David King

Simple Sequence Repeats as Advantageous Mutators in Evolution

by Yechezkel Kashi
Department of Biotechnology and Food Engineering, Technion - Israel Institute of Technology

and David G. King
Department of Anatomy and Department of Zoology, Southern Illinois University

Trends in Genetics 22:253-259 (May 2006).

ABSTRACT
Simple Sequence Repeats (SSRs) often serve to modify the expression of genes with which they are associated. The influence of SSRs on gene regulation, transcription, and protein structure typically depends on the number of repeats, while mutations that add or subtract repeat units are both frequent and reversible. SSRs thus provide a prolific source of quantitative and qualititative variation. Over the past decade, a number of researchers have found that this spontaneous variation has been tapped by natural (as well as artificial) selection to adjust nearly every aspect of gene function. These studies support the hypothesis that SSRs, by virture of their special mutational and functional qualities, play a major role in generating the genetic variation underlying adaptive evolution.

INTRODUCTION
Simple Sequence Repeats (SSRs, also called microsatellites and minisatellites) are mutation-prone DNA tracts composed of tandem repetitions of relatively short motifs. Although SSRs are commonly regarded as "junk" (i.e., with no significant role as genomic information), accumulating evidence for many molecular and phenotypic effects of SSR repeat-number variation has lent growing support to the hypothesis that SSRs could play a positive role in adaptive evolution. Indeed, from an evolutionary perspective, the properties of these remarkable sequences confer virtually ideal "mutator" properties. SSR instability may be indirectly advantageous by supplying abundant quantitative genetic variation with minimal genetic load, while variation in repetition purity and motif length allow site-specific adjustment of both mutation rate and mutation effect.
Here we highlight positive evidence from a few recent reports that support an evolutionary role for SSRs as important sources of adaptive genetic variation, both within and between species. In contrast to many other studies that simply demonstrate effective functional differences between "normal" and "mutant" SSR alleles, these examples offer evidence that common SSRs alleles can offer potential selective advantages. This shall be followed by an overview of the molecular basis for the functional effects of SSRs in both coding and non-coding domains, and a brief consideration of the evolutionary benefit for SSR mutability.

King, D.G., E.N. Trifonov, and Y. Kashi (2006) Tuning Knobs in the Genome: Evolution of Simple Sequence Repeats by Indirect Selection. In: Lynn H. Caporale, ed., The Implicit Genome, Oxford University Press. [Abstract]

King, D.G., and M. Soller (1999) Variation and fidelity: The evolution of simple sequence repeats as functional elements in adjustable genes. In: S.P. Wasser, ed., Evolutionary Theory and Processes: Modern Perspectives, pp. 65-82. Kluwer Academic Publishers, Dordrecht, The Netherlands. [Abstract]

King, D.G. (1994) Triplet repeat DNA as a highly mutable regulatory mechanism. Science 263:595-596. [PDF text]