The influence of mutation, selection and reproductive systems on
within-population
size variation at microsatellite loci was analysed using simulations. Mutation
occurred through either (biased or
unbiased) replication slippage, or unequal recombination between homologous
chromosomes.
Selection acted either on large allele size, or on the difference in size
between the two homologous
alleles of an individual. Reproduction was either sexual (panmictic)
or clonal. Classical population
genetics parameters, such as gene diversity or variance of allele size,
were followed over (generally)
5000 generations for various sets of values of the mutation rate and
strength of selection, in either
clonally or sexually reproducing populations. The reproduction system had
little influence on
genetic parameters, either under neutral conditions or when selection
acted on large allele size.
Selection against difference in allele size strongly constrained
variability in panmictic populations,
whereas a limited influence was observed in clonal populations.
Selection against the difference in
allele size between the two alleles of an individual is an alternative
explanation for the long life
expectancy of microsatellite loci in sexual species. Whether this
selection process actually occurs
can therefore be tested by comparing the allele size distribution
of microsatellite loci between
regions/genomes exhibiting markedly different recombination rates.