Sexual Hieracium pilosella plants are better inter-specific, while apomictic plants are better intra-specific competitors

Apomixis, asexual reproduction through seeds, occurs in over 40 plant families. This widespread phenomenon can lead to the fixation of successful genotypes, resulting in a fitness advantage. On the other hand, apomicts are expected to lose their fitness advantage if the environment changes because of their limited evolutionary potential, which is due to low genetic variability and the potential accumulation of deleterious somatic mutations. Nonetheless, some apomicts have been extremely successful, for example certain apomictic accessions of Hieracium pilosella L. from New Zealand, where the plant is invasive. Here, we investigate whether the success of these apomictic accessions could be due to a fitness advantage by comparing the vegetative competitiveness of apomictic H. pilosella from New Zealand with sexual accessions of H. pilosella from Europe. Sexual and apomictic plants were grown either (A) alone (no competition), (B) in competition with the other type (intra-specific competition), (C) in competition with the grass Bromus erectus (inter-specific competition), and (D) in competition with the other type and the grass B. erectus (intra- and inter-specific competition). To distinguish effects of apomixis and the region of origin, different H. pilosella lineages were compared. Furthermore, experiments were carried out to investigate effects of the ploidy level. We show that sexual plants are better inter-specific competitors than apomicts in terms of vegetative reproduction (number of stolons) and vegetative spread (stolon length), while apomicts do better than sexuals in intra-specific competition. The magnitude of the effect was in some cases dependent on the ploidy levels of the plants. Furthermore, apomicts always produced more stolons than sexuals, suggesting potential displacement of sexuals by apomicts where they co-occur.