Five generations of enforced selfing and outcrossing in Mimulus guttatus: Inbreeding depression variation at the population and family level

Abstract

The focus of this study was to examine the consequences of five sequential generations of enforced selfing and outcrossing in two annual populations of the mixed-mating Mimulus guttatus. Our primary goal was to determine whether purging of deleterious recessive alleles occurs uniformly between populations and among families, and thus gain insights into the mode of gene action (dominance, overdominance, and/or epistasis) governing the expression of inbreeding depression at both the population and family levels across the life cycle. Inbreeding depression was detected across the five-generation breeding program in both populations for germination success, total flower production and adult aboveground biomass. No inbreeding depression was detected for date of first flowering. The serial breeding program minimized selection and may have allowed the random fixation of deleterious recessive alleles. Thus, at the population level this experiment is consistent with (1) dominance with weak selection on partially deleterious recessive alleles; (2) random fixation of deleterious alleles; and (3) overdominance because we observed a steady state of performance following inbreeding and outcrossing in these two populations of M. guttatus. At the family level, however, significantly different maternal line responses (maternal line x pollination treatment and maternal line x pollination treatment x generation) provide a mechanism for the invasion of a selfing variant into the population through any maternal line exhibiting purging of its genetic load. These family level differences in inbreeding depression across the five generations suggest that dominance rather than overdominance underlies the expression of inbreeding depression. No evidence of epistasis was detected at the population level. A significant interaction occurred, however, between the inbreeding coefficient, f2 and maternal families for all traits examined suggesting that epistasis could be playing a role in the expression of inbreeding depression among maternal lines.