Apomixis and Polyembryony: Though most plants show vegetative and sexual reproduction (amphimixis), there are some plants which show certain special types of reproduction called apomixis and polyembryony. Apomixis definition states it to be a type of asexual reproduction, whereas polyembryony is a type of sexual reproduction. The apomixis meaning along with the method is described in the article.

In apomixis, the seeds are produced without the fusion of gametes (or fertilisation) and polyembryony refers to the occurrence of many embryos in the same seed. Apomixis and Polyembryony can be seen mostly in plants. This intriguing reproductive process in botany is explained in detail in this article below with polyembryony and apomixis examples.

What are Apomixis and Polyembryony?

As per apomixis definition, it is a modified form of asexual reproduction in which an egg or other cells associated with the egg (synergids, antipodals, nucellus and integuments) develop into an embryo without fertilisation. The plants formed by apomixis are called apomictic.

Polyembryony is defined “as the phenomenon of having more than one embryo developed from a single fertilised egg or in a single seed.”

Apomixis and Polyembryony Explanation

Check the detailed explanation of Apomixis and Polyembryony below:

Apomixis

1. Apomixis meaning is derived from two Greek words, i.e., “apo” meaning away from and “mixed” meaning the act of mixing or mingling.
2. The term apomixis was given by Winkler \((1908).\)
3. Apomixis is very common in higher plants, especially in some flowering plants in which the embryo may develop from an unfertilised egg cell, or from a cell other than the egg cell within the embryo sac, or from the cell outside the embryo sac.
4. The plant produced is genetically identical to the mother plant.
5. There are more than \(35\) families of plants which are known to be apomictic. Apomictic seeds examples are Gramineae, Rosaceae, Compositae, and Rutaceae.

Polyembryony

1. Polyembryony refers to the formation of more than one embryo in a seed, e.g., citrus, mango, etc.
2. Polyembryony was first discovered by Leeuwenhoek \((1719)\) in citrus fruits and was confirmed later by Schnarf \((1929).\)
3. This phenomenon can be mostly observed in Gymnosperms (conifers) than Angiosperms. In angiosperms, it is generally present as an unusual feature except in a few cases like Citrus, mango, Nicotiana, etc.
4. The causes of polyembryony in Angiosperms are as follows:
   • Cleavage of proembryo, e.g., family Orchidaceae.
   • Development of many embryos from other cells of embryo-sac except egg, e.g., Anemone.
   • Formation of many embryos due to the presence of more than one embryo sac in the same ovule, e.g., Brassica.
   • Formation of many embryos from the structure outside the embryo-sac (adventive polyembryony) like nucellus and integuments, e.g. Citrus, mango, Opuntia.
5. The most important theory for the cause of polyembryony is the Necrohormone theory given by Haberlandt \((1921)\) which states that the stimulus for polyembryony is provided by the degenerating cells of nucellus.

Apomixis and Polyembryony Types

Students can check the types of Apomixis and Polyembryony below:

Apomixis Types

1. There are two main categories of apomixis:
   • Agamospermy: Embryos and seeds are produced without the involvement of fertilization and meiosis.
   • Vegetative propagation: New plants are produced from buds, leaves, stems, etc.
2. For the sake of convenience, P. Maheshwari sub-divided apomixis into three types:
   • Recurrent Apomixis
   • Adventive Apomixis
   • Non-recurrent Apomixis

Recurrent Apomixis

1. In this type of apomixis, a diploid embryo-sac is formed from a megaspore mother cell which has a diploid egg or oosphere.
2. The diploid egg grows parthenogenetically into diploid embryos.
3. It is again divided into two types:
   1. Apospory: In this, the embryo develops from diploid nucellar cells.
   2. Diplospory: In this, the embryo develops from the diploid megaspore mother cell.
4. Recurrent apomixis example – Apple, Pear, Allium, etc.

Non-recurrent Apomixis

1. In this type of apomixis, the megaspore mother cell undergoes the usual meiotic division and a haploid embryo sac is formed, from which the embryo develops from a haploid egg parthenogenetically.
2. But due to various reasons, the fertilization does not take place which is as follows:
   • Absence of pollen tube.
   • No attraction between male and female nuclei.
   • Inability of the tube to discharge its contents.
   • Early degeneration of sperms.
   • Maturation of egg and sperm is not synchronized.
3. It is again divided into two types:
   • Haploid parthenogenesis: In this, the embryo develops from haploid egg cells.
   • Haploid apogamy: In this, the embryo develops from some other haploid cell of embryo-sac or female gametophyte. The plants produced are haploid and sterile.
4. Non-recurrent apomixis example – Banana, Solanum, and Lilium.

Adventive Apomixis (Sporophytic Budding)

1. In adventive embryony apomixis, the embryos arise directly from the diploid sporophytic part of the ovule like the diploid cells of nucellus or integument.
2. In this type, the gametophytic generation is eliminated and thus is also known as sporophytic budding.
3. It is very much close to vegetative propagation, but the plants here retain the seed habit.
4. A zygotic embryo degenerates or competes with an apomictic embryo.
5. Adventive apomixis example – Citrus, Opuntia, and Mango.

Polyembryony Types

Polyembryony is classified into two main types:

1. True polyembryony
2. False polyembryony

True Polyembryony

1. In this type, more than one embryo is formed in the same embryo sac in the ovule.
2. It is again subdivided into four types:
   • Cleavage of zygote or budding: In this type, a single fertilised egg gives rise to several embryos. E.g. Nymphaea, Exocarpus, Orchids, Nicotiana, etc.
   • By fertilisation of synergids: In this type, a number of embryos are formed as a result of the fertilisation of eggs along with synergids. E.g. Sagittaria, Anemone, Aristolochia, etc.,
   • Rosette Polyembryony: In this, additional embryos are formed from the rosette cells in certain gymnosperms.
   • By fertilisation of antipodals: In this type, a number of embryos are formed as a result of the fertilisation of eggs along with antipodal cells. E.g., Ulmus, Paspalum, etc.

False Polyembryony or Pseudopolyembryony

1. In this type, more than one embryo is formed in different embryo-sacs inside the ovule.
2. Polyembryony can also be divided into two other types:
   • Spontaneous Polyembryony: In this type, polyembryony develops naturally.
   • Induced Polyembryony: In this type, polyembryony develops artificially.

Significance of Apomixis and Polyembryony

Apomixis Significance

The significance of apomixis are as follows:

1. Apomixis helps in the rapid multiplication of genetically uniform individuals without any risk of segregation in plant breeding.
2. Apomictic plants help to conserve the genetic structure and can maintain the advantages of heterozygotes.
3. Apomixis can be utilised in plant breeding for recurring production of seeds of \({{\rm{F}}_{\rm{1}}}\) hybrids.
4. Apomixis leads to the formation of infection-embryos.
5. It is responsible for the development of polyploidy.

Polyembryony Significance

The significance of polyembryony is as follows:

1. Polyembryony is used in plant breeding by the horticulturists.
2. Nucellar embryos formed in polyembryony are from diseases and maintain their superiority for a long time.
3. Polyembryony helps in the propagation of the fruit trees such as Citrus and mango.
4. The adventive embryos are also important for providing genetically uniform seedlings in fruit trees.
5. Polyembryony can be used for the development of homozygous diploid.
6. Polyembryony can be used for artificial production of embryos from the eggs or synergids.

Difference Between Apomixis and Polyembryony

The difference between apomixis and polyembryony is as follows:

Apomixis	Polyembryony
It refers to the asexual reproduction that occurs without fertilization and does not involve meiosis.	It refers to the formation of more than one embryo from a single fertilised ovum or in a single seed.
It is a type of asexual reproduction.	It is a type of sexual reproduction.
Fertilization does not occur in apomixis.	Polyembryony results due to fertilization.
In this, the seeds are produced asexually.	In this, the seeds are produced sexually.
The plants produced are genetically identical to the parents.	The plants produced are not genetically identical to the parent.

Do You Know?

1. A special and interesting case of polyembryony is found in Allium odorum, where \(5\) embryos develop by different methods, i.e., zygotic \((1),\) synergid \((1),\) from antipodals \((2)\) and from integuments \((1).\)
2. The maximum number of embryos, i.e., \(40\) are reported inside a single seed of Citrus unshiu.
3. In Lilium, all three types of polyembryony are present.

Summary

Thus, apomixis and polyembryony are the two special modes of reproduction that can be seen in both plants and animals. In apomixis, the seeds are produced without the fusion of gametes (or fertilization) and polyembryony refers to the occurrence of many embryos in the same seed. But the normal type of sexual reproduction involves two regular features, i.e., meiosis and fertilization, so it is also called amphimixis which is completely different from apomixis.

A special and interesting case of polyembryony is found in Allium odorum, where 5 embryos develop by different methods, i.e., zygotic \((1),\) synergid \((1),\) from antipodals \((2)\) and from integuments \((1).\) Maximum number of embryos, i.e., \(40\) are reported inside a single seed of Citrus unshiu.

FAQs on Apomixis and Polyembryony

The most commonly asked questions about Apomixis and Polyembryony are answered here:

Q.1: What is apomixis definition? Ans: Apomixis meaning “a modified form of asexual reproduction in which egg or other cells associated with the egg (synergids, antipodals, nucellus and integuments) develops into an embryo without fertilisation.”

Q.2: What is polyembryony? Ans: Polyembryony is defined as, “The phenomenon of having more than one embryo developed from a single fertilised egg or in a single seed.”

Q.3: What are the causes of apomixis? Ans: a. Cleavage of proembryo. b. Development of many embryos from other cells of embryo-sac except egg. c. Formation of many embryos due to presence of more than one embryo sac in the same ovule. d. Formation of many embryos from the structure outside the embryo sac.

Q.4: What are the three types of apomixis? Ans: a. Recurrent Apomixis b. Adventive Apomixis c. Non-recurrent Apomixis

Q.5: Is apomixis the same as polyembryony? Ans: No, apomixis is not the same as polyembryony. Apomixis is a type of asexual reproduction whereas polyembryony is a type of sexual reproduction.