The degree to which inbreeding purges populations of their genetic load is hotly debated. While often assumed to be efficient, the extent and speed of purging actually depends on many population and genetic parameters. Although inbreeding does expose deleterious recessive mutations to selection, drift acting in small populations also acts to fix the genetic load. We used a fast-cycling variety of the self-incompatible outcrosser, Brassica rapa, to investigate how the genetic load and consequent levels of inbreeding depression vary in response to various histories of inbreeding and population bottlenecks. We obtained 5910 self- and cross-fertilized progeny from parents subject to 1-5 generations of purging. Population sizes were 1 (serial forced selfing), 4, 20, and 100 before selection. For all N>1 populations, we also applied outcrossing alone or in alternation with selfing as well as threshold selection to allow purging. Initial levels of inbreeding depression were high and serial selfing brought accelerating declines in fitness, suggesting synergistic epistasis among deleterious mutations. Surprisingly, outcrossed progeny from crosses between the selfing lines (but not outcrossed progeny from the large N outcrossed populations) also showed large fitness declines over successive generations. Such systematic declines in outcrossed progeny fitness could lead to apparent declines in inbreeding depression even while populations are fixing load mutations. Here, however, selfed fitnesses decline faster than outcrossed fitnesses over successive generations of selfing, causing a rise in inbreeding depression. These results suggest that: 1) Purging in small populations is highly constrained, and 2) The load can increase as well as decrease in small inbred populations, directly affecting demographic performance. Those studying inbreeding depression should examine absolute rather than relative measures of fitness as population-level inbreeding reduces differences between self- and cross-fertilized progeny.

Key words: Brassica rapa, genetic load, inbreeding, purging