THE EFFECT OF INBREEDING IN DIPLOID AND TETRAPLOID POPULATIONS OF EPILOBIUM-ANGUSTIFOLIUM (ONAGRACEAE) - IMPLICATIONS FOR THE GENETIC-BASISOF INBREEDING DEPRESSION

The partial dominance model for the evolution of inbreeding depression predicts that tetraploids should exhibit less inbreeding depression t han their diploid progenitors. We tested this prediction by comparing the magnitude of inbreeding depression in tetraploid and diploid popul ations of the herbaceous perennial Epilobium angustifolium (Onagraceae ). Inbreeding depression was estimated in the greenhouse for three tet raploid and two diploid populations at four life stages. The mating sy stem of a tetraploid population was estimated and compared to a previo us estimate for diploids. Tetraploids showed less inbreeding depressio n than diploids at all life history stages, and these differences were significant for seed-set and cumulative fitness, but not for germinat ion, survival, or plant dry mass at nine weeks. This result suggests t hat the genetic basis of inbreeding depression may differ among life s tages. 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 dipl oids and tetraploids are probably not due to differences in the mating system. Cumulative inbreeding depression, calculated from the four li fe history stages, was significantly higher for diploids ((x) over bar = 0.95) than for tetraploids ((x) over bar = 0.67), supporting the pa rtial dominance model of inbreeding depression.