EVOLUTION OF THE MAGNITUDE AND TIMING OF INBREEDING DEPRESSION IN PLANTS

Estimates of inbreeding depression obtained from the literature were u sed to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decre ase with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreed ing. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germina tion, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mat ing system also were collected and used as a measure of inbreeding his tory. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sa mple of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms bur not gymnosperms examined separatel y. The average inbreeding depression in predominantly selfing species (delta = 0.23) was significantly less (43%) than that in predominantly outcrossing species (delta = 0.53). These results support the theoret ical prediction that selfing reduces the magnitude of inbreeding depre ssion. Most self-fertilizing species expressed the majority of their i nbreeding depression late in the life cycle, at the stage of growth/re production (14 of 18 species), whereas outcrossing species expressed m uch of their inbreeding depression either early, at seed production (1 7 of 40 species), or late (19 species). For species with four life sta ges examined, selfing and outcrossing species differed in the magnitud e of inbreeding depression at the stage of seed production (selfing de lta = 0.05, N = 11; outcrossing delta = 0.32, N = 31), germination (se lfing delta = 0.02, outcrossing delta = 0.12), and survival to reprodu ction (selfing delta = 0.04, outcrossing delta = 0.15), but not at gro wth and reproduction (selfing delta = 0.21, outcrossing delta = 0.27); inbreeding depression in selfers relative to outcrossers increased fr om early to late life stages. These results support the hypothesis tha t most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is ve ry difficult to purge, even under extreme inbreeding.