Fixed and temporary fluctuations in the cell cycle of monomorphic lines of Trypanosoma brucei gambiense

The growth rate of a cloned, monomorphic strain of Trypanosoma brucei gambiense can be changed in both fixed and environmentally induced ways. The purpose of this study was to determine which phases of the cell cycle were affected by these changes in generation time. A slow-growing cloned line of T. b. gambiense and its fast growing, cloned derivative, were collected from immunosuppressed mice at low parasitemias. The percentages of cells in G1, S, and G2-M phases were determined by cytophotometric measurements of nuclear DNA. The percentages of cells in late G2-M and D phases were established by light microscopy. Over 70% of the fixed reduction of generation time in the cell cycle of the fast growing trypanosome occurred during G1, the remainder in G2. Apparently, the parasite had genetically altered a metabolic pathway necessary for both the initiation of DNA replication and karyokinesis. Both fast and slow growing trypanosomes significantly decrease their growth rates at high parasitemia levels in immunosuppressed rodents. To determine which phases of the cell cycle were affected, nuclear DNA of T. b. gambiense collected at low and high parasitemias were compared. Over 60% of the increase occurred during the S-phase, the remainder in G1. Apparently, DNA replication was affected by the parasite load. Finally, these monomorphic strains of T. b. gambiense, like tumors, apparently do not have a noncycling (G0) phase or a restriction point in G1 since slow growing trypanosomes in either optimal or suboptimal conditions did not accumulate in a diploid state or proceed through S to D phases in a constant length of time.