Apomixis

 

The normal sexual cycle called amphimixis involves two important processes: a) Meiosis - through which a diploid sporophytic cell divides and forms four haploid gametophytic cell, 2) Fertilization - in which two haploid gametes of opposite sex fuse and form a zygote re-establishing the diploid sporophytic generation.
This, in a sexual cycle a diploid generation sporophyte alternates with a haploid generation gametophyte in angiosperms. The gametophytic generations are very short and are represented by embryo sac on the female side and microspore pollen grain on the male side. The remaining part of the cycle belongs to the sporophyte generation.

Plants where the usual reproduction has been completely replaced by a type of asexual reproduction are called apomictic and the phenomena is known as apomixis.

 Following Winkler, who introduced the term apomixis may be defined as the substitution of the usual sexual reproduction by a form of reproduction with does not involve meiosis and syngamy.  A species may include sexually as well as a apomitically reproducing individuals.

There are two main categories of apomixis: 1. Vegetative propagation and 2. Agamospermy

Agamospermy: The phenomenon in which the plants have retained seeds as the agent of propagation but the embryo is formed by some process in which normal meiosis and syngamy have been eliminated.

Three different types of agamospermy are recognised: a. Adventive embryony, b. Diplospory, 3. Apospory.

Adventive Embryony: in this type of agamospermy, the embryos arise from diploid sporophytic cells of the ovule lying outside the embryo sac, either from the nucellus or integuments.
In this the gametophytic generation is completely eliminated. The sexual embryo sac develops in normal way and the zygotic embryo either degenerates or competes with the apomictic embryos.
The embryo that is formed shows true morphological features i.e., presence of cotyledons, radicle, plumule, epicotyl and hypocotyl.
A favourite example of adventive Embryony is that of Citrus, in which 3-5 embryos are common. Also occurs in Cactaceae, Euphorbiaceae, Orchidaceae.

Diplospory:
The phenomenon where a diploid embryo sac is formed from a megaspore mother cell without regular meiotic division is called as diplospory.
In this type an archesporium differentiates, but the megaspore mother cell develops into an unreduced embryo sac. Eg., Areva tomentosa, Ixeris

Apospory:
Apospory was first reported by Rosenberg in the Hieracium spp.
A somatic cell in the nucellus directly forms an unreduced embryo sac, and the diploid egg parthenogenetically develops into an embryo.
The MMC goes through the usual meiotic division, but just about this stage a somatic cell situated in the chalazal region begins to enlarge and become vacuolated.
This cell gradually increases in volume, encroaching upon the megaspores and finally crushing them.
Eg., grasses, Crepis, Cliftomia, Malus, Ranunculus.

Parthenogeneis:

Diplospory as well as apospory produce diploid embryosacs. Now, to complete the apomitic cycle without altering the chromosome number of the sporophyte generation, the diploid egg must develop into a embryo without the participation of the male nucleus.

Formation of embryo from an unfertilised egg is called parthenogenesis.

In autonomous apomitics (Compositae and Rubiaceae) the development of embryo is independent of the pollination stimulus.

Howerver, in many apomitic species, the embryo develops only after pollination; the phenomenon is known as pseudogamy, eg., grasses.

Heslop-Harrison has suggested three possible roles of pollination in pseudogamy;

i.  To activate the growth of ovary and ovule,

ii. To supply the male nucleus for endosperm development, and

iii.   To stimulate parthenogenesis.

Importance of Apomixis:

Offers the possibility of indefinite multiplication of favourable biotypes without any variation due to segregation or recombination.