Selective seed abortion induced by nectar robbing in the selfing plant Comastoma pulmonarium

Self-pollination often provides plants with the benefit of reproductive assurance; thus, it is generally assumed that species' interactions that alter floral attractiveness or rewards, such as nectar robbing, will have little effect on the seed production of selfing species. We challenge this view with experimental data from Comastoma pulmonarium, a selfing annual experiencing a high ratio of nectar robbing in the Qinghai-Tibet Plateau. We manipulated robbing (robbed or netted) and pollination mode (hand-selfed or hand-outcrossed) in a factorial design and measured the number of developing ovules and mature seeds, together with seed weight and seed germination, in each treatment. Robbing decreased the number of mature seeds, but not the number of developing ovules, suggesting a negative influence of robbers through indirect effects via selective seed abortion. We found no evidence for early-acting inbreeding depression, but found later-acting inbreeding depression. Our data also suggested that later-acting inbreeding depression of progeny from robbed flowers could be reduced in comparison with that from unrobbed flowers. We suggest that nectar robbing can have both negative and positive effects on the quantity and quality, respectively, of progeny produced in selfing plants, and challenge the view that robbing has no effect on selfing species.