In vitro haploidization of fruit trees

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Thirty years have passed since the pioneering work by Guha and Maheshwari (1964), who obtained haploid embryos from anther cultures of the solanaceous species Datura innoxia. Ten years later, Nitsch and Norreel (1973) described the first successful example of microspore culture with this same species. Since then, in vitro production of haploid plants has been extended to more than 250 species belonging to about 100 genera and over 40 families (Foroughi-Wehr and Wenzel, 1989). However, progress has been achieved only for a limited number of species, essentially confined to the Cruciferae, Gramineae, Ranunculaceae, and Solanaceae (Sangwan-Norreel et al.,1986). For most other species, including the rosaceous fruit crops, androgenesis has been feasible, but the androgenic rates have most often remained very low. There are other approaches to produce haploids. These include the spontaneous occurrence of haploids in angiosperms in vivo (Kimber and Riley, 1963), in vitro gynogenesis (San and Gelebart, 1986), and in situ parthenogenesis induced by irradiated pollen (Raquin, 1985). The induction of haploids via in vitro gynogenesis, first demonstrated for oats (San, 1976) and now extended to a score of species, permits the production of haploids for non-androgenic or male sterile genotypes (e.g., several Compositae such as gerberas, lettuce or sunflower) (San and Gelebart, 1986). The extention of in vitro gynogenesis to a larger range of species, however, is hampered by a lack of efficient strategies for the study of the embryo sac and its evolution (both in vitro and in situ). In addition, for many rosaceous fruit trees (Malus, Pyrus, Prunus), the small size of the embryo sac and the reduced number of female gametophytes constitute a further barrier to the successful exploitation of this approach for the production of haploid plants.