Diversity and dynamics of the minichromosomal karyotype in Trypanosoma brucei

The genome of African trypanosomes contains a large number of minichromosom es. Their only proposed role is in the expansion of the parasites' repertoi re of telomeric variant surface glycoprotein (VSG) genes as minichromosomes carry silent VSG gene copies in telomeric locations. Despite their importa nce as VSG gene donors, little is known about the actual composition of the minichromosomal karyotype and the stability of its inheritance. In this st udy we show, by using high-resolution pulsed-held electrophoresis, that a n on-clonal trypanosome population contains an extremely diverse pattern of m inichromosomes, which can be resolved into less complex close-specific kary otypes by non-selective cloning. We show that the minichromosome patterns o f such clones are stable over at least 360 generations. Furthermore, using DNA markers for specific minichromosomes, we demonstrate the mitotic stabil ity of these minichromosomes within the population over a period of more th an 5 years. Length variation is observed for an individual minichromosome a nd is most likely caused by a continuous telomeric growth of approximately 6 bp per telomere per cell division. This steady telomeric growth, countera cted by stochastic large losses of telomeric sequences is the most likely c ause of minichromosome karyotype heterogeneity within a population. (C) 200 1 Elsevier Science B.V. All rights reserved.