The evolution of nuclear microsatellite DNA markers and their flanking regions using reciprocal comparisons within the African mole-rats (Rodentia: Bathyergidae)

Abstract

Microsatellites are repetitive DNA characterized by tandem repeats of short motifs (2 – 5 bp). High mutation rates make them ideal for population level studies. Microsatellite allele genesis is generally attributed to strand slippage, and it is assumed that alleles are caused only by changes in repeat number. Most analyses are limited to alleles (electromorphs) scored by mobility only, and models of evolution rarely account for homoplasy in allele length. Additionally, insertion/deletion events (indels) in the flanking region or interruptions in the repeat can obfuscate the accuracy of genotyping. Many investigators use microsatellites, designed for a focal species, to screen for genetic variation in non-focal species. Comparative studies have shown different mutation rates of microsatellites in different species, and even individuals. Recent studies have used reciprocal comparisons to assess the level of polymorphism of microsatellites between pairs of taxa. In this study, I investigated the evolution of microsatellites within a phylogenetic context, using comparisons within the rodent family Bathyergidae. Bathyergidae represents a monophyletic group endemic to sub-Saharan Africa and relationships are well supported by morphological and molecular data. Using mitochondrial and nuclear DNA, a robust phylogeny was generated for the Bathyergidae. From my results, I proposed the new genus, Coetomys. I designed species-specific genotyping and microsatellite flanking sequence (MFS) primers for each genus. Sequencing of the MFS provided direct evidence of the evolutionary dynamics of the repeat motifs and their flanking sequence, including rampant electromorphic homoplasy, null alleles, and indels. This adds to the growing body of evidence regarding problems with genotype scores from fragment analysis. A number of the loci isolated were linked with repetitive elements (LTRs and SINEs), characterized as robust phylogenetic characters. Results suggest that cryptic variation in microsatellite loci are not trivial and should be assessed in all studies. The phylogenetic utility of the nucleotide variation of the MFS was compared to the well-resolved relationships of this family based on the 12S/TTR phylogeny. Variation observed in MFS generated robust phylogenies, congruent with results from 12S/TTR. Finally, a number of the indels within the MFS provided a suite of suitable phylogenetic characters.