Apomixis: Embryo Sacs and Embryos Formed without Meiosis or Fertilization in Ovules.

Abstract

The term apomixis has, in the past, been used as a general term for any form of asexual reproduction in plants, including vegetative propagation. This original definition has become more restricted and now covers only those asexual reproductive processes that, paradoxically, occur in the ovule of flowering plants-the structure that has evolved to carry out female sexual reproductive functions in angiosperms (Nogler, 1984; Asker and Jerling, 1992). Apomictic processes mimic many of the events of sexual reproduction and give rise to fertile seeds. An important difference is that the apomictic embryo is derived solely from cells in the maternal ovule tissues rather than from the fusion of male and female gametes. The fertile seeds that result from apomictic reproduction contain embryos that have, barring mutation, a genetic constitution identical to that of the female parent. At least three developmental differences also serve to distinguish apomictic embryo formation from somatic embryogenesis. First, apomictic embryo formation occurs within a differentiated structure. Second, apomictic embryos form directly from a cell located in, or close to, a gametophytic structure, without entering an intervening callus phase, which is often necessary for somatic embryogenesis (Nomura and Komamine, 1985; see also Zimmerman, 1993, this issue). Third, the pattern of embryo formation in apomictic species is often indistinguishable from that which occurs in the nearest sexual relative (Nogler, 1984), which is not always the case for somatic embryogenesis. Apomictic processes have been ObServed in at least 300 plant species spanning 35 different families and are most common in the Gramineae, Compositae, Rosaceae, and Rutaceae (Richards, 1986; Hanna and Bashaw, 1987). With the exception of apple and Citrus, apomixis is not very common in agriculturally important crops. Because apomictically produced embryos are genetically identical to the female parent plant, they are of obvious benefit to agriculture. If apomixis can be introduced into agricultural crops, it can be an inexpensive way to perpetuate a given genotype, preserving even such characters as heterosis through successive generations via seed (Hanna and Bashaw, 1987). Apomictic plants could potentially provide a constant source of renewable seed capable of producing high-yielding food crops, an agricultural trait of great value, particularly for developing countries. Programs aimed at using conventional breeding methods to transfer apomixis to most agricultural crops have been largely unsuccessful, although pearl millet has been an exception (Dujardin and Hanna, 1989). The continuation of such organized breeding programs is important, and a greater understanding of the processes controlling apomixis at the molecular level should facilitate the transfer of apomixis into crops of economic importance. In this review, the different forms of apomixis that occur in ovules will be discussed and compared at a developmental level with sexual reproductive processes. Current mechanistic and genetic knowledge about apomictic processes will be presented. An understanding of apomixis at the molecular level should result in the characterization of genes that control female gametophyte formation and the early events of embryo development in both apomictic and sexually reproducing plants.