Apomixis is not developmentally conserved in related, genetically characterized Hieracium plants of varying ploidy

Apomixis is facultative in characterized members of the genus Hieracium. Th e three components that comprise the apomictic mechanism include apospory f ollowed by autonomous embryo and endosperm formation. The time of aposporou s embryo sac initiation and mode of embryo sac formation are different in H ieracium piloselloides (D3) and Hieracium aurantiacum (A3.4). Genetic studi es have shown that a single dominant locus encodes all three components of apomixis in both species (Bicknell ct al. 2000). We histologically examined a range of related, genetically characterized apomictic Hieracium plants d erived from D3 and A3.4 to assess conservation of the apomictic mechanism i n different genetic backgrounds. The plants varied in ploidy, and also in t he amount of DNA introduced from sexual Hieracium pilosella (P4). An apomic tic hybrid from a cross between the two apomicts was also examined. The dev elopmental processes observed in the parental apomicts were not conserved i n the examined plants and alterations occurred in the components of apomixi s. One plant also exhibited adventitious embryony. The results show that ot her genetic factors can modify apomixis with respect to time of initiation, spatial location, and mode of developmental progression. Both the apomicti c locus and the modifiers are essential for efficient penetrance of the tra it in Hieracium. Some of the findings in Hieracium correspond with observat ions in Ranunculus and this is discussed in terms of models for apomictic d evelopment and the control of apomixis in crops.