The Loss of the Kinetoplast in Trypanosomes, with Special Reference to Trypanosoma evansi

An account is given of the spontaneous occurrence and prolonged maintenance of strains of Trypanosoma evansi devoid of the kinetoplast. The structure and nature of this organelle—which is regarded as a plastid—are discussed and previous work on its loss is reviewed. Among the six akinetoplastic strains studied, two were kept under continuous observation for about 17 years, in the course of which many thousands of trypanosomes were examined, without a single normal flagellate being detected. In these cases, therefore, the aberrant condition had become permanently fixed as a heritable character. Another strain of T. evansi first exhibited considerable fluctuation in the number of akinetoplastic forms (1–70%) but subsequently reverted to the normal state. The perpetuation of the akinetoplastic condition appears to be due to failure of the kinetoplast to divide, with the result that trypanosomes thus incapacitated divide irregularly, giving rise to akinetoplastic forms, which continue to breed true. This may lead to the production, on the one hand, of fluctuating strains, in which the akinetoplastic and normal trypanosomes compete for survival; and, on the other hand, of totally akinetoplastic strains. The loss of the kinetoplast is comparable to the loss of plastids in phytoflagellates. In a discussion of the genetic aspects of this phenomenon it is regarded as a mutation determined by plastogenes, for the mutant trypanosomes appear suddenly, breed true, and give rise to a new race. Moreover, the kinetoplast once lost does not arise de novo. It is suggested that T. equinum had originated from an akinetoplastic strain of T. evansi produced by such mutation.