5 GENERATIONS OF ENFORCED SELFING AND OUTCROSSING IN MIMULUS-GUTTATUS- INBREEDING DEPRESSION VARIATION AT THE POPULATION AND FAMILY LEVEL

The focus of this study was to examine the consequences of five sequen tial generations of enforced selfing and outcrossing in two annual pop ulations of the mixed-mating Mimulus guttatus. Our primary goal was to determine whether purging of deleterious recessive alleles occurs uni formly between populations and among families, and thus gain insights into the mode of gene action (dominance, overdominance, and/or epistas is) governing the expression of inbreeding depression at both the popu lation and family levels across the life cycle. Inbreeding depression was detected across the five-generation breeding program in both popul ations for germination success, total flower production and adult abov eground biomass. No inbreeding depression was detected for date of fir st flowering. The serial breeding program minimized selection and may have allowed the random fixation of deleterious recessive alleles. Thu s, at the population level this experiment is consistent with (1) domi nance with weak selection on partially deleterious recessive alleles; (2) random fixation of deleterious alleles; and (3) overdominance beca use we observed a steady state of performance following inbreeding and outcrossing in these two populations of M. guttatus. At the family le vel, however, significantly different maternal line responses (materna l line x pollination treatment and maternal line x pollination treatme nt x generation) provide a mechanism for the invasion of a selfing var iant into the population through any maternal line exhibiting purging of its genetic load. These family level differences in inbreeding depr ession across the five generations suggest that dominance rather than overdominance underlies the expression of inbreeding depression. No ev idence of epistasis was detected at the population level. A significan t interaction occurred, however, between the inbreeding coefficient, f (2), and maternal families for all traits examined suggesting that epi stasis could be playing a role in the expression of inbreeding depress ion among maternal lines.