Density- and frequency-dependent inbreeding depression in the Australian annual 'Hibiscus trionum' var. 'vesicarius'

Question: Does the magnitude of inbreeding depression fluctuate in response to the density and frequency of inbred and outbred plants in a neighbourhood? Hypothesis: Inbreeding depression is greater when plant density is high and when selfed progeny compete with crossed progeny. Organism: The self-pollinating annual plant 'Hibiscus trionum' var. 'vesicarius'. Methods: We estimated inbreeding depression by comparing selfed and crossed progeny from eight maternal families. We grew plants in monocultures at two densities (1 or 4 plants per pot), and in a replacement series (4 plants per pot) in which the frequencies of the progeny types were manipulated. Results: Contrary to expectations, in monocultures inbreeding depression was less at high than at low density. This occurred because the reduction in fitness between low and high densities was less for selfed progeny than for crossed progeny. In the replacement series, inbreeding depression increased with increasing frequency of crossed progeny. Thus, inbreeding depression cannot be characterized independently of the density and frequency of inbred and outbred plants in a population.