Evolution and function of carbohydrate reserve biosynthesis in parasitic protists

Nearly all eukaryotic cells synthesize carbohydrate reserves, such as glycogen, starch, or low-molecular-weight oligosaccharides. However, a number of parasitic protists have lost this capacity while others have lost, and subsequently evolved, entirely new pathways. Recent studies suggest that retention, loss, or acquisition of these pathways in different protists is intimately linked to their lifestyle. In particular, parasites with carbohydrate reserves often establish long-lived chronic infections and/or produce environmental cysts, whereas loss of these pathways is associated with parasites that have highly proliferative and metabolically active life-cycle stages. The evolution of mannogen biosynthesis in Leishmania and related parasites indicates that these pathways have played a role in defining the host range and niches occupied by some protists. Many medically important protists have inherited the capacity to synthesize glycogen or amylopectin from free-living ancestors.Retention of glycogen/amylopectin biosynthesis is associated with parasitic life styles that include encysted stages and/or slow-growing life-cycle stages that cause long-term persistent infections.Complete loss of carbohydrate reserve biosynthesis in some protists (Plasmodium, Babesia, Theileria, and Trypanosoma) likely reflects multiple factors, including dependence on glycolytic metabolism in some life-cycle stages.A completely new pathway of carbohydrate biosynthesis has evolved in Leishmania and related protists which may have allowed these parasites to establish long-term chronic infections in their vertebrate hosts.