Background: What is Apomixis?
In higher plants, embryogenesis and seed development of all sexual species is the result of a double fertilization process. One sperm cell fertilizes the egg cell from which the embryo develops, while a second sperm cell fertilizes the central cell, that forms the triploid endosperm.
The (different) genomes of both egg and sperm are combined leading to progeny plants with new geno- and phenotypes. These are desired traits during plant breeding to generate new genotypes better adapted to biotic and abiotic environmental factors.
On the other hand, once optimal plant varieties are available, recombination can also be a big disadvantage, especially if elite hybrid genotypes should be maintained.
Asexual reproduction through seeds (apomixis, see also Fig. 1) is the alternative leading to the formation of embryos genetically identical to the mother plant. Because of its economic potential, apomixis research is now receiveing increasing attention from both scientific and industrial sectors (Vielle-Calzada et al., 1996a).
Apomictic processes have been observed in many plant species and are most common in the Gramineae, Compositae and Rosaceae. But with the exception of Citrus, Malus and some forage grasses like Poa and Panicum, apomixis is not very common in agriculturally important crops (Koltunow, 1993).
During sexual megagametophytic development (with the embryo sac as the megagametophyte and the egg cell as megagamete), a sub-epidermal cell of the nucellus tissue differentiates into a megaspore-mother cell which undergoes meiosis I and II to form four megaspores. During the Polygonum type of megasporogenesis and megagametogenesis, which is the most commonly observed type of embryo sac development, three megaspores degenerate and the functional megaspore undergoers three mitotic divisions resulting in an eight celled embryo sac containing the haploid egg cell (for review see Reiser and Fischer, 1993 and Fig. 1). This is the normal way of reproduction of most economically important crop species.
Many species, especially from the Gramineae-family reproduce asexually without fertilization of the egg cell.
Three different forms of this process (apomictic or asexual reproduction) have been described as diplospory, apospory and adventitious embryogenesis (Fig. 1). Apomictic processes can be initiated at several points during gametophytic development. During the two forms of diplospory (mitotic and meiotic), the megaspore-mother cell does not enter meiosis or is arrested at an early stage during meiosis and undergoes only mitotic divisions without a reduction of the genome. In both forms, an embryo sac is formed consisting of an unreduced, diploid egg cell (Leblanc et al.,
۱۹۹۵a). In apospory, aposporous initial cells form from nucellar cells and differentiate after three mitotic divisions into embryo sacs containing a diploid egg cell that directly develops into an embryo without fertilization. In some species fertilization of the central cell might be necessary to develop a fertile seed. Adventitious embryogenesis initiates from the somatic tissues of the ovule, nucellar or integumental cells. These cells develop directly into embryos and compete with the sexual embryo that is formed after fertilization (for review see Koltunow, 1993).
Nearly nothing is known about the genetic regulation of apomixis. Influence of environmental factors have been observed when sexual and asexual reproduction can occur simultaneously. With a few exceptions it has been observed that apomictic species are polyploid, while the sexual varieties of the same species are diploid.
New strategies and methods are now in progress to compare sexual and apomictic varieties of grass species, like e.g. Poa, Paspalum and Brachiaria, and to map the corresponding genes.
Mutants were identified leading to apomictic pathways of reproduction in species that normally reproduce sexually. A fertilization independent endosperm mutation (fie) and a group of fertilization independent seed (fis1-3) mutations have been described recently in Arabidopsis (Ohad et al., 1996; Chaudhury et al., 1997).
Molecular tools have been developed to compare sexual and apomictic ovaries in Pennisetum (Vielle-Calzada et al., 1996b). During the comparison of sexual and parthenogenetic wheat lines one protein (a-Tubulin) was identified that is specifically expressed only in the ovaries of the parthenogenetic lines (Matzk et al., 1997).
In apomictic Tripsacum RFLP and PCR-RAPD markers co-segregating with diplospory have been mapped to the same locus (Leblanc et al., 1995b; Kindinger et al., 1996) and methods are now available to transfer single specific chromosomes between sexually incompatible plants (Ramulu et al., 1996) .
Using novel breeding strategies, combined with novel genetic, molecular and cytological methods it is now possible to attempt to understand the genetic regulation of sexual and asexual ways of reproduction for utilizing apomixis in agriculture and crop improvement of the 21st century.
Fig. 1: Comparison of sexual and apomictic reproduction pathways in angiosperm ovules. Differences are shown between sexual (amphimictic) and apomictic (apospory, diplospory and adventitious embryony) reproduction (modified and supplemented after Koltunow et al. 1995).
Chaudhury, A.M., Ming, L., Miller, C., Craig, S., Dennis, E.S. and Peacock, W.J. (1997) Fertilization-independent seed development in Arabidopsis thaliana. Proc. Natl. Acad. Sci. USA 94: 4223-4228.
Kindinger, B., Bai, D. and Sokolov, V. (1996) Assignment of a gene(s) conferring apomixis in Tripsacum to a chromosome arm: cytological and molecular evidence. Genome 39: 1133-1141.
Koltunow, A.M. (1993) Apomixis: Embryo sacs and embryos formed without meiosis or fertilization in ovules. Plant Cell 5: 1425-1437.
Koltunow, A.M., Bicknell, R.A. and Chaudhury, A.-M. (1995) Apomixis: Molecular strategies for the generation of genetically identical seeds without fertilization. Plant Physiol. 108: 1345-1352.
Leblanc, O., Peel, M.D., Carman, J.G. and Savidan, Y. (1995a) Megasporogenesis and mega-gametogenesis in several Tripsacum species (Poaceae). Amer. J. Bot. 82: 57-63.
Leblanc, O., Grimanelli, D., González-de-León, D. and Savidan, Y. (1995b) Detection of the apomictic mode of reproduction in maize-Tripsacum hybrids using maize RFLP markers. Theor. Appl. Genet. 90: 1198-1203.
Matzk, F., Meyer, H.M., Horstmann, C., Balzer, H.J., Bäumlein, H. and Schubert, I. (1997) A specific alpha-tubulin is associated with the initiation of parthenogenesis in ‘Salmon’ wheat lines. Hereditas 126: 219-224.
Ohad, N., Margossian, L., Hsu, Y.-C., Williams, C., Repetti, P. and Fischer, R.L. (1996) A mutation that allows endosperm development without fertilization. Proc. Natl. Acad. Sci. USA ۹۳: ۵۳۱۹-۵۳۲۴٫
Ramulu, K.S., Dijkhuis, P., Rutgers, E., Blaas, J., Krens, F.A., Verbeek, W.H.J., Colijn-Hooymans and Verhoeven, H.A. (۱۹۹۶) Intergeneric transfer of a partial genome and direct production of monosomic addition plants by microprotoplast fusion. Theor. Appl. Genet. 92: 316-325.
Reiser, L. and Fischer, R.L. (1993) The ovule and the embryo sac. Plant Cell 5: 1291-1301.
Vielle-Calzada, J.P., Crane, C.F. and Stelly, D.M. (1996a) Apomixis: The asexual revolution. Science 274: 1322-1323.
Vielle-Calzada, J.-P., Nuccio, M.L., Budiman, M.A., Thomas, T.L., Burson, B.L., Hussey, M.A. and Wing, R.A. (1996b) Comparative gene expression in sexual and apomictic ovaries of Pennisetum ciliare (L.) Link. Plant Mol. Biol. 32: 1085-1092.