The effect of inbreeding and population hybridization on developmental instability in petals and leaves of the rare plant Silene diclinis (Caryophyllaceae)

Studies of fluctuating asymmetry (FA), a measure of developmental instability (DI), may provide insights into the importance of genetic factors in the long-term survival of small and isolated populations. In the present study of the rare, endemic plant Silene diclinis, I tested how moderate inbreeding within populations (full-sib crosses) and hybridization between populations influenced levels of developmental instability of petals and leaves, using plants derived from controlled crosses and raised under uniform growth conditions. The area and length of petals and leaves were digitized and measured with an image analysis system, but only bifid petals could be tested for true fluctuating asymmetry (normally distributed left-minus-right values with a mean of zero). Based on a bootstrap procedure, I found little evidence for directional asymmetry and antisymmetry in the area and length of the two lobes of the petals. Only the length measurements showed a significant leptokurtic distribution, which may reflect limited resolution (too few classes) in the image analysis system. Randomization tests were performed to test for differences between crossing treatments. Levels of FA of petal area and petal length were significantly higher for both the inbred and the interpopulation hybrid progenies relative to offspring from crosses between unrelated plants from the same population (control). There was no significant treatment effect on DI of leaves. Comparison of plants in the control group revealed that DI of leaf area was significantly higher than FA of petal area, and that these parameters were uncorrelated. This study demonstrates that FA of petals in Silene diclinis is sensitive to moderate levels of inbreeding and outbreeding, and therefore might serve as an indicator of genetic stress.