Mode and tempo of microsatellite length change in a malaria parasite mutation accumulation experiment

Abstract

Microsatellite sequences are widely assumed to evolve neutrally, but also play an important role in bacterial pathogenesis, human disease and transcript abundance. The malaria parasite Plasmodium falciparum genome is extraordinarily AT-rich, containing 132,449 microsatellites-stretches of perfect 1-9 bp repeats between 10-1000bp, which comprise 10.74% of the 23 Mb genome. This project was designed to determine the mode and tempo of microsatellite mutations in malaria parasites. We maintained 31 parasite lines derived from a single 3D7 parasite cell for 114-267 days, with frequent bottlenecking to a single cell to minimize effective population size, allowing us to measure mutations accumulated over ~13,207 mitotic divisions. We Illumina sequenced the genomes of both progenitor and end-point mutation accumulation (MA) parasite lines in duplicate to validate stringent calling parameters. Calls were 99.89% (GATK), 99.99% (freeBayes) and 99.96% (HipSTR) concordant in duplicate sequence runs from independent sequence libraries. We observed 98 microsatellite mutations, giving rates of 2.11 x 10-7 - 1.46 x 10-8 /cell division that were strongly influenced by repeat motif and array length. Mutation rate was low relative to other organisms. However, despite this, in a single infection (1011 parasites) there will be 1.46 x 103 - 2.11 x 104 independent mutations at any single microsatellite locus. Given that many microsatellites are found in promotors, introns, within or close to coding sequences, we suggest that they may be important regulators of transcriptional and phenotypic variation in this pathogen. Author summaryMutation is central to evolution: in pathogens, the rate of mutation may determine how rapidly drug resistance evolves or how effectively pathogens can escape immune attack. Malaria parasites have small extremely AT-rich genomes, and genetic variation in natural populations is dominated by repeat number changes in short tandem repeats (microsatellites) rather than point mutations. We therefore focused on quantifying microsatellite mutation. We established 31 parasite cultures in the laboratory all derived from a single parasite cell. These were maintained for 114-267 days with frequent reductions to a single cell, so parasites accumulated mutations during ~13,207 cell divisions. We sequenced the parasite genomes at the end of the experiment to count the mutations. We highlight several conclusions: like other organisms studied, microsatellite mutation rates are associated with both repeat number and repeat motif. However, 41% of changes resulted from loss or gain of more than one repeat: this was particularly true for long repeat arrays. Unlike other eukaryotes, we found no insertions or deletions that were not associated with repeats or homology regions. Overall, we found that microsatellite mutation rates in malaria were amongst the lowest recorded and comparable to those in another AT-rich protozoan (the slime mold Dictyostelium).