Spontaneous chromosome rearrangements in the protozoan Giardia lamblia: estimation of mutation rates

Subcloned lines of the WB strain of Giardia lamblia contain polymorphic ribosomal RNA (rRNA) encoding chromosomes (Le Blancq et al., Nucl. Acids Res. 1991, 19, 4405–4412). We show that in a continuously propagated culture of G.lamblia trophozoites the proportion of trophozoites with rearranged rRNA encoding chromosomes gradually increases, consistent with the high mutation rate of about 1% per cell per division cycle. This conclusion is based on the finding in one experiment that after about 8 division cycles 20% of the population consisted of independent mutants, while after ∼ 100 dMsion cycles 87.5% of the population were independent mutants. In a second experiment, approximately 38% and 71.5% of the trophozoites were independent mutants after ∼9 and ∼100 division cycles, respectively. The data show that the genome of the WB strain of G.lamblia has a highly recombinogenic phenotype. Extensive karyotype heterogeneity has also been observed among recently isolated G.lamblia strains obtained from a defined geographic area (Korman et al., J. Clin. Invest. 1992, 89, 1725–1733) suggesting that a high mutation rate might also occur in vivo.