The effect of drought stress on inbreeding depression in four populations of the Mediterranean outcrossing plant Crepis sancta (Asteraceae)

The effect of physiological stress on the magnitude of inbreeding depression in plants has been the subject of few studies and is currently controversial because of contradictory results. We measured the inbreeding depression at three drought stress levels, precisely defined by a preliminary physiological experiment. We also tested the hypothesis that more highly self-compatible populations exhibit reduced inbreeding depression due to purging of deleterious mutations. The study was conducted on two populations of the annual and allogamous plant Crepis sancta collected from the French Mediterranean region and two other populations from marginal areas with various self-incompatibility levels. Drought stress did not increase inbreeding depression in terms of plant mortality but significantly increased the inbreeding depression for the date of first flowering, number of heads per plant and relative growth rate. The most self-fertile marginal population showed an absence of inbreeding depression in most of the measured traits indicating that purging could have taken place in this population. The three others populations showed relatively low and similar estimates of inbreeding depression (delta approximate to 0.35). The relatively low values obtained compared to the results found in allogamous plants suggests that the absence of competition for C. sancta in our experiment probably underestimated the effects of inbreeding in natural populations where competition occurs.