Sexual Reproduction in Flowering Plants · Lesson 6 of 9
Outbreeding, Pollen–Pistil Interaction and Controlled Pollination
“How plants limit selfing, recognise compatible pollen, deliver sperm cells and enable controlled crosses.”
• Explain timing, spatial, genetic and sexual systems that discourage self-fertilisation. • Describe how the pistil accepts or rejects pollen. • Trace pollen-tube growth from stigma to synergid and the roles of the two male gametes. • Explain emasculation, bagging and controlled pollen transfer.
Why flowering plants often limit self-fertilisation
Self-fertilisation can provide reproductive assurance when mates or pollinators are scarce, but repeated close inbreeding may increase the expression of harmful recessive alleles and reduce offspring performance. This is one reason many flowering plants have evolved outbreeding devices that reduce the probability of self-fertilisation and promote genetic mixing. These devices do not all operate at the same stage. Some prevent pollen reaching a suitable stigma, while others act after pollen lands through biochemical recognition of genetically incompatible pollen.
Timing and spatial mechanisms
If the anther releases pollen before the stigma becomes receptive, or the stigma becomes receptive before the anther releases pollen, self-transfer within a flower is reduced. This difference in timing is commonly called dichogamy. Another strategy, spatial separation of the anthers and stigma within a flower, is often called herkogamy. These are mechanisms that make autogamy less likely, rather than universal guarantees that no self-fertilisation will ever occur. They can sometimes permit geitonogamy because pollen from another flower on the same plant may still be transferred.
Flower sexuality affects which selfing routes are possible. A monoecious plant has separate male and female flowers on the same individual, as in maize or castor. This prevents autogamy within one bisexual flower but does not prevent pollen transfer between male and female flowers of that plant. A dioecious species has separate male and female plants, as in papaya, and this arrangement prevents both autogamy and geitonogamy. Neither condition eliminates the need for a pollen-transfer agent where pollen cannot reach the stigma unaided.
| Device | How it works | Autogamy prevented? | Geitonogamy prevented? |
|---|---|---|---|
| Dichogamy | Anther release and stigma receptivity differ in time | Often reduces | Not necessarily |
| Herkogamy | Anthers and stigma separated spatially | Often reduces | Not necessarily |
| Self-incompatibility | Genetic recognition blocks self-pollen fertilisation | Yes, if the system is effective | Yes, if genetically self pollen is rejected |
| Monoecy | Male and female flowers on same plant | Yes | No |
| Dioecy | Male and female flowers on different plants | Yes | Yes |
Self-incompatibility is a genetic recognition system
Self-incompatibility is not the same as pollen simply failing to land on a stigma. It is a genetically controlled interaction between pollen and pistil that prevents pollen recognised as incompatible from completing fertilisation. In different species, the incompatibility reaction may inhibit germination at the stigma or arrest growth of the pollen tube within the style. Compatible pollen is allowed to germinate and grow. This explains why a stigma covered in apparently healthy self-pollen may produce no seed: transport can succeed while the subsequent recognition step fails.
The pollen–pistil interaction involves chemical signals and recognition molecules on pollen and maternal tissues. A compatible stigma permits hydration and growth of a pollen tube through an exine aperture. The vegetative cell and its tube then extend through stigma and style tissues toward an ovary. If the grain was shed in a two-celled state, its generative cell divides during this interval to produce two male gametes. If it was shed three-celled, those male gametes are already present. Their destination is an ovule, not the ovary wall.
Following the pollen tube into an ovule
Once the pollen tube reaches the ovary, it enters a suitable ovule, commonly through the micropyle. The tube is directed to the egg apparatus by signals from the female gametophyte, including the synergids; the filiform apparatus participates in its reception. Typically the tube enters one synergid and releases two male gametes. One is directed toward the egg cell and the other toward the central cell containing the polar nuclei. This is why a flowering plant can perform two fertilisation fusions even though the tube carries only two sperm cells.
Controlled crosses: emasculation and bagging
Plant breeders can cross selected parents by deliberately controlling which pollen reaches a receptive stigma. If the intended female parent has bisexual flowers, the anthers are carefully removed from an unopened flower bud before they dehisce; this step is emasculation. The bud is then enclosed in a suitable breathable bag, often made from paper, to prevent stray pollen from reaching its stigma. When the stigma is receptive, mature pollen from the chosen male parent is applied and the flower is bagged again while development proceeds.
If the female parent has naturally unisexual female flowers, there are no anthers to remove, so emasculation is unnecessary. Bagging can still prevent unwanted pollen deposition. A controlled cross therefore has three distinct purposes: remove or bypass self pollen, exclude foreign pollen, and introduce only the intended pollen when the stigma is receptive. A documented controlled cross can help breeders combine selected characteristics, but hybrid progeny still require biological compatibility and successful fertilisation.
Pollination is deposition; compatible pollen germination initiates pollen-tube growth; fertilisation occurs only when male gamete nuclei fuse with nuclei in the female gametophyte. Self-incompatibility can stop the pathway after deposition.
Quick Quiz
Quiz
Which outbreeding device stops autogamy but does not necessarily stop geitonogamy?
What does self-incompatibility directly prevent?
Where does the pollen tube commonly enter an ovule?
Why is emasculation performed in a controlled cross with a bisexual female-parent flower?
A receptive stigma rejects viable self pollen after deposition. What is the most plausible mechanism?
Practice Problems
- Explain why dioecy prevents both autogamy and geitonogamy while monoecy prevents only the first.
- Trace the route and fate of pollen from a compatible stigma to the egg apparatus.
- Write a controlled pollination protocol for a bisexual flower and explain why each step matters.
- A breeder uses a female flower with no stamens. Which steps of artificial hybridisation can be omitted and which remain necessary?
Dioecious plants have male and female flowers on different individual plants. An individual plant cannot provide both pollen and a receptive stigma, so within-flower and same-plant transfer are impossible. Monoecious plants produce distinct male and female flowers on one plant. This prevents transfer within a bisexual flower but pollen can still move between its flowers.
Key Takeaways
• Selfing may cause inbreeding depression; many flowers possess outbreeding devices. • Different timing and position of anthers and stigmas can reduce autogamy. • Monoecy blocks autogamy but not geitonogamy; dioecy blocks both. • Self-incompatibility genetically rejects certain self-pollen before fertilisation. • Compatible pollen forms a tube that reaches the ovule through the micropyle. • The pollen tube enters a synergid and delivers two male gametes. • Emasculation removes anthers; bagging excludes unwanted pollen in controlled crosses.