differentiate between hybrid and apomictic seeds
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Interspecific hybrids result from crossing two related, but different, plant species. ... Intraspecific hybrids are the result of crossing two different varieties of the same species. The term "hybrid" is also used to signify intraspecific hybrids obtained from crossing two inbred lines of the same crop.
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Who will benefit from apomixis?
By
Ross A. Bicknell and Katie B. Bicknell
Keywords: Apomixis, Technology transfer, Hybridization, Sterile seeds.
Correct citation: Bicknell, R.A. and Bicknell, K.B. (1999), "Who Will Benefit from Apomixis?" . Biotechnology and Development Monitor, No. 37, p. 17-20.
The potential benefits of clonal plant reproduction by apomixis are numerous. Farmers could profit from apomictic hybrid varieties that proliferate their superior characteristics and may make the regular purchase of new hybrid seed obsolete. Commercial plant breeders and National Agricultural Research Systems (NARS) could employ apomixis to economize variety development. The distribution of benefits, however, will depend on the parties controlling the use of this technology.
For agricultural purposes, both sexual and asexual crop reproduction have their specific limitations (see box). It would clearly be valuable if the advantages of clonal uniformity could be combined with the cost effectiveness and utility of seed propagation. In a small number of plant species this happens naturally through a process termed apomixis. In these plants, an embryo forms without the fertilization of an egg cell by a pollen cell. As the viable seeds are produced asexually, they are genetically identical to each other and to the mother plant, leading to the formation of clonal populations. There are many different apomictic mechanisms that have been observed in different plant species. Some plants reproduce by ‘autonomous apomixis’ where the asexual formation of seed takes place without a requirement for pollination. Other plants continue to require either cross- or self-pollination to stimulate seed formation and/or to ensure the development of the nutritive embryo sac (‘pseudogamous apomixis’). Apomixis and sexual reproduction are not mutually exclusive, and in facultative apomictics both mechanisms of reproduction occur in the same plant.
Natural apomicts include plants like the common dandelion (Taraxacum species) and the silvery cinquefoil (Potentilla argentea). Few crops, however, are apomictic. Of those that are, the majority are either tropical fruit trees, such as citrus and mango, or forage species, such as Poa pratensis and Brachiaria decumbens.
Plant reproduction: Sexual versus asexual
Plants are either reproduced sexually by seed, or by some method of asexual propagation (cloning). In sexual reproduction, male and female gametes, the pollen and the egg cell, are produced separately with half the normal chromosome number. Their combination during fertilization gives rise to the development of a seed that carries a unique combination of the genes derived from both parents. It is this recombination that causes variability in a sexually propagated population, as expressed in characteristics such as plant height, vigour, seed size and nutritional composition. Seeds are physiologically robust, naturally primed for growth and adapted for field emergence. Sexual reproduction and genetic uniqueness appear to have provided most species with evolutionary advantages. In agriculture, however, it can be often regarded as undesirable, since it causes variation that can negatively effect production practices and the quality of the harvested and processed product.
Asexual reproduction, in contrast, provides the advantages of absolute crop uniformity. The genetic make-up of the parents is identical to the progeny, so a single desirable plant can become the basis of a new variety. The efforts essential for sexually propagated plants to ‘fix’ characteristics to ensure ‘true breeding’ are therefore unnecessary. Consequently, cloning makes the development of new varieties more time and cost effective. Many economically important fruiting plants, such as date palms and grapevines, have been propagated by vegetative means for hundreds, sometimes even thousands of years. Similarly, many root and bulb crops are cloned by natural means, such as cassava, potato and garlic. More recently, technologies such as tissue culture and cutting propagation have greatly expanded the number of species that can be cloned routinely. Despite these opportunities, however, cloning is currently only economic for plants that either use a convenient natural mechanism or that have a high unit value. Absent from this list are crops such as maize, rice, wheat, millet, sorghum, most pulse species, and the majority of economically important forage, fibre and timber species.