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African trypanosomes are protozoan parasites that cause sleeping sickness in humans through a tsetse fly vector. The procyclic form of Trypanosoma brucei has a single, attached flagellum that describes a helical path along the cell from posterior to anterior. During division, a specific flagellum-flagellum connection is elaborated between the new and old flagellum. This connector was present only during cell duplication and was found to be involved in the replication of the helical cell pattern and polarity. This finding implicates the concept of cytotaxis in cell morphogenesis in trypanosomes.
A procyclic (tsetse form) trypanosome has a single flagellum that emerges from an invagination of the plasma membrane termed the "flagellar pocket," close to the posterior end of the cell. The flagellar pocket is the sole site for membrane traffic to and from the cell surface. The flagellum is attached to the cell body throughout most of its length and follows a left-handed helical path toward the anterior end. Along the region of attachment, known as the flagellum attachment zone (FAZ) (1, 2), the membranes of the flagellum and cell body are intimately apposed. Inside the cell, this region of apposed membranes is associated with two internal cytoskeletal structures-the FAZ filament and a subset of four specialized microtubules-both of which run from the basal body to the anterior end of the cell, underlying the flagellum. The flagellar cytoskeleton is cross-linked to the cell body cytoskeleton through the FAZ, and the arrangement of these structures confers the typical polarity and helicity of a trypanosome cell. During cell duplication, unique mechanisms such as the interaction between the flagellum and the mitochondrial DNA are involved in the rigorous replication of this cell pattern and polarity (3).
The emergence of a short new flagellum (Fig. IA) from the flagellar pocket is one of the earliest events in cell duplication of T. brucei. Later, separation of the basal bodies (which subtend the proximal ends of the flagella) produces a second flagellar pocket for the new flagellum (Fig. 113) and mediates the segregation of the replicated mitochondrial DNA in the kinetoplasts (3). The new flagellum emerges closer to the posterior end of the cell and is always on the left side of the old flagellum (Fig. 113) when the cell is...