Microsatellites, transposable elements and the X chromosome.

Variability at microsatellite (MS) loci is generally perceived as resulting from an interaction between mutation and genetic drift and, to a lesser extent, selection and recombination. Less investigated has been the reason for MS accumulation in genomes. We present here a simple model that could account for the variation in density of MS loci, assuming that they are created either through replication slippage or in association with transposable elements. Microsatellites then evolve under the forces cited above. We use this framework to revisit two results obtained from high-density genomic maps of the human and mouse genomes built with thousands of CA repeats: MS loci are (1) less variable and (2) less dense on the X chromosome than on autosomes. The first result is most likely explained by differential mutation on the X chromosome and the autosomes. The second result may be explained by differential mutation, provided the distributions of MS loci are still not at equilibrium. Selection, acting either directly on large allele size or indirectly on the transposable elements associated with MS, may explain the same result. The framework developed here is a first step toward more rigorous models, calling for additional data.