Outbreeding Devices

Continued, generation-after-generation self-pollination tends to reduce genetic diversity and can lead to inbreeding depression, a decline in vigour and fitness in the offspring. To counter this, plants have evolved a range of structural and physiological 'contrivances' that discourage or altogether prevent self-pollination and instead promote cross-pollination, thereby preserving genetic diversity that is essential fuel for natural selection and long-term evolutionary adaptability.

Unisexuality : When a plant bears only male flowers or only female flowers (never both on one flower), self-pollination within a flower is structurally impossible. Depending on whether both flower types occur on the same individual plant or on separate individual plants, such species are termed monoecious (e.g., maize, which bears separate male and female inflorescences on one plant) or dioecious (e.g., mulberry, papaya, where individual plants are entirely male or entirely female).

Dichogamy : In a bisexual flower the anthers and the stigma mature at different times, so self-pollen never meets a receptive stigma of the same flower. It is of two types:

1. Protandry : the androecium matures earlier than the gynoecium, e.g. the disc florets of sunflower.

2. Protogyny : the gynoecium matures earlier than the androecium, e.g. Gloriosa.

Prepotency : Pollen grains arriving from other flowers germinate and grow their pollen tubes more rapidly on the stigma than the flower's own pollen does, so that cross-pollen effectively outcompetes self-pollen even when both land on the same stigma, as in apple.

Heterostyly (heteromorphy) : In species like Primula (primrose), a population contains two or three distinct floral forms in which the relative heights of the stigma and anthers differ between forms. Because pollen from one form's anthers is positioned to match only a stigma at the same relative height (found on a different plant), effective self-pollination within one flower, or even between two flowers of the same form, is hindered.

Herkogamy : A purely mechanical/structural barrier physically separates the anther and stigma within a bisexual flower so that self-pollen cannot easily contact the flower's own stigma, as in Calotropis, where the pentangular stigma sits above the level of the anthers (pollinia).

Self incompatibility (self sterility) : A genetic mechanism, mediated by specific recognition proteins, actively inhibits the germination or growth of a flower's own pollen on its own stigma, even though the pollen and stigma are structurally in contact, as in tobacco and tea (Thea).