Understanding apomixis: recent advances and remaining conundrums.

It has been 10 years since the last review on apomixis, or asexual seed formation, in this journal (Koltunow, 1993). In that article, emphasis was given to the commonalties known among apomictic processes relative to the events of sexual reproduction. The inheritance of apomixis had been established in some species, and molecular mapping studies had been initiated. The molecular relationships between apomictic and sexual reproduction, however, were completely unknown. With research progress in both sexual and apomictic systems in the intervening years, subsequent reviews on apomixis in the literature have considered the economic advantages of providing apomixis to developing and developed agricultural economies (Hanna, 1995; Savidan, 2000a) and strategies to gain an understanding of apomixis by comparison with sexual systems (Koltunow et al., 1995a; Grimanelli et al., 2001a). The potential to ‘‘synthesize apomixis’’ in agricultural crops in which it is currently absent by modifying steps in sexual reproduction and the possible ecological consequences of the release of ‘‘synthesized apomicts’’ in nature also have been discussed (van Dijk and van Damme, 2000; Grossniklaus, 2001; Spillane et al., 2001). Recently, comparative developmental features of apomixis have been considered in light of the now considerable knowledge accumulated about ovule and female gametophyte development, and seed formation in sexual plants (Koltunow and Grossniklaus, 2003). In this review, we focus on the initiation and progression of apomixis in plants that naturally express the trait. Since 1993, there has been a growing understanding of the complexity that underlies apomixis; some contentious issues have been resolved and others raised. There also have been significant advances in terms of new model systems and approaches being used to study apomixis. We structure the wider discussion around the knowledge of apomixis we have accumulated from our study of Hieracium species, or hawkweeds, a model system we established, and consider additional factors that should be taken into account to induce apomixis in crops. The continued comparative analyses of apomictic and sexual reproduction at the fundamental level in appropriate model systems remains essential for the development of successful strategies for the greater application and manipulation of apomixis in agriculture.