Successful transfer of apomixis leads to heritable semi‐sterility in experimental apomictic hybrids of <i>Hieracium</i> s.str. (Asteraceae)

Abstract Apomixis, the asexual reproduction through seeds, has repeatedly evolved in angiosperms, but its developmental pathways and inheritance are still incompletely understood. Here, we investigated the potential for infectious apomixis via pollen transfer in Hieracium , the largest apomictic genus with nearly obligate diplosporous apomixis and autonomous endosperm development. We tested whether pollen from apomictic polyploids can transfer apomixis to sexual diploids and assessed the fertility and reproductive mode of the resulting hybrids and their progeny. We performed controlled reciprocal crosses between sexual diploid H. alpinum and apomictic tetraploid H. nigrescens , followed by back‐crosses and reciprocal F 2 crosses. Flow cytometry was used to determine the ploidy of seedlings and of both embryo and endosperm in seeds to infer reproductive pathways. Gene flow was unidirectional from apomicts to sexuals. Crosses yielded rare triploid F 1 hybrids (~7% of the progeny), morphologically intermediate between the parents. F 1 triploids reproduced exclusively by apomixis but showed low and variable seed set (~30%, semi‐sterility). The same pattern was inherited in the F 2 progeny arising by apomixis from the F 1 . We provide the first experimental evidence of infectious transfer of apomixis in Hieracium s.str., leading to the formation of morphologically distinct, exclusively apomictic and likely reproductively isolated triploid lineage. However, as these apomicts were semi‐sterile, their long‐term evolutionary significance is uncertain. Further research should focus on the incidence of infectious apomixis and semi‐sterile apomicts in natural populations as these may represent an early stage in the evolution of new apomictic lineages with more stable expression of apomixis.