Natural populations of Mimulus guttatus usually exhibit moderate levels of inbreeding, with selfing rates of 10%-30%. When inbreeding depression is present, the inbreeding coefficient F is reduced as selection acts. Such changes of F, as monitored by genetic markers, can be used as a non-manipulative method to infer the strength of selection against inbred individuals in natural populations. For this, changes of F should ideally be measured across the entire life cycle (e.g., parent-seedling-parent), but if inbreeding equilibrium is assumed, only parent F and selfing rate is required. Another approach for obtaining in-situ estimates of inbreeding depression is to regress fitness (fecundity) on F, using estimates of individual inbreeding coefficients and an estimate of the actual variance of F among individuals. The slope of the regression is directly related to the strength of inbreeding depression. Also, from this regression one can calculate the "inbreeding load" of the population, or the reduction in population mean fitness due the expression of inbreeding depression. A multigenerational study comparing M. guttatus with the more highly selfing M. platycalyx found inbreeding depression to be substantial in nature, and its level higher in M. guttatus. The presence of inbreeding load M. guttatus is also documented. The variety of mating systems, morphologies and life histories in Mimulus makes this genus an ideal model system for studies in evolution and population genetics.

Key words: genetic markers, inbreeding depression, Mimulus guttatus, monkeyflower