Sexual reproduction in field populations of the facultative apomict, Hieracium pilosella

Hand pollination under New Zealand field conditions of capitula from each o f six populations of Hieracium pilosella (maternal parent) with the closely related but morphologically distinct H. aurantiacum produced F-1 hybrids i n every case. The frequency of F-1 hybrid offspring among the populations r anged from 0.2 to 21.69/0. Involucral bract characteristics, leaf shape, an d petal colour were the most useful characters for identifying hybrid proge ny. Most F-1 hybrids were able to set seed by apomixis, but a sexual indivi dual was also recovered. The discovery that both tetraploid and pentaploid H. pilosella can reproduce sexually, following artificial pollination under field conditions, in New Zealand has implications for the bio-control of t his species. Sexually reproducing plants are more likely to evolve resistan ce to host-specific bio-control agents than are apomicts. These results are part of a larger, ongoing research programme investigating the role of the environment in the expression of apomixis.