Hypostatic modifiers cause variation in degree of copper tolerance in Mimulus guttatus

Previous research into copper tolerance in the monkey flower Mimulus guttatus has found that: (i) it is primarily determined by a single dominant gene; and (ii) there is variation between tolerant plants that has been ascribed to ‘modifiers’. Modifiers can be either nonspecific, which act additively on both tolerant (T) and nontolerant (NT) genotypes, or specific, which act only on the tolerant genotype, and are thus hypostatic to the tolerance locus. We show here that there are hypostatic modifiers of tolerance in this species. Two selection lines that differ in degree of tolerance (and thus in the presence of putative modifiers) were crossed to a single NT plant. The F 1 s were selfed to produce F 2 s that segregated 3:1 T:NT. NT F 2 individuals were crossed to a single homozygous T plant of low tolerance. The families differed in tolerance, showing that the NT F 2 individuals differed in genes that only have an effect on tolerance phenotype in the presence of the tolerance gene. F 3 individuals from F 2 s of contrasting phenotype were crossed to a second tester tolerant plant, and these F 3 s also varied, confirming the presence of the specific modifiers. There was no evidence of the segregation that would suggest a single modifier gene, but there is evidence from the F 2 s of at least one additive, nonspecific modifier in addition to the specific modifiers.