THE EFFECT OF INBREEDING IN DIPLOID AND TETRAPLOID POPULATIONS OFEPILOBIUM ANGUSTIFOLIUM(ONAGRACEAE): IMPLICATIONS FOR THE GENETIC BASIS OF INBREEDING DEPRESSION

The partial dominance model for the evolution of inbreeding depression predicts that tetraploids should exhibit less inbreeding depression than their diploid progenitors. We tested this prediction by comparing the magnitude of inbreeding depression in tetraploid and diploid populations of the herbaceous perennial Epilobium angustifolium (Onagraceae). Inbreeding depression was estimated in the greenhouse for three tetraploid and two diploid populations at four life stages. The mating system of a tetraploid population was estimated and compared to a previous estimate for diploids. Tetraploids showed less inbreeding depression than diploids at all life history stages, and these differences were significant for seed-set and cumulative fitness, but not for germination, survival, or plant dry mass at nine weeks. This result suggests that the genetic basis of inbreeding depression may differ among life stages. The primary selfing rate of the tetraploid population was r = 0.43, which is nearly identical to that of a diploid population (r = 0.45), indicating that differences in inbreeding depression between diploids and tetraploids are probably not due to differences in the mating system. Cumulative inbreeding depression, calculated from the four life history stages, was significantly higher for diploids ( \stackrel{?}{x}=0.95 ) than for tetraploids ( \stackrel{?}{x}=0.67 ), supporting the partial dominance model of inbreeding depression.