EVIDENCE FOR THE PARTIAL DOMINANCE OF VIABILITY GENES CONTRIBUTING TOINBREEDING DEPRESSION IN MIMULUS-GUTTATUS

The relative importance of different modes of gene expression of viabi lity genes contributing to inbreeding depression was investigated in t he wild plant, Mimulus guttatus. Viability genes were identified by se lf-fertilizing 31 outbred plants, each heterozygous for three to nine unlinked allozyme markers, and analyzing segregation ratios of selfed progeny at maturity for deviations from 1:2:1 ratios. In this study, 2 4 linkages of viability genes to marker loci were detected. To infer t he nature of gene action for these viability genes, a ''model-free'' g raphical method was developed that examines the ''space'' of segregati on ratios allowed by each of seven selection models (i.e., overdominan ce, complete recessivity, partial recessivity, additivity, partial dom inance, complete dominance and underdominance). Using this method, we found that, of 24 linkages detected, 18 were consistent with either pa rtial dominance, complete dominance or underdominance. Six were consis tent with either partial recessivity, complete recessivity or overdomi nance. This finding indicates that, in these chromosomal segments iden tified by allozyme markers, partial dominance plays the predominant ro le in inbreeding depression. This is inconsistent with either the domi nance or overdominance hypotheses proposed to account for inbreeding d epression.