An Unusually Low Microsatellite Mutation Rate in Dictyostelium discoideum, an Organism With Unusually Abundant Microsatellites

Abstract

The genome of the social amoeba Dictyostelium discoideum is known to have a very high density of microsatellite repeats, including thousands of triplet microsatellite repeats in coding regions that apparently code for long runs of single amino acids. We used a mutation accumulation study to see if unusually high microsatellite mutation rates contribute to this pattern. There was a modest bias toward mutations that increase repeat number, but because upward mutations were smaller than downward ones, this did not lead to a net average increase in size. Longer microsatellites had higher mutation rates than shorter ones, but did not show greater directional bias. The most striking finding is that the overall mutation rate is the lowest reported for microsatellites: $1 3 10 À6 for 10 dinucleotide loci and 6 3 10 À6 for 52 trinucleotide loci (which were longer). High microsatellite mutation rates therefore do not explain the high incidence of microsatellites. The causal relation may in fact be reversed, with low mutation rates evolving to protect against deleterious fitness effects of mutation at the numerous microsatellites.M ICROSATELLITES, also known as simple sequence repeats, are long stretches of a short (1-6 bp), tandemly repeated DNA unit, such as the motif CAA repeated 20 times. Microsatellites are common throughout eukaryotic genomes and their lengths are often highly polymorphic, making them powerful markers for use in genetic mapping

Evidence for a recent population bottleneck in an Apicomplexan parasite of caribou and reindeer, Besnoitia tarandi

“…Even assuming that the slowest reported rates of microsatellite mutation (McConnell et al, 2007) apply to B. tarandi, this parasite is 50% likely to have undergone a population bottleneck during or since the last glacial period. Accepting more typical estimates of microsatellite mutational rate (10 À5 -10 À4 ) provides 95% confidence that Finnish and Canadian populations could have remained independent for no longer than 3000-30,000 years.…”

The Rate and Spectrum of Microsatellite Mutation in Caenorhabditis elegans and Daphnia pulex

“…First, although few thorough experimental investigations have been performed with other unicellular organisms, the predominantly single-celled slime mold Dictyostelium discoideum exhibits patterns of microsatellite instability (McConnell et al 2007) that are not discernibly different from those of yeast (Figure 1). Second, several surveys have been performed on small di-and trinucleotide repeats in Drosophila melanogaster (Schlö tterer et al 1998;Schug et al 1998;Fernando Vázquez et al 2000).…”

The Rate and Effects of Spontaneous Mutation on Fitness Traits in the Social Amoeba,Dictyostelium discoideum

“…The MA experiment was previously described in a study of the mutation rate at microsatellite loci (Mcconnell et al 2007). We initiated the MA experiment from a single cell of the laboratory-generated strain AX4 (Knecht et al 1986) of Dictyostelium discoideum , provided by Gad Shaulsky at Baylor College of Medicine.…”