Authors: *Kana Ram Kumawat, Madhu Choudhary and Ravi Kumawat
Department of Plant Breeding and Genetics, S.K.N. Agriculture University
Jobner-303329, Jaipur, Rajasthan, India
*Email: kanaramkumawat8@gmail.com
Apomixis refers to the occurrence of a sexual reproductive process in the place of normal sexual processes involving reduction division and fertilization. In other words apomixis is a type of reproduction in which sexual organs of related structures take part but seeds are formed without union of gametes. Seeds formed in this way are vegetative in origin.
When apomixis is the only method of reproduction in a plant species, it is known as obligate apomixis. On the other hand, if gametic and apomictic reproduction occurs in the same plant, it is known as facultative apomixis.
Apomixis is widely distributed among higher plants. More than 300 species belonging to 35 families are apomictic. It is most common in Gramineae, Compositae, Rosaceae and Rutaceae. Among the major cereals maize, wheat and pearl millet have apomictic relatives. Here, a brief account of apomixis, is furnished only from the point of view of breeding.
Types of apomixis>
1. Recurrent Apomixis:
An embryo sac develops from the megaspore mother cell where meiosis is disturbed or from some adjoining cell. Consequently, the egg-cell is diploid. The embryo subsequently develops directly from the diploid egg-cell without fertilization. Somatic apospory, diploid parthenogenesis and diploid apogamy are recurrent apomixis. However, diploid parthenogenesis / apogamy occur only in aposporic (somatic) embryo-sacs. Therefore, it is the somatic or diploid aposory that constitutes the recurrent apomixis. Such apomixis occurs in some species of Crepis, Taraxacum, Paa (blue grass) andAllium (onion) without the stimulus of pollination. Malus (apple), and Rudbeckia where pollination appears to be necessary, either to stimulate embryo development or to produce a viable endosperm.
2. Non -recurrent Apomixis:
An embryo arises directly from normal egg-cell (n) without fertilization. Since an egg cell is haploid, the resulting embryo will also be haploid. Haploid parthenogenesis and haploid apogamy, and androgamy fall in this category. Such types of apomixis are of rare occurrence. They do not perpetuate and are primarily of genetic interest as in com.
3. Adventive Embryony:
Embryos arise from a cell or a group of cells either in the nucellus or in the integuments, e.g. in oranges and roses. Since it takes place outside the embryo sac, it is not grouped with recurrent apomixis, though this is regenerated with the accuracy. In addition to such embryos, regular embryo within the embryo sac may also develop simultaneously, thus giving rise to poly-embryony condition, as in Citrus and Opuntia.
4. Vegetative apomixis:
In some cases like Poa bulbosa and some Allium, Agave and grass species, vegetative buds or bulbils, instead of flowers are produced in the inflorescence. They can also be reproduced without difficulty. However, Russian workers do not group this type of vegetative reproduction with apomixis. Now, different apomictic phenomena in each of the recurrent and non-recurrent apomicts are considered in relation to the development of the embryo sac or embryo.
Advantages of apomixis in plant breeding
The two sexual processes, self-and cross fertilization, followed by segregation, tend to alter the genetic composition of plants reproduced through amphimixis. Inbreeding and uncontrolled out breeding also tend to break heterozygote superiority in such plants. On the contrary, apomicts tend to conserve the genetic structure of their carriers. They are also capable of maintaining heterozygote advantages generation after generation. Therefore, such a mechanism might offer a great advantage in plant breeding where genetic uniformity maintained over generation for both homozygosity and heterozygosity is the choicest goal. Additionally, apomixis may also affect an efficient exploitation of maternal influence, if any, reflecting in the resultant progenies, early or delayed because it causes the perpetuation of only maternal individuals and maternal properties due to prohibition of fertilization. Maternal effects are most common in horticultural crops, particularly fruit trees and ornamental plants. Thus, in short the benefits of apomixis, insofar as their utility in plant breeding is concerned, are:
- - Rapid multiplication of genetically uniform individuals can be achieved without risk of segregation.
- - Heterosis or hybrid vigour can permanently be fixed in crop plants, thus no problem for recurring seed production of F1 hybrids.
- -Efficient exploitation of maternal effect, if present, is possible from generation to generation.
References:
1. Singh, B.D. (2015) Plant Breeding Principles and Methods. 10th Revised Edition, Kalyani publishers, New Delhi.
2. Internet (IASRI)
About Author / Additional Info:
Post-Graduated in subject of Plant Breeding and Genetics from SKNAU, Jobner