A novel selection regime for differentiation defects demonstrates an essential role for the stumpy form in the life cycle of the African trypanosome

A novel selection scheme has been developed to isolate bloodstream forms of Trypanosoma brucei, which are defective in their ability to differentiate to the procyclic stage. Detailed characterization of one selected cell line (defective in differentiation clone 1 [DID-1]) has demonstrated that these cells are indistinguishable from the wild-type population in terms of thei r morphology, cell cycle progression, and biochemical characteristics but a re defective in their ability to initiate differentiation to the procyclic form. Although a small proportion of DiD-1 cells remain able to transform, deletion of the genes for glycophosphatidyl inositol-phospholipase C demons trated that this enzyme was not responsible for this inefficient differenti ation. However, the attenuated growth of the Delta-glycophosphatidyl inosit ol-phospholipase C DiD-1 cells in mice permitted the expression of stumpy c haracteristics in this previously monomorphic cell line, and concomitantly their ability to differentiate efficiently was restored. Our results indica te that monomorphic cells retain expression of a characteristic of the stum py form essential for differentiation, and that this is reduced in the defe ctive cells. This approach provides a new route to dissection of the cytolo gical and molecular basis of life cycle progression in the African trypanos ome.