1. Spatial Separation of Male and Female Parts:
* Dichogamy: This involves timing the maturation of male (pollen) and female (stigma) parts differently.
* Protandry: Male parts mature first (e.g., lilies, sunflowers).
* Protogyny: Female parts mature first (e.g., some orchids, willows).
* Unisexuality: Plants have either male or female flowers (e.g., corn, squash).
2. Structural Barriers:
* Self-incompatibility: This is a genetic mechanism where a plant's pollen is rejected by its own stigma. This can be controlled by:
* Gametophytic Self-Incompatibility (GSI): Pollen grain itself determines compatibility.
* Sporophytic Self-Incompatibility (SSI): The stigma recognizes and rejects pollen based on the genotype of the parent plant that produced it.
* Physical barriers: Some flowers have structures that physically prevent self-pollination, such as:
* Anthers positioned below the stigma: Pollen falls away from the stigma (e.g., some grasses).
* Stigma lobes that curve away from the anthers: Preventing self-pollination (e.g., some orchids).
3. Chemical cues:
* Pollen-stigma recognition: Chemical compounds on the stigma can recognize and reject pollen from the same plant.
* Pollinator behavior: Some plants produce different scents or nectar rewards for pollinators depending on whether the flower is self-compatible or not, influencing pollinator behavior and promoting cross-pollination.
4. Other mechanisms:
* Self-pruning: Plants may abort their own flowers or pollen before they can fertilize the same plant.
* Plant architecture: Some plants have separate male and female individuals, or their flowers are positioned in a way that minimizes self-pollination.
Why is self-fertilization avoided?
* Increased genetic diversity: Cross-pollination with other individuals leads to a broader range of genetic combinations, enhancing adaptability to changing environments and disease resistance.
* Reduction of inbreeding depression: Self-fertilization can lead to accumulation of harmful recessive genes, which can reduce fitness.
Exceptions:
While many plants actively avoid self-fertilization, some species have evolved to be self-compatible or even self-fertilizing. This can be beneficial in environments where pollinators are scarce or unreliable.
In conclusion, plants employ a diverse array of mechanisms to avoid self-fertilization, ensuring genetic diversity and maintaining the health and fitness of their species.
