Monogenic inheritance of apomixis in two Hieracium species with distinct developmental mechanisms

Apomixis, the asexual formation of seed, has been known in angiosperms for more than a century yet the genetic mechanisms that control this trait rema in poorly understood. Most members of the genus Hieracium are apomicts, for ming predominantly asexual seed. Some purely sexual forms, however, also ex ist. In this paper we present a study of the inheritance of apomixis using two apomictic species of Hieracium which utilize very different forms of me gagametogenesis. In both cases the progeny inherited apomixis as a monogeni c, dominant trait that could be transferred by both haploid and diploid mal e gametes. In common with observations from other systems, no diploid apomi ctic progeny were recovered from these crosses. This appears to have been c aused by selection against the survival of diploid zygotes, rather than aga inst the mediation of haploid gametes as has been noted in other systems. C rosses between the two apomicts showed that the dominant determinants in th e two forms examined were closely linked, possibly allelic. The significanc e of these data is discussed with respect to current theories on the associ ative link between gametophytic apomixis and polyploidy.