinalis) that is the most common flagellated protozoan found in the human digestive tract and is often a cause of diarrhea in hikers. Giardia was first described about 1681 by van Leeuwenhoek. The parasite is tear-drop shaped, about 12 μm long, and is binucleate. The stationary phase trophozoites arrest in the G2 phase with a ploidy of 8N (2 nuclei, each with 4N ploidy). The haploid number of chromosomes is 5 and the C value is 10–12 Mb. Each cell has 4 pairs of flagella, but lacks conventional mitochondria. However, the cells each contain several dozen mitosomes (q.v.). These provide clusters of iron and sulfur atoms that the Giardia need to synthesize ATP (q.v.). Giardia trophozoites form a Golgi apparatus as they differentiate into cysts, but no Golgi can be identified within non-cysting cells. Comparative analyses of the 16 S-like RNAs of Giardia and several other eukaryotes show that Giardia represents the earliest diverging lineage in the eukaryotic line of descent yet encountered. See Classification, Protoctista, Archaeprotista; Chronology, 1677, van Leeuwenhoek; Archaeprotista, ribosomes.
Subjects: Genetics and Genomics.