Outbreeding devices and pollen-pistil interaction

Most flowering plants have bisexual flowers, so pollen easily reaches the stigma of the same flower; continued self-pollination leads to inbreeding depression. Plants have evolved devices that discourage selfing and favour cross-pollination.

Device 1: pollen release and stigma receptivity are not synchronised; either pollen is shed before the stigma is receptive or the stigma is receptive long before pollen is shed. Device 2: anther and stigma sit at different positions. Both prevent autogamy.

Device 3: self-incompatibility, a genetic mechanism that stops pollen from the same flower or the same plant from fertilising ovules by blocking pollen germination or pollen tube growth in the pistil.

Device 4: unisexual flowers. Male and female flowers on the same plant (monoecious, as in castor and maize) prevent autogamy but not geitonogamy. Male and female flowers on separate plants (dioecious, as in papaya) prevent both.

The pistil can recognise pollen as compatible (right type, same species) or incompatible. Compatible pollen is accepted and post-pollination events follow; incompatible pollen is rejected by blocking germination on the stigma or tube growth in the style. This recognition is a chemical dialogue between pollen and pistil components.

After compatible pollination the grain germinates on the stigma and sends out a pollen tube through one germ pore; the grain's contents move into the tube, which grows through the stigma and style to the ovary.

If pollen was shed 2-celled, the generative cell divides into two male gametes while the tube grows; 3-celled pollen carries both gametes from the start. The tube enters the ovule through the micropyle and a synergid through the filiform apparatus, which guides its entry.

Pollen-pistil interaction covers all events from pollen landing on the stigma to the pollen tube entering the ovule. Understanding it helps breeders obtain desired hybrids even from incompatible crosses.

In artificial hybridisation only the chosen pollen may reach the stigma. For a bisexual female parent, the anthers are removed from the bud with forceps before they dehisce (emasculation), and the flower is covered with a bag, usually of butter paper (bagging). When the stigma is receptive, pollen from the male parent is dusted on it and the flower is rebagged.

If the female parent has unisexual flowers, emasculation is not needed: the female buds are simply bagged before opening, pollinated when receptive, and rebagged.