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Apomixis, Polyembryony, Practice Problems, FAQs

Apomixis, Polyembryony, Practice Problems, FAQs

How are seeds produced? We all know that seeds mature from ovules post fertilisation and have the growing embryo within themselves. It is widely known that sexually reproducing plants propagate via seeds and asexually reproducing plants propagate via vegetative parts. But, what if I told you that seeds can be produced even without sexual reproduction and fertilisation in plants? You might not believe me but this is a real phenomenon and is known as apomixis.

Let me shock you with another piece of information. Did you know that even plants can have twins? A single seed can carry more than one embryo just like a human mother with twins carries two embryos in her womb. This phenomenon is known as polyembryony.

In this article, we are gonna learn about both apomixis and polyembryony.

Table of content

  • Apomixis
  • Types of apomixis
  • Benefits of apomixis
  • Polyembryony
  • Types of polyembryony
  • Practice problems
  • FAQs

Apomixis 

It is the process of production of new individuals or seeds containing embryo directly from the diploid cells of the nucellus or integument, diploid egg or some other gametophytic cell, without involving fertilisation. The term was coined by Hans Karl Albert Winkler. The term apomixis comes from the Greek words apo meaning without and mixis meaning mingling. It is a type of asexual reproduction that mimics sexual reproduction. Examples: Poa (meadow grasses), hieracium (hawkseed). The offspring produced by apomixis are genetically identical to each other.



                                  Fig: Examples of plants that undergo apomixis

Types of Apomixis

Apomixis can be broadly classified as vegetative propagation and agamospermy. 

Vegetative Apomixis

In this apomixis type, vegetative bulbils or buds are produced in the inflorescence instead of flowers. They are easily reproducible and are seen in certain plants like Fragaria, Agave, Poa bulbosa, etc.

Agamospermy

In agamospermy, there is no formation of haploid gametes. It results in the formation of seeds and embryos without meiosis and fertilisation. Agamospermy can be of the following types -

Adventive embryony

It is a type of sporophytic apomixis, where embryos are produced directly from the nucellus or the integument of the ovule. The embryo develops by mitotic division and forms a bud-like structure. Simultaneous fertilization in the adjoining sexual embryo sac is required to form viable seeds. The developing embryos grow towards it to obtain nutrients and signals from the embryo sac.



                                                           Fig: Adventive embryony

Recurrent agamospermy

In recurrent agamospermy, a diploid embryo sac with a diploid egg or ovum is formed due to defective meiosis. The diploid egg develops into a diploid embryo without fertilisation, by the process of parthenogenesis. Thus, this process is also known as diploid parthenogenesis. This includes the following types -

Apospory

In apospory, the diploid gametophyte develops from the sporophyte tissue, without the involvement of meiosis. In this process the embryo sac or gametophyte develops from a diploid sporophytic cell of the ovule such as the nucellus. Aposporous initial cells differentiate from the nucellus and undergo mitosis to produce an embryo sac. Sometimes multiple embryo sacs may be found. The initiation of the apospory embryo sac and the sexual one can occur simultaneously or it can displace or interrupt sexual embryo sac formation.

Diplospory

In diplospory, diploid megaspore mother cells give rise to the embryo directly by mitotic division. The megaspore mother cell arises from one of cells of the nucellus lying at the micropylar end. The megaspore mother cell undergoes three mitotic divisions to form eight nuclei. The embryo sac is unreduced and has the same number of chromosomes and genetic material.



                                                     Fig: Megaspore mother cell

Non-recurrent agamospermy

In non-recurrent apomixis, both the egg cell and embryo are haploid and the embryo is developed directly from an egg cell without fertilisation. Thus, it is also known as haploid parthenogenesis.

Apogamy

In apogamy the embryo is formed from any cell of the embryo sac other than the egg, without fertilisation. The embryo generally arises from the synergids or antipodal cells. This is commonly seen in orchids belonging to the family Asteraceae.

Benefits of Apomixis

1. Apomixis helps to indefinitely maintain hybrid vigour in plants. Hybrid varieties cannot maintain the vigour beyond a single generation because sexual reproduction in such plants would result in random segregation of genes and the hybrid vigour will be lost. But producing identical clones by apomixis helps to preserve hybrid vigour as there is no mixing of genes.

2. Hybrid seeds cannot be collected from the hybrid plants and have to be produced every year. This is because sexual reproduction would lead to loss of hybrid vigour. Thus, apomixis is an effective alternative for raising hybrid plants.

3. Embryos developed through apomixis are devoid of infectious diseases.

Polyembryony

In polyembryony, more than one embryo occurs in a seed. It is seen in seeds of mango, orange, etc. 



                              Fig: Polyembryony in orange seeds

Polyembryony in humans results in the formation of two identical twins. It occurs in both plants and animals. The nine-banded armadillo is an excellent example of an animal showing polyembryony. It is a mammal found in certain parts of America and gives birth to identical quadruplets.

Polyembryony in Plants

The formation of more than one embryo from a single seed is known as polyembryony. Antonie van Leeuwenhoek was the first person to discover this natural phenomenon in Citrus in the year 1719.

Polyembryony can be naturally occurring or can be induced. Polyembryony in plants can be considered true if extra embryos arise in the same embryo sac in which the zygotic embryo has developed. It is considered as false if more than one embryo develops in different embryo sacs inside the same ovule. 

Types of Polyembryony 

Polyembryony can be of three types -

Simple polyembryony

It is due to the fertilisation of more than one egg cell in an ovule. This can arise when the embryo sac contains more than one egg or the ovule possesses more than one embryo sac. The latter is a case of false polyembryony. Examples include Citrus, Poa, etc.

Cleavage polyembryony

This is caused due to cleavage or splitting of the proembryo into two or more parts and branching of the proembryo. This can also be caused due to splitting of the suspensor. This is more common in gymnosperms and in angiosperms, it is commonly seen in orchids. This is a type of true polyembryony.

Adventive polyembryony

In this case, the embryos develop from diploid sporophytic cells of nucellus or integument which penetrate into the embryo sac, for example, mango. This is a type of true polyembryony.



                                            Fig: Adventive polyembryony

Embryo from synergids

In this type of true polyembryony, the synergids form embryos with or without fertilisation, For example - Sagittaria.

Embryo from antipodal cells

This is also a type of true polyembryony in which the extra embryos arise from the antipodal cells, e.g, Ulmus americana.

Embryo from endosperm

In this type of true polyembryony, the embryo develops from one of the cells of the endosperm, for example, Balanophora.

Practice problems

Q 1. In which of the following apomictic processes, embryos develop directly from the sporophytic tissue of nucellus or integument?

a. Adventive embryony
b. Apospory
c. Diplospory
d. Apogamy

Answer: In adventive embryony, embryos are produced directly from the nucellus or the integument of the ovule which penetrate into the embryo sac. The embryo develops by mitotic division and forms a bud-like structure. Simultaneous fertilization in the adjoining sexual embryo sac is required to form viable seeds. 

Thus, the correct option is a.

Q 2. One of these statements about apomixis is incorrect

a. formation of seeds without fertilisation.
b. Meiotic division and fertilisation of gametes takes place to form a zygote.
c. Offspring produced are genetically identical.
d. both b and c.

Answer: Apomixis is the process of production of new individuals or seeds containing embryos developed directly from the diploid cells of the nucellus or integument, diploid egg or some other gametophytic cell, without involving fertilisation or meiotic division. It is a type of asexual reproduction that mimics sexual reproduction. The offspring produced by apomixis are genetically identical to each other.

Thus, the correct option is b.

Q 3. In diplospory, the embryo develops from the

a. Nucellus
b. Integument
c. Synergids
d. Megaspore mother cell

Answer: In diplospory, diploid megaspore mother cells give rise to the embryo directly by mitotic division. The megaspore mother cell arises from one of cells of the nucellus lying at the micropylar end. The megaspore mother cell undergoes three mitotic divisions to form eight nuclei. 

Thus, the correct option is d.

Q 4. Cleavage polyembryony is most commonly seen in

a. Citrus
b. Mango
c. Orchids
d. Balanophora

Answer: Cleavage polyembryony is a type of true polyembryony caused due to cleavage or splitting of the proembryo and its branching. This can also be caused due to splitting of the suspensor. This is more common in gymnosperms and in angiosperms, it is commonly seen in orchids. 

Thus, the correct option is c.

FAQs

Q 1. Which type of apomixis can lead to polyembryony?
Answer: Adventive embryony or sporophytic budding can lead to adventive polyembryony. In adventive embryony, extra embryos are produced directly from the nucellus or the integument of the ovule which penetrate into the embryo sac. 

Q 2. What is amphimixis?
Answer: Amphimixis is the process of formation of new individuals through the process of sexual reproduction, that is, by the process of gamete formation and fertilisation is known as amphimixis.

Q 3. Is parthenogenesis the same as apomixis?
Answer: Apomixis is the process of production of new individuals or seeds containing embryos developed directly from the diploid cells of the nucellus or integument, diploid egg or some other gametophytic cell, without involving fertilisation or meiotic division. Parthenogenesis is a type of apomixis in which the embryo develops from the egg cell without fertilisation. It can be haploid parthenogenesis where the haploid egg develops into haploid embryo, or diploid parthenogenesis in which the diploid egg develops into diploid embryo.

Q 4. How is apomixis different from parthenocarpy?
Answer: Apomixis is the process by which seeds containing embryos are developed without fertilisation. In parthenocarpy, seedless fruits are developed without fertilisation.

Related links

Double fertilisation

Microsporogenesis 

Megasporogenesis 

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