Authors: Jitendra Kumar Meena and Rahul Kumar
1Division of genetics 2Division of Vegetable Science,
Indian Agricultural Research Institute, Pusa, New Delhi-110012
Apomixes is a form of asexual reproduction that occurs via seeds in which embryo’s develop without fertilization .It is observed in more than 300 species in 30 out of 460 angiosperm families, but it occurs most frequently in Compositae, Rosaceae and Graminae family of plant kingdom. It was considered as an obstacle for plant breeding. Indeed, the presence of apomixis in the species Hieracium, in which Mendel was asked to prove the genetic principles he had developed in peas, caused him to withdraw from the scientific world due to his failure to do so . In recent years, however, apomixis is seen as a way to maintain superior genotypes as clones of seeds. As such, apomixis has been a major area of investigation in plant genetics. Genetically apomixes involves the following two development changes from the normal reproduction. First one is a suitable modification /failure of meiosis during megasporogenesis and another one isinitiation of mitosis in the unfertilized 2n egg cell or some other 2n cell of embrosac.Gametophytic apomixes is controlled by the dominant gene or a few closely linked genes on the same chromosome.apospory is due to dominant gene in ranunculasspecies.nuclear adventive in citrus is controlled by single dominant gene. In Panicum maximum dominant manofactorial control of apomixes reported by Savidan.
Applications of Apomixes:
- Fixation of heterosis–obligate apomixes results in complete fixation of heterosis. If apomixes is facultative, heterosis can be fixed only partly e.g.,Bahia grass( P.notatum) and Buffle grass( P.ciliare).
Sexual reproducing line (male parent) x Apomictic line (female parent)
Maintained and multiplied through seeds
- Production of homozygous line-some types of apomixes involve parthanogenetic development of reduced egg cells followed by development of embroys from secondary diploid cells of the embrosac derived through fusions. e.g., Rubus and Sorgum.
- Production of vybrids- a vybrid is the progeny obtained from a cross between two facultative apomictics; the vybrids itself reproduce through facultative apomixes and is maintained by harvesting the seeds of only F1-like apomictic plants in every generation.e.g.,Sorghum.
- Advetiveembryony for uniform root stock and virus free scion in Citrus and Merion variety in Poapratensis.
- Adventitious embryony: it is the sporophytic form of agamospermy in which embryo develops directly from of vegetative cells of the ovule such as nucellus, integument and chalaza.e.g. Mango,Citrus Orchid etc.
- Gametophytic apomixes: in this, embryos develop without fertilization from egg cell or other cells of embryosacs. The embrosacs are produced from unreduced gametophyte.
- Apospory – the megaspore mother cell usually degenerates and unreduced embryosac is formed from a somatic cell like that of nucleus or integument cell in the ovule by a series of mitotic divisions.e.g.,Grass family.
- Diplospory – the unreduced embrosac are formed from megaspore mother cells by circumvention of meiosis.
Advantages of Apomixes
- Obligate apomixes permits fixation of heterosis in hybrids .Therefore farmers can resow the seeds produced by apomictic hybrids.
- The new hybrids variety could be multiplied from few hybrid seeds ina manner as a pureline.
- Apomixes provides the mean for the commercial production of F1 hybrids in crops where CMS/fertility restoration system is not available.
Disadvantages of Apomixes
1. Tedious and time consuming especially in case of facultative apomixis.
2. In case of facultative apomixes the proportion of sexual progeny is affected by environment factors.
3. The genetic base of apomixes is not clear in most of cases.
4. In the absence of morphological markers linked with apomictic development, maintenance of apomictic stocks become difficult.
Techniques to screen Apomixis
1. Cytological analysis of embroysacdevelopment.
2. Callose fluorescence in combination with clearing of ovules can be used to detect diplospory.
3. Uniformity of progenies from cross pollinated parents is the best indication. Also other screening methods are maternal phenotype among the progeny of F1 crosses, limited or no genetic variation in F2 progeny, high speed fertility in aneuploidy, triploid and wide crosses and multiple seedling per seed.
4. DNA marker analysis .For example in Penisetum two molecular markers are used to screen apomictic lines.
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