Apomixis in Plants

Apomixis is the formation of an embryo without the fusion (mixing) of gametes. Apomixis example is Taraxacum officinale. The term was first coined by Albert Winkler in 1908 when he noticed a single female Alchornea ilicifolia plant producing seeds even when there were no male equivalents in the United Kingdom. In this article, we will learn about the apomixis definition, types, characteristics, advantages, and disadvantages of apomixis in plants in detail.

What is Apomixis?

Apomixis Definition – Apomixis is the asexual reproduction in plants where seeds are produced without fertilization, leading to genetic clones of the parent plant.

The seeds formed by Apomixis are known as apomictic seeds. Since fertilization is absent, the progeny formed is a maternal clone. It is useful for producing a large number of genetically similar plants without losing its characteristics.

In higher plants like angiosperms, the term apomixis is known as agamospermy and is a common occurrence. Many plant families including the Gramineae, Rosaceae, and Compositae are known to be apomictic. Apomixis is seen in major grains like wheat and maize.

Apomixis

Apomixis Example

The examples of Apomixis are:

• Taraxacum officinale: Diplospory
• Hieracium praealtum: Apospory
• Garcinia mangostana
• Bahia grass (Paspalum notatum): Obligate apomixis

Types of Apomixis

There are several types of apomixis in plants. The basic classification of apomixis is gametophytic and sporophytic apomixis based on the origin of the embryo. The gametophytic apomixis is caused by the direct development of an embryo from the egg cell of an unreduced embryo sac. Based on the precursor cell used to form the embryo sac, it can divided into diplospory and apospory.

Sporophytic apomixis is formed by the direct development of an embryo from the mitotic division of the somatic cell of the ovule. Sporophytic apomixis is a typical phenomenon in citrus plants in which sexual reproduction coexists with adventitious embryony.

Gametophytic Apomixis

• Diplospory: It is a type of gametophytic apomixis where the unreduced embryo sacs are formed from the megaspore mother cell in which the meiosis is suspended. From the above diagram (fig 2), we can see that an unreduced embryo sac develops from a megaspore mother cell. The unreduced embryo sac contains of diploid egg which develops into an embryo. It can be observed in Tripsacum, Erigeron, and Taraxacum officinale species.
• Apospory: It is a type of gametophytic apomixis where an unreduced embryo sac originates from somatic cells instead of germ line cells of the ovule. From the above diagram, we can see the development of unreduced embryo sacs from nuclear cells known as aposporos initial cells. It is seen in Panicum, Poa, Pennisetum, and Hieracium spp.

Sporophytic Apomixis

Adventitious Embryony: It is also known as nucellar embryony. It is a type of sporophytic apomixis where the embryo is formed from a cell or a group of cells either in the nucellus or in the integuments (somatic cells) of the ovule. It takes place outside the embryo sac. It can also cause a condition called polyembryony, where multiple embryos are formed in the same ovary. This condition is due to the simultaneous development of an embryo from sexual reproduction within the same ovary. It is commonly seen in citrus fruits.

In most apomictic plants, both sexual and asexual reproductive processes take place simultaneously in the same ovule; this is known as facultative apomixis. In others, gametophytic apomixis replaces amphimixis and this is considered obligate apomixis. In both diplosporous and aposporous ovules, the endosperm can grow spontaneously without fertilization or by pseudo-fertilization, supplying nutrition for the embryo.

Vegetative Apomixis

This type of apomixis is rare in nature. So it is not frequent in fruit crops. But in certain situations, such as Poa bulbosa and various Agave, and grass species, vegetative buds or bulbils are formed instead of flowers in the inflorescence. The bulblet reproduction can also be considered as vegetative apomixis.

Based on the egg cell ploidy, apomixis can also be classified into recurrent and non-recurrent apomixis:

• Recurrent Apomixis: The embryo develops directly from a diploid megaspore mother cell or egg cell without fertilisation. Here both the egg and embryo are diploid(2n). In general, somatic apospory (apospory- development of a gametophyte from a sporophyte without meiosis seen in certain bryophytes), diploid parthenogenesis, and diploid apogamy (apogamy- development of a sporophyte from a gametophyte without fertilization seen in some pteridophytes) are considered recurrent apomixis. It is observed in Taraxacum, Poa, and Allium spp.
• Non-recurrent Apomixis: The embryo develops from a haploid egg cell without being fertilized. This sort of apomixis is very rare. The egg cell and embryo are haploid(n) in nature. This category includes generative apospory, haploid parthenogenesis, and haploid apogamy.
• Parthenogenesis: This is a type of apomixis that can occur in plants, animals or microbes. It is the development of the unfertilised egg into an embryo. Parthenogenesis in honey bees is a common example.

General Characteristics of Apomixis

The characteristics of Apomixis are:

• It is formed by asexual reproduction.
• The process of meiosis is absent.
• There is no fusion of gametes .
• There is no genetic variation which means the progeny produced is genetically identical to the parent (maternal) plant.
• It is commonly seen in many angiosperms.

Applications of Apomixis

Apomixis has been a very useful tool in agricultural biotechnology. Currently, two ways have been used to introduce apomixis into agronomically significant crops, that is the introgression of apomixis genes from natural apomicts and the introduction of synthetic apomixis by molecular engineering.

• The use of natural apomixis or introgression of apomixis genes in agriculture helps the development of pure lines faster.
• It ensures a reduction of cost for plant breeding and fast reproduction.
• Synthetic apomixis: It is caused by combining several components of apomixis to form an apomictic method of development, and has the potential to transform agriculture in the future.
• High-frequency synthetic apomixis has been observed in hybrid rice.
• The progeny of clonal plants with hybrid vigour can sustainably pass on across several generations.

What are the Advantages of Apomixis?

There are several advantages of apomixis as it surpasses the requirements of sexual reproduction:

• In plants, sexual reproduction promotes genetic variation but it also seems to have a lot of expenditure in time and energy. So in certain higher plants apomixis helps to resolve such problems by reproducing asexually.
• It is very helpful in preserving hybrid vigour and protecting superior genes.
• Recently, apomixis has been a very useful tool in agricultural biotechnology. Currently, two ways have been used to introduce apomixis into agronomically significant crops, that is the introgression of apomixis genes from natural apomicts and the introduction of synthetic apomixis by molecular engineering.
• The use of natural apomixis or introgression of apomixis genes in agriculture helps the development of pure lines faster. It ensures a reduction of cost for plant breeding and fast reproduction.

What are the Disadvantages of Apomixis?

Like any other process, apomixis has some disadvantages as well. Those are listed below:

• It does not cause genetic variations thus, it cannot adapt to certain changes in the environment
• It will hinder the process of evolution.
• It may transfer the harmful mutations occurring in the parent cells to progeny cells.

Conclusion – Apomixis in Plants

Apomixis is a seed-developing mechanism without fertilisation. Apomixis example includes several flowering plants or angiosperms like Gramineae, and Rosaceae families. It is beneficial to plants for producing a large number of seeds and sustaining their genetic stability for several generations. Different types of apomixis have different mechanisms for inducing apomixis. The different types of apomixis in plants are gametophytic apomixis consisting of diplospory and apospory, sporophytic apomixis, vegetative apomixis, recurrent and non-recurrent apomixis. There are also certain disadvantages of apomixis such as less resistance to changes in the environment and lack of genetic variations.

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FAQs on Apomixis

What do you Mean by Apomixis in Plants?

Apomixis is a form of producing an embryo without fertilisation in plants. The progeny formed is a clone of the parent plant.

What are the Several Types of Apomixis in Plants?

The different types of apomixis in plants are gametophytic apomixis consisting of diplospory and apospory, sporophytic apomixis, vegetative apomixis, recurrent and non-recurrent apomixis.

Is Apomixis Beneficial?

In plants, it helps to save the loss of energy from sexual reproduction. It is very helpful in preserving hybrid vigour and protecting superior genes.

Why do Farmers Plant Apomixis Seeds?

The use of apomixis seeds in agriculture helps farmers by saving hybrid seeds and maintaining their best genetic variety. It also ensures a reduction of cost for plant breeding and fast reproduction.

What are the Drawbacks of Apomixis?

It does not have genetic variations thus it cannot adapt to changes in the environment. It may also pass the harmful mutations occurring in the parent cells to progeny cells.

What is the Application of Apomixis in Plant Breeding?

Apomixis in plant breeding helps in the production of genetically identical plants, ensuring desirable traits are preserved and transferred consistently across generations.

What are the Causes of Apomixis?

The causes of apomixis include genetic factors, environmental conditions, and hormonal regulation, leading to the development of embryos without fertilization in plants.

What is Apomixis and its Significance?

Apomixis is a form of asexual reproduction in plants where seeds are produced without fertilization. This process is significant in agriculture for preserving and tranferring valuable plant characteristics, contributing to crop improvement and hybrid vigor.

What is the Difference Between Apomixis and Parthenocarpy?

Apomixis is a form of asexual reproduction in plants where seeds are produced without fertilization ensuring genetic uniformity, while parthenocarpy is the development of fruit without fertilization leading to seedless fruit production.

Why is Apomixis Important in Plant Breeding?

Apomixis is crucial in plant breeding because it allows for the preservation of desirable traits in plants without the need for sexual reproduction which ensures genetic uniformity in offspring, which can be beneficial for producing high-yielding and disease-resistant crops.

What is Recurrent Apomixis?

Recurrent apomixis refers to a phenomenon where apomixis occurs repeatedly across generations in a plant species allowing for the continous asexual reproduction without the need for fertilization.