AN EVALUATION OF EVOLUTIONARY CONSTRAINTS ON MICROSATELLITE LOCI USING NULL ALLELES

A test to evaluate constraints on the evolution of single microsatelli te loci is described. The test assumes that microsatellite alleles tha t share the same flanking sequence constitute a series of alleles with a common descent that is distinct fi om alleles with a mutation in th e flanking sequence. Thus two or more differ ent series of alleles at a given locus represent the outcomes of different evolutionary process es. The higher rate of mutations within the repeat region (10(-9) or 1 0(-4)) compared with that of insertion/deletion or point mutations in adjacent nanking regions (10(-9)) or with that of recombination betwee n the repeat and the point mutation (10(-6) for sequences 100 bp long) provides the rationale for this assumption. Using a two-phase, stepwi se mutation model we simulated the evolution of a number of independen t series of alleles and constructed the distributions of two similarit y indices between pairs of these allele series. Applying this approach to empirical data fi-om locus AG2H46 of Anopheles gambiae resulted in a significant excess of similarity between the main and the null seri es, indicating that constraints affect allele distribution in this loc us. Practical considerations of the test are discussed.