A weak effect of background selection on trinucleotide microsatellites in maize.

Artificial selection during the domestication of maize is thought to have been predominantly positive and to have had little effect on the surrounding neutral diversity because linkage disequilibrium breaks down rapidly when physical distance increases. However, the degree to which indirect selection has shaped neutral diversity in the maize genome during domestication remains unclear. In this study, we investigate the relationship between local recombination rate and neutral polymorphism in maize and in teosinte using both sequence and microsatellite data. To quantify diversity, we estimate 3 parameters expected to differentially reflect the effects of indirect selection and mutation. We find no general correlation between diversity and recombination, indicating that indirect selection has had no genome-wide impact on maize diversity. However, we detect a weak correlation between heterozygosity and recombination for trinucleotide microsatellites deviating from the stepwise mutation model and located within genes (rho = 0.32, P < 0.03). This result can be explained by a background selection hypothesis. The fact that the same correlation is not confirmed for nucleotide diversity suggests that the strength of purifying selection at or near this class of microsatellites is higher than for nucleotide mutations.