Abstract
Giardia is a worldwide spread protozoan parasite colonizing in small intestines of vertebrates, causing Giardiasis. The controversy about whether it is an extremely primitive eukaryote or just a highly evolved parasite has become a fetter to its uses as a model for both evolutionary and parasitological studies for years. Glycerophospholipid (GPL) synthesis is a conserved essential cellular process, and thus may retain some original features reflecting its evolutionary position, and this process should also have undergone parasitic adaptation to suit Giardia’s dietary lipid-rich environment. Thus, GPL synthesis pathways may be a perfect object to examine the controversy over Giardia. Here, we first clarified Giardia’s previously confusing GPL synthesis by re-identifying a reliable set of GPL synthesis genes/enzymes. Then using phylogenetic and comparative genomic analyses, we revealed that these pathways turn out to be evolutionarily primitive ones, but with many secondary parasitic adaptation ‘patches’ including gene loss, rapid evolution, product relocation, and horizontal gene transfer. Therefore, modern Giardia should be a mosaic of ‘primary primitivity’ and ‘secondary parasitic adaptability’, and to make a distinction between the two categories of features would restart the studies of eukaryotic evolution and parasitic adaptation using Giardia as a model system.
Highlights
Giardia is a widespread intestinal protozoan parasite in human and many other vertebrates, causing one of the most common parasitic diseases – giardiasis
GPL biosynthesis genes/enzymes re-identified in Giardia
Through our strict identification process, as many as eighteen homologous GPL biosynthesis enzymes were re-identified in Giardia
Summary
Giardia is a widespread intestinal protozoan parasite in human and many other vertebrates, causing one of the most common parasitic diseases – giardiasis. Giardia was once regarded as one of the earliest divergent eukaryotes due to having many so-called ‘simple’ and ‘prokaryote-like’ features: 1) the simplicity in cellular structures, such as lack of some cellular organelles (e.g. mitochondrion) and poorly developed endomembrane system; 2) the prokaryote-like metabolic pathways; 3) the basal position on molecular phylogenetic trees. The bone of contention is whether the simple or prokaryote-like features are a reflection of Giardia’s primitivity or just the consequences of parasitic reduction or adaptation. This debate has become a fetter to its uses as a model system for both evolutionary and parasitological studies for years. It is not until 2013 that the overall GPL synthesis pathways of prokaryotes and eukaryotes are nearly explicit[20,21,22], which had seriously limited the previous identification of the homologous genes/enzymes in Giardia, and the previously reconstructed pathways could not be complete and were even inconsistent in different literatures
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