Introduction

Pollination is the procedure of transfer of pollen from an anther of a plant to the stigma of the same or the other plant. This transfer can occur in several ways. Sometimes by wind, sometimes by water and by insects also. Successful pollination leads to fertilization and the formation of seeds. (1). This article is about cross pollination vs self pollination.

Types of pollination

Pollination is primarily two types. These are

• Cross-pollination
• Self-pollination

Cross-pollination

When the pollen of one plant is transferred into the stigma of a flower of another plant, it is called cross-pollination. It is also known as allogamy. The word ‘allo’ means ‘other’ and ‘gamy’ means ‘union.

 Process

Cross-pollination can occur both naturally and artificially. 

• After the maturation of the anther, the pollen sac (a sac-like structure that contains matured pollens) bursts open. 
• The pollens are released from the pollen sac. Stigma of another flower receives the pollen(s). The pollen(s) then travels through the style by forming a pollen tube (a tube-like structure that helps the pollen to reach the ovary of the flower).
• The pollen(s) then fertilize with the egg and form a seed. 
• There are many ways through which pollen(s) can reach the stigma of another flower. The agent which helps the pollen to transfer is called a pollinator. It can be wind or water or animals like bees, insects, bats, birds, molluscs, and even humans.
• If the pollinators are living organisms, we call them biotic agents. Non-living agents like water or wind are regarded as abiotic agents.(2)

Self-pollination

When the pollen of a flower is transferred to the stigma of the same flower or the flower of the same plant, the process is called self-pollination. This process is faster and simpler than cross-pollination. But new characters do not evolve in self-pollination.

Process

In self-pollination, the pollen sac bursts open to liberate anthers. Those anthers fall upon the stigma of the same flower or different flowers of the same plant.

One such mechanism is cleistogamy, where pollination occurs even before the flowers open. There are some flowers whose flowers never open, here self-pollination is a must.(1)

Advantages of cross-pollination

• The unisexual plants can reproduce only through cross-pollination.
• Male and female gametes of two different plants mate with each other. Only the dominant characters are expressed (as per Mendel’s law of dominance).
•  New characters can be evolved.
• Variation can be found which helps in evolution.

Disadvantages of cross-pollination

• Numerous pollens are wasted.
• Undesirable characteristics can be expressed.
• The possibility of failure is much higher.

Advantages of self-pollination

• The number of offsprings is greater as the wastage of pollen grains are less.
• Pollinators are not needed.
• The characters are exactly as same as parents.

Disadvantages of self-pollination

• The vigour (strength of the plant) of the plant is reduced.
• As new characters are not produced, the chances of going extinct are higher.

Ecological and evolutionary impact of cross and self-pollination

In self-pollination, the number of plants produced is much higher than cross-pollination. However, no new characters are developed in self-pollination. Hence, the chances of the plant going extinct is much higher. The development of new characters brings variation in plants. It helps the plants to survive the ever-changing environment. Thus, self-pollination has no such ecological impact. But cross-pollination helps a plant species to evolve.

The offspring remains completely similar to the parents in self-pollination. The chances of production of new variety or species is very low. Whereas, in cross-pollination, the chances of evolution of new variety are much higher. Thus, cross-pollination can promote ecological diversity (3)

Human impact of cross and self-pollination

Self-pollination mostly occurs naturally. All humans can do is brush pollen in the stigma. There are few plants where the pollen does not reach the stigma easily despite being in the same flower. That is where humans can help.

Cross-pollination always needs a pollinator. So, humans can be excellent pollinators here. One can collect the pollens from the anthers and brush them on the flower of another plant. Monosexual plants are always dependent on cross-pollination.

Breeding strategies: Exploiting pollination methods

While performing the breeding, the pollination strategies have always been used. One such example is Mendel’s experiment. In the first filial generation, Mendel performed cross-pollination. He removed the anthers of bisexual plants so that pollens from other plants fused with the egg. In the second filial generation, he let plants perform self-pollination.

These techniques have largely been used. Till today breeders mostly use cross-pollination to generate desirable characteristics in the plants, especially crops.

Effect on biodiversity

Cross-pollination helps to promote biodiversity. New characters are introduced in plants. Often, these variations lead to mutation, and new varieties or species are produced. This is how it helps in biodiversity.

Self-pollination on the other hand has no such impact.

Examples of cross-pollination crops in agriculture

• Maize (Zea mays)
• Rye (Secale cereale)
• Bajra (Pennisetum americanum)
• Cabbage (Brassica oleracea)

Examples of self-pollination in agriculture

Some examples of self-pollination in agriculture are-

• Wheat (Triticum aestivum)
• Paddy (Oryza sativa)
• Tomato (Solanum lycopersicum)
• Potato (Solanum tuberosum)

Differences between Cross-pollination and self-pollination 

Q&A

1. Which is better self-pollination or cross-pollination?

Cross-pollination is better than self-pollination. As the pollen of one plant fuses with the egg of other plants, there are chances to evolve new characters in offspring.

 2. What are the advantages of cross-pollination?

The advantages are-

• New characters are produced in offspring.
• It has a positive impact on biodiversity and evolution.
• This is the only mode of sexual reproduction in unisexual plants.

3. What is the difference between cross and self-fertilisation?

The primary difference is that in self-pollination the pollen of one plant falls on the stigma of the same flower or another flower in the same plant. While in cross-pollination the pollen of one plant falls on the stigma of another flower of another plant.

 4. What are examples of self-pollinating plants?

Examples of self-pollinating plants are

• Potato (Solanum tuberosum)
• Paddy (Oryza sativa)
• Wheat (Triticum aestivum)

Summary

• Pollination is the strategy of transferring the pollen grain of one flower to the stigma of the same or any other flower.
• It is mainly two types- self-pollination and cross-pollination.
• In self-pollination, the pollen grain falls on the stigma of the same flower or any other flower of the same plant. While in cross-pollination the pollen grain transfers to the flower of the other plant.
• Due to the fusion between two different plants, cross-pollination is more developed and can form new characters.
• The offspring remain true to the type of self-pollination.
• Cross-pollination has more impact on evolution and ecological diversity than self-pollination.
• In breeding techniques, cross-pollination is used primarily to introduce desirable characteristics in crops.
• Rye, Cabbage, and Maize are some cross-pollinating plants.
• Some plants where self-pollination was used are wheat, paddy, potato, etc.

Written By: Pritha Chowdhury