Outbreeding Devices

Most flowering plants (angiosperms) produce hermaphrodite (bisexual) flowers — flowers carrying both male (anther) and female (stigma) parts. Pollen grains can therefore easily reach the stigma of the same flower (autogamy) or another flower on the same plant (geitonogamy).

• Prolonged self-pollination leads to inbreeding depression - a gradual decline in biological fitness, vigour, and reproductive capacity due to the accumulation of harmful recessive alleles in homozygous individuals.
• To counter this, flowering plants have evolved a suite of outbreeding devices (contrivances) - structural, temporal, or genetic mechanisms that discourage self-pollination and encourage cross-pollination (xenogamy), thereby maintaining genetic diversity.

In a bisexual flower, the anther and stigma mature at different times, ensuring that self-pollen is not available when the stigma is receptive - or vice versa. This is purely a time-based device.

In some bisexual flowers, the anther and stigma are physically placed at different positions within the same flower, creating a natural mechanical barrier that prevents pollen from reaching the stigma of the same flower.

Mechanism: A structural barrier - either a difference in height, orientation, or organ morphology - physically separates the male and female organs. Pollination can only occur via a pollinator that bridges this gap from a different flower.

Examples:

• In Calotropis - the pentangular stigma (pollinium) is positioned above the level of the anthers
• In family Caryophyllaceae - the style is much longer than the stamens
• In Gloriosa - anthers dehisce at a distance so stigma is out of reach of its own pollen

A genetic mechanism in which pollen from the same flower - or from another flower on the same plant - is recognised and rejected by the pistil. The self-pollen either cannot germinate on the stigma, or the pollen tube is arrested inside the style before reaching the ovule.

Molecular Mechanism: Plants use S-alleles (S-locus genes). If the S-allele carried by the pollen matches that of the stigma tissue, the plant biochemically blocks fertilisation at one of two stages:

• Stage 1 (Stigma level): Pollen germination is inhibited on the stigma surface
• Stage 2 (Pistil level): Pollen tube growth is arrested within the style or pistil

Examples: Thea (Tea), Nicotiana (Tobacco), Petunia, Solanum melongena (Brinjal)

When a plant bears only male flowers (staminate) or only female flowers (pistillate), self-pollination becomes structurally impossible. Bisexual flowers are absent; the two sexes are separated at the flower level.

Pollen grains from another flower or different plant germinate more rapidly on the stigma than pollen grains from the same flower. This gives foreign (cross) pollen a competitive physiological advantage, even when self-pollen is also present.

Mechanism: The stigma's biochemical environment preferentially activates cross-pollen tube growth. Self-pollen does not receive the same stimulatory signals, so it germinates more slowly and is outcompeted.

Examples: Malus domestica (Apple), Grape (Vitis)

In some plant species, individual plants produce flowers in two (dimorphic) or three (trimorphic) morphological forms that differ in the relative lengths of the style and filaments. This ensures that pollen from one morph can only effectively reach the stigma of the other morph, promoting cross-pollination.

Classic Example: Primula (Primrose):

When a pollinator visits both flower types, pollen is deposited at different body positions - and each position matches the stigma height of the reciprocal morph. Only pollen deposited at the matching level germinates successfully (controlled by S-alleles associated with style morphology).

Heterostyly: A. long-styled flower; B. short-styled flower