Inbreeding depression in Solanum carolinense (Solanaceae), a species with a plastic self-incompatibility response

BACKGROUND: Solanum carolinense (horsenettle) is a highly successful weed with a gametophytic self-incompatibility (SI) system. Previous studies reveal that the strength of SI in S. carolinense is a plastic trait, associated with particular S-alleles. The importance of this variation in self-fertility on the ability of horsenettle to found and establish new populations will depend, to a large extent, on the magnitude of inbreeding depression. We performed a series of greenhouse and field experiments to determine the magnitude of inbreeding depression in S. carolinense, whether inbreeding depression varies by family, and whether the estimates of inbreeding depression vary under field and greenhouse conditions. We performed a series of controlled self- and cross-pollinations on 16 genets collected from a large population in Pennsylvania to obtain progeny with different levels of inbreeding. We grew the selfed and outcrossed progeny in the greenhouse and under field conditions and recorded various measures of growth and reproductive output. RESULTS: In the greenhouse study we found (1) a reduction in flower, fruit and seed production per fruit in inbred (selfed) progeny when compared to outbred (outcrossed) progeny; (2) a reduction in growth of resprouts obtained from rhizome cuttings of selfed progeny; and (3) an increase in the ability to self-fertilize in the selfed progeny. In the field, we found that (1) outcrossed progeny produced more leaves than their selfed siblings; (2) herbivory seems to add little to inbreeding depression; and (3) outcrossed plants grew faster and were able to set more fruits than selfed plants. CONCLUSION: Solanum carolinense experiences low levels of inbreeding depression under greenhouse conditions and slightly more inbreeding depression under our field conditions. The combined effects of low levels of inbreeding depression and plasticity in the strength of SI suggest that the production of selfed progeny may play an important role in the establishment of new populations of S. carolinense.