MARKER-BASED INFERENCES ABOUT EPISTASIS FOR GENES INFLUENCING INBREEDING DEPRESSION

We describe a multilocus, marker-based regression method for inferring interactions between genes controlling inbreeding depression in self- fertile organisms. It is based upon selfing a parent heterozygous for several unlinked codominant markers, then analyzing the fitness of pro geny marker genotypes. If loci causing inbreeding depression are linke d to marker loci, then viability selection is manifested by distorted segregation of markers, and fecundity selection by dependence of the f ecundity character upon the marker genotype. To characterize this sele ction, fitness is regressed on the proportion of loci homozygous for m arkers linked to deleterious alleles, and epistasis is detected by non linearity of the regression. Alternatively, fitness can be regressed o n the proportion of heterozygous loci. Other modes of selection can be incorporated with a bivariate regression involving both homozygote an d heterozygote marker genotypes. The advantage of this marker-based ap proach is that ''purging'' is minimized and specific chromosomal segme nts are identified; its disadvantage lies in low statistical power whe n linkage is not strong and/or the linkage phase between marker and se lected loci is uncertain. Using this method, in the wildflower Mimulus guttatus, we found predominant multiplicative gene interaction determ ining fecundity and some negative synergistic (nonmultiplicative) inte raction for viability.