Evolutionary Theory and Processes: Modern Perspectives

pp 65-82

Variation and Fidelity: The Evolution of Simple Sequence Repeats as Functional Elements in Adjustable Genes

  • David G. King
  • , Morris Soller

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The functional properties of simple sequence repeats (SSRs) support an expanded understanding of evolution’s effect on mutability. These DNA tracts are characterized by high rates of gain or loss in the number of tandem repetitions of a short DNA motif. Such mutations are remarkable for being frequent, site-specific and readily reversible. Furthermore, many SSRs are functionally integrated into the genome, so that such changes in tract length can exert a quantitative regulatory effect on gene transcription activity. Although the characteristic mutability of SSRs increases the site-specific rate of mutation, the quantitative effect minimizes the probability of significantly deleterious outcome. Such mutable sites can thus create a favorable balance between the costs and the benefits of mutability.

SSR alleles which undergo changes in length can nevertheless retain their characteristic mutability. Consequently, selection for favorable alleles will indirectly select the mutability function by which those alleles arose. Conversely, SSR alleles can undergo site specific modification of their mutability, for example by mutation which alters the purity of motif repetition, while retaining their characteristic effect on phenotype and fitness. Indirect selection can thereby shape the degree of mutability for any gene that includes a functional SSR.

Genes associated with SSRs may be favored by indirect selection whenever quantitative variation in the affected traits can provide a population with genetic resiliency for adaptation, especially in fluctuating or heterogeneous environments. Such “adjustable genes” may provide a prolific and evolutionarily significant source of quantitative genetic variation.