Effects of ants on the reproductive success of Euphorbia cyparissias and associated pathogenic rust fungi

Ants are common visitors to the flowers of Euphorbia cyparissias, and also often forage on E. cyparissias stems that are infected by rust fungi of the species complex Uromyces pisi. These fungi sterilise their host, produce n ectar and require insects for their sexual reproduction. Our objective was to determine whether ant visits enhance the sexual reproduction of either E . cyparissias or the rust fungi. Uromyces pisi is known to be obligately ou tcrossing, whereas a breeding system experiment established that E. cyparis sias can self, but sets more seeds when outcrossed. We used insect exclusio n experiments to test whether ants fertilise the rust fungi and to determin e whether ants are pollinators of E. cyparissias. These experiments showed that insect pollination is necessary for seed set and that ants can pollina te the flowers. However, ants do not fertilise the rust fungi.