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

Abstract

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 nectar 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 outcrossing, whereas a breeding system experiment established that E. cyparissias can self, but sets more seeds when outcrossed. We used insect exclusion experiments to test whether ants fertilise the rust fungi and to determine whether ants are pollinators of E. cyparissias, These experiments showed that insect pollination is necessary for seed set and that ants can pollinate the flowers. However, ants do not fertilise the rust fungi.