Spatially and temporally varying selection on intra-population QTL for a life history tradeoff in Mimulus guttatus

Abstract

Why do populations remain genetically variable despite strong continuous natural selection? Mutation reconstitutes variation eliminated by selection and genetic drift, but theoretical and experimental studies each suggest that mutation-selection balance insufficient to explain extant genetic variation in most complex traits. The alternative hypothesis of balancing selection, wherein selection maintains genetic variation, is an aggregate of multiple mechanisms (spatial and temporal heterogeneity in selection, frequency-dependent selection, antagonistic pleiotropy, etc.). Most of these mechanisms have been demonstrated for Mendelian traits, but there is little comparable data for loci affecting quantitative characters. Here, we report a three-year field study of selection on intra-population Quantitative Trait Loci (QTL) of flower size, a highly polygenic trait of Mimulus guttatus. The QTL exhibit antagonistic pleiotropy: alleles that increase flower size reduce viability but increase fecundity. The magnitude and direction of selection fluctuates yearly and on a spatial scale of meters. This study provides direct evidence of balancing selection mechanisms on QTL of an ecologically relevant trait.