Plant Propogation and Nursery Management

Apomixis

In some species of plants, an embryo develops from the diploid cells of the seed and not as a result of fertilization between ovule and pollen. This type of reproduction is known as apomixis and the seedlings produced in this manner are known as apomicts.

• Apomictic seedlings are identical to mother plant and similar to plants raised by other vegetative means, because such plants have the same genetic make-up as that of the mother plant.
• Such seedlings are completely free from viruses. Plants that produces only apomictic embryo and are known as obligate apomicts and those which produce both apomictic and sexual seedlings are called facultative apomicts.



Plate 4.1 Apomixis in black berry

Types of Apomixis: Maheshwari (1950) classified apomixis into four groups:

1. Recurrent Apomixis: In this type of apomixis, the embryo develops from the diploid egg cell or from the diploid cells of the embryo sac without fertilization. As a result, the egg has normal diploid number of chromosomes, just like the mother plant (Fig.4.1).The species, where recurrent apomixis commonly occur are, Parthenium, Rubus, Malus, Allium , Rudbeckia, Poa, Taraxacum ,etc.



Fig.4.1: Recurrent apomixis

2. Non-Recurrent Apomixis: In this case, the embryo develops either from the haploid egg cell or from some other haploid cells of the embryo sac. In this case, haploid plants are produced, which contain only one set of chromosome of the mother plant. Hence, the haploid plants are sterile in nature and cannot be normally perpetuated into the next generation. Non-recurrent apomixis occurs only in a few species such as Solanum nigrum, Lilium spp. etc.
3. Nucellar Embryony or Adventitious Embryony: In this type of apomixes, the embryos arise from diploid sporophytic cells outside the embryo sac i.e. cells of the nucellus, integuments etc. This type of apomixis is quite common in citrus and certain varieties of mango, where fertilization occurs normally and sexual plus a number of apomictic (nucellar) embryos develop(Fig.4.2).



Fig.4.2: Nucellar Embryony or Adventitious Embryony

4. Vegetative apomixis or bulbils: In some species of plants, such as Allium , Agave, Poa etc., the flowers in an inflorescence are replaced by bulbils or vegetative buds, which sprout, while still on the mother plant and turn into new daughter plants (Fig.4.3).



Fig.4.3: Vegetative apomixis or bulbils

Advantages

• Assured reproduction in the absence of pollinators, such as in extreme environments
• Maternal energy not wasted in unfit offspring (cost of meiosis)
• Some apomictic plants (but not all) avoid the male energy cost of producing pollen
Disadvantages

• Can't control accumulation of deleterious genetic mutations
• Usually restricted to narrow ecological niches lack ability to adapt to changing environments