Giardia's primitive GPL biosynthesis pathways with parasitic adaptation 'patches': implications for Giardia's evolutionary history and for finding targets against Giardiasis.

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.