Outbreeding Devices: How Plants Prevent Self-Pollination

Outbreeding devices are special mechanisms in flowering plants that prevent self-pollination and encourage cross-pollination. Even when a flower has both male and female parts, nature often favours pollen from a different plant to increase genetic variation.

These devices matter because cross-pollination produces healthier offspring with better adaptability. Plant breeders study outbreeding mechanisms to design hybrid seed production and to maintain diversity in crops and wild species.

Key Concepts

Dichogamy: Anthers and stigma mature at different times in the same flower. It includes protandry and protogyny.

Herkogamy: Physical barrier between anther and stigma in the same flower prevents self pollen contact.

Self-incompatibility: Genetic mechanism where pollen from the same plant fails to fertilise even if it reaches the stigma.

Monoecy: Male and female flowers separate but on the same plant, as in maize and castor.

Dioecy: Male and female flowers on different plants, as in papaya, ensuring cross-pollination.

Heterostyly: Different lengths of style and stamen in flowers of the same species to promote cross transfer.

How It Works

1. In protandry, anthers release pollen before the stigma becomes receptive, so self pollen is wasted.
2. In protogyny, the stigma is receptive first and may be fertilised by outside pollen before own pollen matures.
3. Physical separation places anthers and stigma at different heights or behind a barrier.
4. Self-incompatibility allows the stigma to recognise self pollen and block pollen tube growth.
5. Unisexual flowers make self-pollination within a flower impossible.
6. Together these mechanisms force pollinators to carry pollen between plants.

Worked Example

In a self-incompatible population, a breeder makes 100 pollinations using self pollen and 100 using cross pollen. Self pollinations produce only 2 seeds in total, average 0.02 seeds per pollination. Cross pollinations produce 1800 seeds, average 18 seeds per pollination. Ratio of cross to self success is 1800 divided by 2 equals 900 times higher. The near complete failure of selfing shows self-incompatibility is a powerful outbreeding device.

Common Misconceptions

• Bisexual flowers always self pollinate. Many have timing or genetic barriers that prevent it.
• Self-incompatibility means the plant is sterile. It is fertile with pollen from a genetically different plant.
• Dioecy and monoecy are the same. Dioecy separates sexes on different plants, monoecy keeps them on one plant.

Real-World Applications

Self-incompatibility is used to produce hybrid seeds without manual removal of anthers in some vegetables. Dioecy in papaya requires farmers to keep some male plants for fruit set in females. Dichogamy in avocado affects orchard design, where type A and type B varieties are planted together. Breeders exploit herkogamy to reduce selfing in experimental crosses.

Outbreeding devices are evolutionary solutions to avoid inbreeding. By separating sexes in time, space, or genetics, plants ensure pollen travels and offspring gain the benefits of variation.