SEXUAL AND APOMICTIC DEVELOPMENT IN HIERACIUM

Most members of the genus Hieracium are apomictic and set seed without fertilization, but sexual forms also exist. A cytological study was c onducted on an apomictic accession of H. aurantiacum (A3.4) and also H . piloselloides (D3) to precisely define the cellular basis for apomix is. The apomictic events were compared with the sexual events in a sel f-incompatible isolate of H. pilosella (P4). All plants were maintaine d as vegetatively propagated lines each derived from a single plant. S exual P4 exhibited characteristic events of polygonum-type embryo sac formation, showed no latent apomitic tendencies, and depended upon fer tilization to set seed. In contrast, D3 and A3.4 were autonomous aposp orous apomicts, forming both embryo and endosperm spontaneously inside an unreduced embryo sac. The two apomicts exhibited distinct mechanis ms, but variation was also observed within each apomictic line. Seeds from apomicts often contained more than one embryo. A degree of develo pmental instability was also observed amongst germinated seedlings and included variation in meristem and cotyledon number, altered phyllota xis, callus formation, and seedling fusion. In most cases abnormal see dlings developed into normal plants. Such phenomena were not observed following germination of hybrid seeds derived from crosses between sex ual P4 and the apomictic plants. The three plants can now be used in i nheritance studies and also to investigate the molecular mechanisms co ntrolling apomixis.