LIFETIME ESTIMATES OF BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT RAPHANUS-SATIVUS

Studies of inbreeding depression in plant populations have focused pri marily on comparisons of selfing versus outcrossing in self-compatible species. Here we examine the effect of five naturally occurring level s of inbreeding (f ranging from 0 to 0.25 by pedigree) on components o f lifetime fitness in a field population of the self-incompatible annu al. Raphanus sativus. Pre- and postgermination survival and reproducti ve success were examined for offspring resulting from compatible cross -pollinations. Multiple linear regression of inbreeding level on rates of fruit and seed abortion as well as seed weight and total seed weig ht per fruit were not significant. Inbreeding level was not found to a ffect seed germination, offspring survival in the field, date of first flowering, or plant biomass (dry weight minus fruit). The effect of i nbreeding on seedling viability in the greenhouse and viability to flo wering was significant but small and inconsistently correlated with in breeding level. Maternal fecundity, however, a measure of seed yield, was reduced almost 60% in offspring from full-sib crosses (f = 0.25) r elative to offspring resulting from experimental outcross pollinations (f = 0). Water availability, a form of physiological stress, affected plant biomass but did not affect maternal fecundity, nor did it inter act with inbreeding level to influence these characters. The delayed e xpression of strong inbreeding depression suggests that highly deleter ious recessive alleles were not a primary cause of fitness loss with i nbreeding. Highly deleterious recessives may have been purged by bottl enecks in population size associated with the introduction of Raphanus and its recent range expansions. In general, reductions in total rela tive fitness of greater than 50% associated with full-sib crosses shou ld be sufficient to prohibit the evolution of self-compatibility via t ransmission advantage in Raphanus.