01 Reproduction in Lower and Higher Plants - part 11 - Apomixis ,Parthenocarpy and Polyembryony

Apomixis :

Phenomenon of formation of embryo(s) through asexual method of reproduction without formation of gametes and the act of fertilization.
Unusual sexual reproduction where there is no meiosis and syngamy.
Embryo develops in the ovule and ovule developes to form seed .
In apomixis, when a gametophyte organ or cell produces embryo like structure without fertilization, it is termed as apogamy.
Similarly when diploid sporophyte cell produces a diploid gametophyte without undergoing meiosis is called apospory,
e.g. Orange, Mango.

Main categories of apomixis are:

a. Recurrent apomixis :

In this type, the embryo sac generally rise either from anarchesporial cell or from some other part of the nucellus.
In diplospory, the unreduced embryo sac is derived from the diploid megaspore mother cell e.g. Taraxacum.
In apospory, the nucellar cells give rise to apomictic embryo sac.

b. Non-recurrent apomixis :

In this type, megaspore mother cell undergoes usual meotic division and a haploid embryo sac is formed.
Here, the embryo arises either from the egg by parthenogenesis or from some other haploid cells of gametophyte through apogamy.
Plants produced by this method are generally sterile and do not reproduce sexually, e.g. Nicotiana.

c. Adventive Embryony :

In this type, embryos may develop from somatic nucellus or integuments along with normal zygotic embryo.
It is common in Mango, Orange, Lemon, etc.
It gives rise to a condition called polyembryony.
Genetically identical plants can be produced effectively and rapidly by apomixis.

Parthenocarpy :

This term is coined by Noll (1902).
It is the condition in which fruit is developed without the process of fertilization.
It occurs naturally in some varities of Pineapple, Banana, Papaya,etc.
In these plants, it seems that the placental tissue in the unfertilized ovary produces auxin IAA (Indole-3 Acetic Acid) which is responsible for enlargement of ovary into fruit.
The fruit resembles the normally produced fruit but it is seedless.

Polyembryony :

It is the development of more than one embryos, inside the seed and the condition is described as polyembryony.
It was first noticed by Leeuwenhoek (1719) in the seeds of Citrus genus.
It is the occurrence of more than one embryo in a seed which consequently results in the emergence of multiple seedlings.
The additional embryos result from the differentiation and development of various maternal and zygotic tissues associated with the ovule of seed.
Polyembryony may be true or false depending upon whether many embryos arise in the same embryo sac or in different embryo sacs in the same ovule.
In adventive polyembryony, an embryo develop directly from the diploid cell of nucellus and integuments as in Citrus.
In cleavage polyembryony, zygote proembryo sometimes divides (cleaves) into many parts or units.
Each unit then developes into an embryo.
Polyembryony increases the chances of survival of the new plants.
Nucellar adventive polyembryony is of great significance in horticulture.

Parthenogenesis is the development of embryo directly from egg cell or a male gamete. It is a kind of apogamy.
Agamospermy : Here plants produce seeds. But embryo, inside it, is produced without (omitting) meiosis and syngamy.
Parthenocarpy can be induced artificially by - spraying of gibberellins, delaying pollination, use of foreign pollens, etc.
Genetically uniform parental type seedlings are obtained from nucellar embryos.

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