Inbreeding depression under intraspecific competition in a highly outcrossing population of Crepis sancta (Asteraceae): Evidence for frequency-dependent variation

Inbreeding depression is a major selective factor acting to maintain outcro ssing in hermaphroditic plants. Recently it has been shown that environment al conditions may greatly affect the levels of inbreeding depression. In th is study, the effects of intraspecific competition, from either crossed or inbred progeny, and plant density on the expression of inbreeding depressio n were estimated for the allogamous colonizing plant Crepis sancta (Asterac eae). The population used in this experiment showed a very high outcrossing rate (t = 0.99). Inbreeding depression was measured for germination, survi val to reproduction, number of capitula, and a multiplicative fitness estim ate. We found that inbreeding depression on survival, the number of capitul a, and total fitness was the greatest when inbred plants compete with outbr ed plants. The effect of density on inbreeding depression was less evident, The major implication of our study is that the relative fitness of selfed progeny is strongly influenced by the type of competitors (outbred or inbre d). These results support the hypothesis that inbreeding depression varies according to the density and frequency of outbred plants and suggest that i t could be considered as a density- and frequency-dependent phenomenon.