The evolution of microsatellites

This thesis investigates the evolutionary dynamics of microsatellite loci, and their use as measures of genetic variability for conservation studies. An understanding of the dynamic mutation process at microsatellite loci is developing, but at present it is not known what factors limit array size change - how microsatellites are born and how they die. A novel approach to this question is made possible by the analysis of recently produced large scale genome sequence data. Chapter 2 proposes a lower size limit for slippage mutation - the birth of a microsatellite, based on the distribution of short tandem repeat array sizes in the genomes of the yeast, nematode and human. Chapter 3 presents a simple model to show how the two components of dynamic mutation, slippage mutation and unequal exchange, could combine to set an intrinsic limit on array size increase - the death of a microsatellite. Microsatellite variation can be used as a surrogate for overall genomic diversity to test the hypothesis that frequent extinction and colonisation in fragmented populations can drastically reduce levels of genetic variability. Three microsatellite loci were cloned from the declining British butterfly Plebejus argus to examine the genetic effects of population turnover in this species. Two related studies were performed. Chapter 5 shows that serial colonisation events in an introduced P. argus metapopulation have caused significant losses of genetic variation. In Chapter 6, levels of variability and patterns of differentiation at microsatellite loci are used to show that turnover within natural populations has not led to genetic impoverishment of the species. Both studies show that the microsatellite data are consistent with studies of allozyme loci in the same populations