Double Fertilisation: Definition, Process & Significance

Double Fertilisation: Do you know fertilisation takes place twice in plants? Yes, and this process is known as Double fertilisation, which is commonly seen in flowering plants, i.e. Angiosperms. Angiosperms are the most diverse group of terrestrial plants, and the flowers are considered to be the reproductive parts of the angiosperms. These flowers may be unisexual or bisexual. The process of double fertilisation is not possible without microsporogenesis (formation of pollen), megasporogenesis (formation of embryo sac) and pollination (transfer of pollen from the anther to the stigma either of the same flower or of the different flower but of the same species).

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm are produced by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic formation of the triploid endosperm is twice that of the embryo.

What is Double Fertilisation?

Double Fertilisation is defined as a complex process in which one of the haploid male gametes fuses with the haploid egg nucleus to form the diploid zygote, i.e. syngamy and the other haploid male gamete fuses with the diploid polar nucleus to form the triploid endosperm, i.e. triple fusion. The concept of double fertilisation was discovered by the Russian scientist S.G. Nawaschin in \(1898\) in Lilium and Fritillaria.

Events That Take Place Before Double Fertilisation

1. In the process of double fertilisation, flowers play a significant role as they contain the male and female reproductive structures called stamen and pistil, respectively.
2. These reproductive structures are responsible for producing the haploid male and female gamete. 
3. Three main events should occur as a prerequisite for double fertilisation to take place, i.e. development of male gametophyte, development of female gametophyte and pollination.

Fig: Structure of a Mature Flower

Development of Male Gametophyte

1. Microsporogenesis: The process of formation of the male gametophyte (microspore) is called microsporogenesis.
2. In this process, one pollen mother cell or microspore mother cell (PMC or MMC) undergoes meiosis to produce four haploid pollen or microspores.
3. When the pollen matures, the pollen nucleus divides into two nuclei, namely, the smaller generative nucleus (male nucleus) and the larger tube nucleus (vegetative nucleus).  
4. In over \(60\%\) of angiosperms, the pollen grains are transferred to the stigma at this two-celled stage, while in the remaining \(40\%,\) pollen is released in a three-celled stage.

Fig: Structure of a Pollen Grain

Development of Female Gametophyte and Megasporogenesis

1. Megasporogenesis: The process of formation of a megaspore from a megaspore mother cell is called megasporogenesis.
2. An ovary may contain one or many ovules. Each ovule contains a megaspore mother cell that is a large diploid cell.
3. Megaspore mother cell undergoes meiotic division and gives rise to four haploid megaspores, out of which only one remains functional and the other three
4. The functional haploid megaspore grows and undergoes \(3\) mitotic divisions to produce eight haploid nuclei. The structure formed is known as the embryo sac or female gametophyte.

Fig: Development of Embryo Sac.

Pollination

1. In the flowering plants or angiosperms, fertilisation takes place after the process of pollination, i.e., the transfer of pollen grains from the anther to the stigma of the same or of the different flowers but of the same species.
2. The transfer of pollen grains can take place naturally or with the help of pollinating agents like honey bees, butterflies, birds, wind, water, etc.

Double Fertilisation Process

Pollen–Pistil Interaction: The events from pollen deposition on the stigma until the pollen tube enters the ovule are together referred to as pollen–pistil interaction. This interaction is a dynamic process.

1. Pollination does not guarantee the transfer of the right type of pollen of stigma. Often, pollen of the wrong type, either from other species or from the same plant (if it is self–incompatible), also lands on the stigma.
2. The pistil has the ability to recognize the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible).
3. The ability of the pistil to recognize the pollen followed by its acceptance or rejection is the result of a continuous chemical dialogue between pollen grain and the pistil.
4. After the pollen grains land on the stigma, the vegetative cell forms the pollen tube.
5. The pollen tube emerges from the germ pore as a short cytoplasmic outgrowth initially and is called a germ tube.
6. The pollen tube development is directed towards the ovary by chemicals, calcium-boron-inositol sugar complex.
7. The synergids in the embryo sac help in directing the movement of the pollen tube towards it, and hence pollen tube development is positively chemotropic and negatively aerotropic (grows away from the air).
8. The pollen tube produces certain enzymes that digest the tissues of the stigma and style and enables the passage of the pollen tube to the ovule.
9. The pollen tube has three nuclei- the vegetative or tube nucleus and two male nuclei produced by the mitotic division of the generative nucleus.
10. The tube nucleus present at the tip of the pollen tube helps in directing the growth of the pollen tube and ultimately disintegrates.
11. Fertilisation involving the carrying of male gametes by pollen tube is called siphonogamy.

Entry of Pollen Tube

1. The pollen tube enters the embryo-sac via three routes, i.e., through the micropyle (porogamy) as seen in Lily, through the chalaza (chalazogamy or basigamy) as seen in Casurina or through the integuments (mesogamy) as seen in Populus Cucurbita, etc.
2. In the majority of the flowering plants, it is porogamy.

Fig: Possible Ways by which Pollen Tube can Enter Embryo Sac

Entry of Pollen Tube in the Embryo Sac

1. After entering the ovule, the pollen tube grows towards the egg apparatus because of the hormones secreted by the synergids that attract the growth of the pollen tube.
2. One of the synergids starts degenerating, and the pollen tube enters into the embryo-sac through the degenerating synergids.
3. The tip of the pollen tube swells and bursts after reaching inside the embryo-sac, releasing all the contents, including both male gametes inside the embryo-sac.
4. Two dark granules appear in the region of degenerating synergids known as \(X\)-bodies, which are formed by the degenerating nucleus of tube cell and synergids.
5. Before or after the entrance of the pollen tube into the embryo sac, the two polar nuclei of the central cell fuse together to form the diploid nucleus known as the secondary nucleus or definitive nucleus.

Fig: Events Occurring after Entry of Pollen Tube into the Embryo Sac

Double Fertilisation Events

1. First Fertilisation: One of the male gametes \(\left(n \right)\) fuses with the egg nucleus \(\left(n \right)\) to form the diploid zygote \(\left({2n} \right).\) This process is the first fertilisation process or also known as syngamy.
2. Second Fertilisation: The other male gamete moves to the central cell where it fuses with the two polar nuclei or the secondary diploid nucleus (formed by the fusion of \(2\) polar nuclei) and forms a triploid nucleus called primary endosperm nucleus (PEN). This process is known as the second fertilisation process or triple fusion.
3. Since two sets of fertilisation take place, that is syngamy and triple fusion; it is called double fertilisation. It is unique to flowering plants. 

Fig: The process of Double Fertilisation

Events That Take Place After Double Fertilisation

After the completion of the double fertilisation process, the following events occur in the flower:

1. Development of Endosperm: Endosperm develops from the triploid primary endosperm nucleus (PEN) by the mitotic divisions. The development of endosperm begins just before the embryo development, which is of three types, i.e., nuclear, cellular, and helobial.

Fig: Types of Endosperm Development

2. Development of zygote into an embryo: The zygote formed as a result of the fusion of the haploid male gamete with the haploid female gamete develops at the micropylar end of the embryo sac. The zygote develops into an embryo by mitotic divisions. A mature embryo may have one cotyledon (monocot) or two cotyledons (dicot), a plumule at the apical end and a radicle towards the micropylar end.

Fig: a) A Typical Dicot Embryo, b) L.S. of a Monocot Embryo

3. Formation of seed: An ovule undergoes a series of changes after fertilisation, as a result of which seed is formed. The two integuments develop into the two seed coats, i.e., testa and tegmen. So, a seed can be defined as a mature fertilized ovule that possesses an embryo, stored food material in the form of endosperm in certain seeds and a protective seed coat. The seeds can be broadly classified into three types as follows:
a. Ex-albuminous seeds or non-endospermic seeds: The endosperm may be completely consumed by the developing embryo as in certain dicot seeds like pea, bean, etc.
b. Albuminous seeds or endospermic seeds: Sometimes, the endosperm may persist in the mature seeds and is not completely utilized by the embryo-like in coconut, cereals, etc.
c. Perispermic seeds: Seeds in which remains of nucellus are seen. The residual, persistent nucellus is called perisperm, e.g., Black pepper, Beet.

4. Formation of Fruit: A true fruit develops from a ripened ovary. As the seed develops, the ovary wall matures, forms a pericarp and most of the floral parts like sepals, petals, stamens, style and stigma wither away and generally fall off. So, fruit can be defined as a mature ovary containing seeds.

Significance of Double Fertilisation

The significance of double fertilisation are as follows:

1. It helps to maintain the diploid number of chromosomes.
2. It is important for the continuation of the species.
3. It produces genetic variations by mixing the characters from the two parents.
4. It results in the formation of an endosperm which provides nutrition to the developing embryo.

Summary

Double fertilisation is a complex process of the fusion of haploid male gametes with the haploid egg nucleus to form the diploid zygote and the fusion of another haploid male gamete with the diploid polar nucleus to form the triploid primary endosperm nucleus. It is commonly seen in flowering plants (or angiosperms). Microsporogenesis, megasporogenesis and pollination are prerequisite processes before double fertilisation can proceed. The pollen-pistil interaction is responsible for the germination of the pollen tube that is positively chemotropic and negatively aerotropic. 

Fertilisation involving the carrying of male gametes by pollen tube is called siphonogamy. After the completion of the double fertilisation process, the development of endosperm and zygote takes place, leading to the formation of seeds and fruit.

Important Points About Double Fertilisation

Double fertilisation was discovered more than a century ago by Sergei Navashin in Kiev in the Russian Empire and by Leon Guignard in France. Each independently discovered the other. The first observations of Lilium martagon and Fritillaria tenella double fertilisation, which were made using classical light microscopes, were used. Due to the limitations of the light microscope, there were many unanswered questions regarding the process of double fertilisation. However, with the development of the electron microscope, many questions were answered. Most notably, observations made by W. Jensen’s group showed that the male gamete did not have any cell walls and that the plasma membrane of gametes is close to the plasma membrane of the cell that surrounds them inside the pollen grain.

In Vitro Double Fertilization

In vitro double fertilization is often used to study molecular interactions as well as other aspects of gamete fusion in flowering plants. One of the major obstacles to developing in vitro double fertilization between male and female gametes is the binding of the sperm in the pollen tube and the egg in the embryo sac. Controlled fusion of egg and sperm with poppy seeds has already been achieved.

FAQs

Q.1. What is double fertilisation?
Ans: Double Fertilisation is defined as a complex process in which one of the haploid male gametes fuses with the haploid egg nucleus to form the diploid zygote, i.e. syngamy and the other haploid male gamete fuses with the diploid polar nucleus to form the triploid endosperm, i.e. triple fusion.

Q.2. Who discovered double fertilisation in flowering plants?
Ans: The concept of double fertilisation was discovered by the Russian scientist S.G. Nawaschin in \(1898\) in Lilium and Fritillaria.

Q.3. Why is fertilisation in flowering plants called double fertilisation?
Ans: Fertilisation in flowering plants is called double fertilisation because the process of fertilisation takes place twice in these plants.
First fertilisation or syngamy: One of the male gametes \(\left(n \right)\) fuses with the egg nucleus \(\left(n \right)\) to form thediploid zygote \(\left({2n} \right).\)
Second fertilisation or triple fusion: Another male gamete moves to the central cell where it fuses with the two polar nuclei or the secondary diploid nucleus (formed by the fusion of \(2\) polar nuclei) and forms a triploid nucleus called the primary endosperm nucleus (PEN).

Q.4. Why is double fertilisation important?
Ans: 1. It helps in maintaining the diploid number of chromosomes.
2. It is helpful for the continuation of the species.
3. It produces genetic variations by mixing the characters from the two parents that lead to the evolution of a new species.
4. It results in the formation of an endosperm which provides nutrition to the developing embryo.

Q.5. What are the events that take place after double fertilisation?
Ans: a. Development of endosperm.
b. Development of zygote into an embryo.
c. Formation of seed.
d. Formation of fruit.

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