Protoplasma (1999) 208:196-205 PROTOPLASMA 9 Springer-Verlag 1999 Printed in Austria Apomixis for crop improvement K. S. Ramulu*, V. K. Sharma, T. N. Naumova, P. Dijkhuis, and M. M. van Lookercn Campagne Center for Plant Breeding and Reproduction Research, Agricultural Research Department, Wageningen Received November 6, 1998 Accepted March 29, 1999 Summary. Apomixis is a genetically controlled reproductive process by which embryos and seeds develop in the ovule without female meiosis and egg cell fertilization. Apomixis produces seed progeny that are exact replicas of the mother plant. The major advantage of apomixis over sexual reproduction is the possibility to select individuals with desirable gene combinations and to propa- gate them as clones. In contrast to clonal propagation through somatic embryogenesis or in vitro shoot multiplication, apomixis avoids the need for costly processes, such as the production of artificial seeds and tissue culture. It simplifies the processes of com- mercial hybrid and cultivar production and enables a large-scale seed production economically in both seed- and vegetatively prop- agated crops. In vegetatively reproduced plants (e.g., potato), the main applications of apomixis are the avoidance of phytosanitary threats and the spanning of unfavorable seasons. Because of its potential for crop improvement and global agricultural production, apomixis is now receiving increasing attention from both scientific and industrial sectors. Harnessing apomixis is a major goal in applied plant genetic engineering. In this regard, efforts are focused on genetic and breeding strategies in various plant species, com- bined with molecular methods to analyze apomictic and sexual modes of reproduction and to identify key regulatory genes and mechanisms underlying these processes. Also, investigations on the components of apomixis, i.e., apomeiosis, parthenogenesis, and endosperm development without fertilization, genetic screens for apomictic mutants and transgenic approaches to modify sexual reproduction by using various regulatory genes are receiving a major effort. These can open new avenues for the transfer of the apomixis trait to important crop species and will have far-reaching potentials in crop improvement regarding agricultural production and the quality of the products. Keywords: Asexual reproduction; Apomixis; Meiotic mutants; Apomeiotic mutants; Genetic screening; Fertilization-independent seed mutants. * Correspondence and reprints: Centrum voor Plantenveredelings- en Reproduktieonderzoek, Dienst Landbouwkundig Onderzoek, Postbus 16, 6700 AA Wageningen, The Netherlands. Introduction In angiosperms, two pathways of reproduction through seed exist: sexual, or amphimictic, and asexual, or apomictic. In the sexual pathway of reproduction, meiosis and double fertilization are the two important processes allowing genetic recombination, segrega- tion, and the maintenance of ploidy level. Apomixis deviates from sexual reproduction in several aspects: modification or complete omission of meiosis, for- mation of unreduced megaspores, parthenogenetic development of embryo, and autonomous, or pseudo- gamous, endosperm development (Hanna 1991, Sybenga 1992, Willemse and Naumova 1992, Koltunow et al. 1995, Ramulu et al. 1998). Apomixis produces embryos or seed progeny that are exact genetic repli- cas of the mother plant. Apomixis is currently receiv- ing increasing attention from both scientific and industrial sectors (Jefferson and Bicknell 1996, Vielle Calzada et al. 1996, Grossniklaus et al. 1998a, Maheshwari et al. 1998). This is due to recent scientific advances and a broader awareness of the large poten- tial that apomixis may have on crop improvement and agricultural production. The main advantage of apomixis over sexual reproduction is the possibility to select individual plants with superior characteristics and to propagate them clonally through seeds. When apomixis is engineered into the crops in a controlled way, its impact on agriculture will be sweep- ing and profound. It will allow clonal seed production and enable the efficient and consistent production of high-quality seeds, fruits, and vegetables at lower costs. The use of apomixis in cultivar development will enhance genetic diversity; each apomictic plant from sexual x apomictic cross is potentially a unique cul-