THE INBREEDING DEPRESSION COST OF SELFING: IMPORTANCE OF FLOWER SIZE AND POPULATION SIZE IN COLLINSIA PARVIFLORA (VERONICACEAE)

Inbreeding depression should evolve with selting rate when frequent inbreeding results in exposure of and selection against deleterious alleles. The selting rate may be modified by plant traits such as flower size. or by population characteristics such as census size that can affect the probability of biparental inbreeding. Here we quantify inbreeding depression(delta) among different populatio sizes of Collinsia parviflora, a wildflower with interpopulation variation in flower size, by comparing fitness components and multiplicative fitness of experimentally produced selfed and outcrossed offspring. Selfed offspring had reduced multiplicative fitness compared to outcrossed offspring but inbreeding depression was low in all combinations of population size and flower size (delta <= 0.05) except in large populations of large-flowered plant (delta=0.45). The decrement to multiplicative fitness with inbreeding was not affected by population size nested within flower size but differed between small- and large-flowered plants: small-flowered populations had lower overall inbreeding depression (delta=0.45). The decrement to multiplicative fitness with inbreeding was not affected by population size nested within flower size, but differed between small- and large-flowered plants: small-flowered populations had lower overall inbreeding depression (delta=0.04) compared to large-flowered population (delta=0.25). The difference in load with flower size suggests that either selection has removed deleterious recessive alleles or these alleles have become fixed in small-flowered, potentially more selfing population, but that purging has not occured to the same extent in presumably outcrossing large-flowered populations.