Another theoretical source of haploid plants in angiosperms is the female egg nucleus or ovum; this is contained within the nucellus of an ovule in a specialised cell (the megaspore or embryo sac). The ovum cannot be separated readily from other associated nuclei in the megaspore and so haploid plants can normally be produced from it, only by stimulating the development of unfertilised ovules into seedlings. In some species [e.g. Gerbera jamesonii (Sitbon, 1981; Meynet and Sibi, 1984); maize (Truong-Andre and Demarly, 1984); sugar beet (Hosemans and Bossoutrot, 1983); onion (Keller, 1990)], some haploid plants can be obtained by culturing unpollinated ovules, ovaries or flower buds. In some other plants (Pavlova, 1986), larger numbers of haploids are obtained if ovaries are pollinated by a distantly-related species (or genus) or with pollen which has been irradiated with X- or y-rays. Successful pollination results in stimulation of endosperm growth by fusion of one of the generative nuclei of the pollen tube with the central fusion nucleus of the megaspore, but fusion of the other generative nucleus with the egg cell does not occur and the egg cell is induced to grow into a seedling without being fertilised (gynogenesis). An alternative technique, which has resulted in haploid Petunia seedlings (Raquin, 1986) is to treat ovaries with y-rays and then pollinate them with normal pollen. Gynogenesis has so far been employed much less frequently than androgenesis for the production of haploids. A review of progress in this area has been provided by Yang and Zhou (1990).

Haploid cells and haploid plants produced by androgenesis or gynogenesis have many uses in plant breeding and genetics (Vasil and Nitsch, 1975). Most recent research on anther culture has concentrated on trying to improve the efficiency of plantlet regeneration in economically important species. Haploid plants of cereals are particularly valuable in breeding programmes, but in the Gramineae, the frequency and reliability of recovery through anther culture is still too low for routine use.

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