INBREEDING DEPRESSION IN 2 SPECIES OF MIMULUS (SCROPHULARIACEAE) WITHCONTRASTING MATING SYSTEMS

We examined the effect of self- and cross-pollination on germination s uccess, flowering probability, pollen and ovule production, survivorsh ip, and adult aboveground biomass in two species of Mimulus with contr asting mating systems: the highly selfing M. micranthus and an outcros sing population of M. guttatus. Cross-pollinations were performed both within and between populations in order to examine the scale at which the genetic load is distributed. We found significant inbreeding depr ession in M. guttatus in four of the six traits, with the highest inbr eeding depression observed in biomass (68% and 69% based on within- an d between-population crosses, respectively) and lowest in ovule produc tion (21% based on between-population crosses only). M. micranthus dis played significant inbreeding depression in only two of the six traits examined. Again, we observed the highest inbreeding depression in bio mass (47-60% based on within- and between-population crosses, respecti vely), but both traits showing significant differences between self an d outcross progeny expressed lower inbreeding depression than in M. gu ttatus. We detected no significant inbreeding depression for either po llen or ovule production in M. micranthus. An estimate of total inbree ding depression based on the multiplicative effects of all traits was also lower in M. micanthus than in M. guttatus. Our results are consis tent with the expected purging of generic load in populations with hig h selfing rates. The absence of inbreeding depression in M. micranthus pollen and ovule production, two traits with strong links to fitness in a selfing annual, further suggests the important role of directiona l selection in determining the population's genetic load. Comparison o f cross-pollinations made within and between populations revealed litt le evidence of divergence of genetic load among the M. micranthus and M. guttatus populations examined.