Apomixis - Vegetative/asexual propagation

Apomixis

In certain plants, meiosis and syngamy are interrupted and still a suitable embryo is formed inside the confines of the seed coat.

Such asexual seeds that produce progeny identical to the female parent are called Apomictic seeds and the phenomenon is called Apomixis. (Apo = away from + mixis = act of mixing or mingling).

At the turn of the last century, it becomes known that certain male-sterile biotype of Taraxacum and Hieracium set ample seeds even when no pollen plants were growing far and wide, suggesting their apomictic nature. This was experimentally confirmed simply by cutting off the upper half of the unopened flower buds with a razor so that, the anthers, stigma, and most of the corolla were removed. Even after this brutal emasculation, seed set and seed germination were as good as from intact and amply pollinated heads and the plants derived from such seeds were genetically identical to the respective mother plant. The offspring produced by apomixis are genetically identical to the mother plant. Along these lines, if apomixis can be presented in crop plants, it would give a cheap method to propagate a given genotype saving such characters as heterosis through progressive seed generations. Apomixis would also simplify commercial hybrid seed production because isolation would not be necessary to produce F, or maintain and multiply parental lines.

However, apomixis is not very usual in the major crop plants. Inspired by the potential benefits of apomictic reproduction, plant breeders are making serious attempts to introduce this transformation.

Objectives:

• Fixation of Heterosis
• Fixation of Heterozygous state
• Rapid Production of Homozygous Lines
• Production of the phenotypically stable population called Vybrids

Types of apomixis:

    1. Recurrent apomixis

In this type of apomixis, the embryo sac develops from the diploid egg mother cell or from some other diploid cells of the embryo sac without fertilization. Thus, the egg has the typical diploid number of chromosomes, equivalent to in the mother plant. The embryo subsequently develops from an egg nucleus without fertilization. This series of events is known to occur in some species of Crepis, Poa Taraxacum, and Allium without the stimulus of pollination, while in others, like Parthenium, Rubus, Malus (apple) the stimulus of pollination appears to be necessary for the developing embryo or to produce a viable endosperm

    2. Non-recurrent apomixis

In this type of apomixis, the embryo grows directly from the haploid egg cell (haploid parthenogenesis) or some other haploid cells of the embryo sac (haploid apogamy) without fertilization, and subsequently, the embryo developed is also haploid. This type of apomixis occurs very rarely and is primarily of genetic interest. The plants produced are haploid and sterile. It is common in Solanum nigrum and Lilium species etc.

    3. Adventitious Embryony or Nucellar Embryony

Adventitious embryony is also called as nucellar embryony or nucellar budding. In this type, the embryo does not develop from the cells of the embryo sac but it develops from a cell or a group of cells either of nucellus or integuments. The cells of integument or nucellus are diploid and hence the resultant embryo is also diploid. Such embryos usually develop outside the embryo sac in addition to the regular embryo. Adventitious embryony occurs in many plant species but is most prominent in tropical and sub-tropical tree species, like citrus and mango.

    4. Vegetative apomixis or bulbils

In this type, vegetative buds or bulbils are produced in the inflorescence in place of flowers. These buds or bulbils may sprout into new plants while they are still attached to the mother plants. This type of apomixis is quite common in Allum, Agave, Poa, and Dioscorea and in some grasses.