EQUILIBRIUM DISTRIBUTIONS OF MICROSATELLITE REPEAT LENGTH RESULTING FROM A BALANCE BETWEEN SLIPPAGE EVENTS AND POINT MUTATIONS

We describe and test a Markov chain model of microsatellite evolution that can explain the different distributions of microsatellite lengths across different organisms and repeat motifs, Two key features of thi s model are the dependence of mutation rates on microsatellite length and a mutation process that includes both strand slippage and point mu tation events. We compute the stationary distribution of allele length s under this model and use it to fit DNA data for di-, tri-, and tetra nucleotide repeats in humans, mice, fruit flies, and yeast, The best f it results lead to slippage rate estimates that are highest in mice, f ollowed by humans, then yeast, and then fruit flies, Within each organ ism, the estimates are highest in di-, then tri-, and then tetranucleo tide repeats. Our estimates are consistent with experimentally determi ned mutation rates from other studies. The results suggest that the di fferent length distributions among organisms and repeat motifs can be explained by a simple difference in slippage rates and that selective constraints on length need not be imposed.