GENETIC-ANALYSIS OF INBREEDING DEPRESSION CAUSED BY CHLOROPHYLL-DEFICIENT LETHALS IN MIMULUS-GUTTATUS

Two Oregon populations of the partially self-fertilizing Mimulus guttatus, located on Iron Mountain and Cone Peak, were surveyed for the frequency of recessive chlorophyll-deficient lethals by selfing plants collected as seed from the wild. Allelism tests were used to determine the number of different loci with lethal alleles present in the carriers isolated in the population surveys. The frequency of carriers was 0.065 for the Iron Mountain population (23/356) and 0.024 for the Cone Peak population (8/327). Allelism tests of the 31 carriers isolated from both populations revealed the existence of 26 different independently acting loci and two different duplicate locus systems with lethal alleles. These results indicate that the component of inbreeding depression caused by chlorophyll-deficient lethals is not due to single locus heterozygote advantage and is probably caused by mutation-selection balance at many loci. Estimates of the genomic and per locus mutation rates for this class of lethals are in close agreement with those obtained in studies of chlorophyll-deficient lethals in several agricultural plant species. Genetic analysis of a California population of M. guttatus, reported in the literature to exhibit an unusually high per locus mutation rate and segregation distortion for a chlorophyll-deficient lethal, revealed that chlorophyll-deficiency is inherited as a duplicate gene system. This mode of inheritance, not recognized previously, can explain the appearance of unusually high mutation rates and segregation distortion.