Microsatellite evolution in vertebrates: Inference from AC dinucleotide repeats

We analyze published data from 592 AC microsatellite loci from 98 species i n five vertebrate classes including fish, reptiles, amphibians, birds, and mammals. We use these data to address nine major questions about microsatel lite evolution. First, we find that larger genomes do not have more microsa tellite loci and therefore reject the hypothesis that microsatellites funct ion primarily to package DNA into chromosomes. Second, we confirm that micr osatellite loci are relatively rare in avian genomes, but reject the hypoth esis that this is due to physical constraints imposed by flight. Third, we find that microsatellite variation differs among species within classes, po ssibly relating to population dynamics. Fourth, we reject the hypothesis th at microsatellite structure (length, number of alleles, allele dispersion, range in allele sizes) differs between poikilotherms and homeotherms. The d ifference is found only in fish, which have longer microsatellites and more alleles than the other classes. Fifth, we find that the range in microsate llite allele size at a locus is largely due to the number of alleles and se condarily to allele dispersion. Sixth, length is a major factor influencing mutation rate. Seventh, there is a directional mutation toward an increase in microsatellite length. Eighth, at the species level, microsatellite and allozyme heterozygosity covary and therefore inferences based on large-sca le studies of allozyme variation may also reflect microsatellite genetic di versity. Finally, published microsatellite loci (isolated using conventiona l hybridization methods) provide a biased estimate of the actual mean repea t length of microsatellites in the genome.