INBREEDING DEPRESSION IN 2 MIMULUS TAXA MEASURED BY MULTIGENERATIONALCHANGES IN THE INBREEDING COEFFICIENT

In mixed-mating plant populations. one can estimate the relative fitne ss of selfed progeny w by measuring the inbreeding coefficient F and s elfing rate s of adults of one generation, together with F of adults i n the following generation (after selection). In the first application of this multigenerational method, we estimated F and s for adults ove r three consecutive generations in adjacent populations of two annual Mimulus taxa: the outbreeding M. guttatus and the inbreeding M. platyc alyx. This gave estimates of w for the last two generations. Although average multilocus selfing rates were high in both taxa (0.63 in M. gu ttatus; 0.84 in M. platycalyx), the relative fitness of selfed progeny averaged only 0.19 in M. guttatus and 0.32 in M. platycalyx. An alter native estimator for w that incorporates biparental inbreeding gave ev en lower estimates of w. These values are significantly below the 0.5 threshold thought to favor selfing, and show that partially selfing po pulations can harbor substantial genetic load. In accordance with the purging hypothesis, the more highly selfing M. platycalyx showed margi nally lower inbreeding depression than M. guttatus in both years (P = 0.08). Inbreeding depression and selfing rates also varied among years in concert among taxa. Several sources of bias are discussed, but com puter simulations indicate it is unlikely that w is biased downwards b y linkage of marker loci to load loci.