Outbreeding devices, pollen-pistil interaction and artificial hybridisation

Why flowers avoid self-pollination, and the four devices

• Most flowers are bisexual, so their own pollen easily reaches their stigma. Continued self-pollination causes inbreeding depression (loss of vigour).
• Timing differs: pollen is released before the stigma is receptive, or the stigma becomes receptive well before the pollen is released.
• Position differs: the anther and stigma are placed apart, so pollen cannot easily reach the stigma of the same flower.
• Self-incompatibility: a genetic mechanism in which the pistil stops self-pollen from germinating, or stops its pollen tube from growing.
• Unisexual flowers: if male and female flowers are on the same plant (monoecious, e.g. castor, maize), autogamy is prevented but geitonogamy is not. If they are on different plants (dioecious, e.g. papaya), both are prevented.

Pollen-pistil interaction

1. 1
   Pollen lands on the stigma. The pistil recognises whether the pollen is compatible (the right kind) or not. Chemicals of the pollen interact with those of the pistil.
2. 2
   Incompatible pollen is rejected: its germination on the stigma, or the growth of its tube in the style, is stopped.
3. 3
   Compatible pollen germinates. A pollen tube grows out through one of the germ pores, and the contents of the pollen grain move into it.
4. 4
   The tube grows through the stigma and style to the ovary. In 2-celled pollen, the generative cell divides into two male gametes on the way; 3-celled pollen carries them from the start.
5. 5★
   The tube enters the ovule through the micropyle, then enters a synergid, guided by the filiform apparatus.

★ key step

Artificial hybridisation: emasculation, bagging and tagging (spotting)

Breeders use this to cross two chosen parents, making sure that only the desired pollen reaches the stigma. Your textbook describes emasculation and bagging; tagging comes from your practical syllabus.

1. 1
   Choose a female parent and a male parent that have the characters you want to combine.
2. 2
   Emasculation: if the female parent has bisexual flowers, remove the anthers from the flower bud with forceps before they dehisce.
3. 3
   Bagging: cover the emasculated flower with a bag, usually of butter paper, so no unwanted pollen reaches its stigma.
4. 4★
   When the stigma becomes receptive, dust it with pollen collected from the chosen male parent, and rebag the flower.
5. 5
   Tagging (practical step, not in the textbook text): tie a label to the flower stalk with the date and the names of both parents, so the cross can be traced.
6. 6
   If the female parent has unisexual flowers, no emasculation is needed: bag the female flower buds before they open.

★ key step

Activity: spotting pollen germination on the stigma (slide or electron micrograph)

• Identify the spot as pollen grains germinating on the stigma.
• Note the features: many pollen grains stuck to the stigma surface, some with pollen tubes growing down into the style.
• Comment: this shows a compatible pollination, the first step of pollen-pistil interaction. The tubes will carry male gametes to the ovules.