Apomixis and Polyembryony: Differences, Types, Significance

Apomixis and polyembryony are two different but related biological processes that result in the production of offspring without fertilization. Apomixis is a type of asexual reproduction where seeds are produced without gametic fusion. While polyembryony is a process in which multiple embryos are produced from a single fertilized egg. Both apomixis and polyembryony have important implications for plant breeding and agriculture. 

What is Apomixes?

The term apomixes were coined by Hans Winker, who was a famous botanist in 1908. Apomixes are derived from the two Greek words that are: apo + mixes, where “apo” means away from, and “mixes” means mixing. Apomixes can be defined as asexual reproduction in which fertilization doesn’t occur. However, still, it produces an embryo and seed, or it can be defined as the type of reproduction in plants in which there is no fusion of sexual gametes but still produces an embryo and seed. The most common example of Apomixis is apomictic parthenogenesis. In apomictic parthenogenesis, the new generation of offspring will be the full clone of the mother as it involves the direct development of the egg cells into an embryo without fertilization.

Types of Apomixis 

Apomixis can be classified based on the following criteria:

1. Cell involved
2. Occurrence
3. Frequency

Based on the cells involved, apomixes are of four types

1. Parthenogenesis: It is a type of apomixes in which the growth and development of an embryo take place in a gamete without the process of fusion of gametes.
2. Diplospory: In this type of apomixes, the embryo sac develops directly from the megaspore mother cell (also known as the archesporial cell) through mitosis and/or meiosis that has been terminated. 
3. Apospory: It is a type of apomixis in which the embryo sac develops from nucellar cells, and as a result, the gametophyte develops without the involvement of meiosis.
4. Adventive embryonic: It is also known as a nucellar embryo or sporophytic apomixis. In this type of apomixis, the embryo develops from a diploid sporophyte; without meiosis or the formation of a female gametophyte, there is no embryo sac to be seen.

Based on occurrence, apomixes are of two types

1. Recurrent apomixes: It can also be referred to as gametophytic apomixis. In recurrent apomixis, the embryo and egg cell are diploid, and the embryo sac develops from the megaspore mother cell. The chromosome number in the megagametophyte is equal to that of the mother plant, and this results from incomplete meiosis.
2. Non-Recurrent apomixes: In non-recurrent apomixis, the embryo grows without fertilization directly from the haploid egg cell (haploid parthenogenesis) or from other haploid embryo sac cells (haploid apogamy), and as a result, the embryo produced is similarly haploid.

Based on frequency, apomixes are of two types

1. Obligate apomixes: It is a type of apomixes in which there is the production of a new generation of offspring through the process of only apomixes. 
2. Facultative apomixes: It is a type of apomixes in which there is no assurance that a new generation of offspring will occur through apomixes or sexual reproduction. 

Importance of Apomixes

1. Apomixis aids in the quick multiplication of genetically identical individuals in plant breeding without taking the risk of segregation.
2. Apomixes create embryos without infections.
3. Apomixes cause the growth of polyploidy.

What is Polyembryony?

In the year 1719, Antonie van Leeuwenhoek first discovered this natural phenomenon. Polyembryony is the term used to describe the formation of multiple embryos within a single ovule, seed, or fertilized ovum. The embryos are genetically distinct from the parents but identical to one another since they develop from the same egg. Significant differences between polyembryony, budding, and sexual reproduction are the genetic diversity between the offspring and the parents and the similarity among siblings. Polyembryony is so abundant in some gymnosperms that it might be considered a key characteristic of this group.

Types of Polyembryony

There are two different kinds of polyembryony:

• Induced Polyembryony: This category contains situations when polyembryony was caused through an experiment.
• Spontaneous Polyembryony: This refers to all instances of polyembryony that develop spontaneously.

According to Webber (1940), there are three different forms of polyembryony.

1. Cleavage Polyembryony: Several embryos are created from a single fertilized egg.
2. Simple Polyembryony: This method of fertilization results in the development of many embryos.
3. Rosette Polyembryony: In certain gymnosperms, the rosette cells divide to form additional embryos; this process is referred to as rosette polyembryony.

Ernst and Schnarf’s classification of polyembryony

• True polyembryony: True polyembryony occurs when many embryos develop in the same ovule’s embryo sac. The extra embryo(s), are produced either by the cleavage of the egg or by the antipodal cells or synergids that are created by the antipodal cells or synergies.
• False polyembryony: This condition occurs when more than one embryo develops inside the ovule in several embryo-sacs.

Importance of Polyembryony

1. Fruit trees can be multiplied with the aid of polyembryony.
2. The growth of homozygous diploids is possible through polyembryony.
3. Horticulturists use polyembryony in plant breeding.

FAQs on Apomixis and Polyembryony

Q1: Write the classification of the flower.

Answer:

 A flower is classified into two types: complete flowers and incomplete flowers. Complete flowers have all four parts, while incomplete flowers do not.

Q2: Define apomixes.

Answer:

Apomixes is an asexual reproduction in which fertilization does not occur, derived from the Greek words “apo” and “mixes”.

Q3: Define polyembryony.

Answer:

Polyembryony is the formation of multiple embryos within a single ovule, seed, or fertilized ovum, which are genetically distinct from the parents.

Q4: Write the difference between recurrent and non-recurrent apomixes.

Answer:

Recurrent apomixis produces diploid embryos and egg cells, while non-recurrent apomixis produces haploid embryos from haploid egg cells or other embryo sac cells.