Outbreeding, the pollen-pistil interaction and hybridisation

Why plants avoid their own pollen

Self-pollination is easy and sure, but if it goes on for many generations, the offspring become less fit. This is inbreeding depression. Most flowering plants have ways to avoid it, and these ways are called outbreeding devices. Four are described.

1. Different timing. The pollen is released before the stigma is ready, or the stigma is ready long before the pollen is released. In either case, the flower cannot pollinate itself. This is dichogamy.
2. Different positions. The anther and the stigma lie at different places in the flower, so the pollen does not touch the stigma of its own flower. Some books call this herkogamy.
3. Self-incompatibility. This is a genetic mechanism. The pistil recognises the pollen of the same flower or plant and stops it, either by stopping its germination or by stopping the growth of its tube in the style.
4. Unisexual flowers. A plant with male flowers and female flowers cannot pollinate itself within one flower. A monoecious plant has both kinds on one plant, as in castor and maize. It prevents autogamy, but not geitonogamy. A dioecious plant has male flowers on one plant and female flowers on another, as in papaya. It prevents both autogamy and geitonogamy.

This part is not in the NEET syllabus text, but A-Level and IB ask it. In some plants, such as the primrose, two kinds of flower exist on different plants: one with a long style and short stamens, and one with a short style and long stamens. This is heterostyly, and it is an example of different positions.

The pollen-pistil interaction

Pollination does not guarantee a seed. After the pollen lands, a dialogue between the pollen and the pistil begins, and it is called the pollen-pistil interaction. The pistil can tell a compatible pollen from an incompatible one, perhaps from chemicals that the pollen releases.

• If the pollen is compatible, the pistil accepts it and helps it.
• If the pollen is incompatible, the pistil rejects it, either on the stigma, by stopping germination, or in the style, by stopping the pollen tube.

What happens after a compatible pollination?

1. The pollen grain germinates on the stigma. It makes a pollen tube through one of its germ pores. The contents of the grain move into the tube.
2. The tube grows through the stigma and the style and reaches the ovary.
3. In a plant that sheds two-celled pollen, the generative cell divides in the tube and forms two male gametes. In a plant that sheds three-celled pollen, the tube carries the two male gametes from the start.
4. The tube enters the ovule through the micropyle. Then it enters one of the synergids through the filiform apparatus. The filiform apparatus guides the tube.

Everything from the landing of the pollen on the stigma to the entry of the tube into the ovule is the pollen-pistil interaction.

Artificial hybridisation

This part is not in the NEET syllabus text, but boards, A-Level and IB plant breeding ask it.

A plant breeder wants to cross two chosen parents, and no other pollen may reach the stigma. The breeder uses two steps.

• Emasculation: removing the anthers from a bisexual flower bud with forceps, before they burst. It stops the flower from pollinating itself.
• Bagging: covering the flower with a bag, usually of butter paper, so unwanted pollen cannot reach the stigma.

When the stigma is ready, the breeder dusts it with ripe pollen of the chosen male parent. The flower is bagged again, and the fruit is allowed to grow. If the female parent has unisexual flowers, emasculation is not needed. The female buds are only bagged before they open.

You should find that a rejected tube grows at the same rate as an accepted one for a while, and then stops. The style is the place where the pistil makes its decision.

Try it in Python

The program gives the time that a pollen tube needs to cross the style, and the length that a rejected tube reaches.

Practise with new numbers

Common mistakes

• Saying that monoecious plants prevent geitonogamy. They prevent autogamy only. Dioecious plants prevent both.
• Thinking that pollination always means fertilisation. An incompatible pollen is rejected.
• Saying that the tube enters the ovule through the chalaza. It enters through the micropyle in most plants.
• Saying that the tube enters the egg directly. It enters a synergid and releases the male gametes there.
• Mixing up emasculation and bagging. Emasculation removes the anthers. Bagging covers the flower.

Remember these

• Outbreeding devices: different timing, different positions, self-incompatibility and unisexual flowers.
• Castor and maize are monoecious (no autogamy). Papaya is dioecious (no autogamy and no geitonogamy).
• Pollen-pistil interaction: recognition, acceptance or rejection, germination, a tube through the style, entry through the micropyle and a synergid.
• Emasculation and bagging are the two steps of artificial hybridisation.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name four outbreeding devices.

2. Why does a dioecious plant prevent both autogamy and geitonogamy?

3. What is self-incompatibility?

4. Trace the path of a pollen tube from the stigma to the egg apparatus.

5. What are emasculation and bagging, and why are they done?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.
• Urry L. A. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 38.