Outbreeding devices (contrivances)

 Outbreeding devices (contrivances)

• Mechanisms that helps in preventing self pollination are called Outbreeding devices.

• To promote cross pollination and increase  genetic diversity.

• plants have modified  a wide variety of sexual strategies.

• Genetic diversity is an essential to evolution.

• Continued self pollination causes inbreeding depression. 

• Thus plants have developed many devices to promote cross pollination

Outbreeding devices are.

1)      Unisexuality :

2)      Dichogamy :

                                i.            Protandry

                              ii.            Protogyny

3)      Prepotency :

4)      Heterostyly (heteromorphy):

5)      Herkogamy :

6)      Self incompatibility (self sterility):.

For video lecture watch this video 

1. Unisexuality :

• Plant bears either male or female flowers.

• It is also called as dioecism

• As flowers are unisexual, self pollination is not possible.

• Plants may be monoecious, e.g. Maize or dioecious, e.g. Mulberry, Papaya.

• Anthers and stigmas mature at different times in a bisexual flower so as to prevent self  pollination.

• It can be further divided into two types:

2. Dichogamy :

                                i.            Protandry: Androecium matures earlier than the gynoecium, e.g. in the disc florets of sunflower.

                              ii.            Protogyny : Gynoecium matures earlier than the androecium, e.g. Gloriosa.

3. Prepotency :

Pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower, e.g. Apple.

4. Heterostyly (heteromorphy):

• In some plants like Primula (Primrose, there are two or three  types of flowers in which stigmas and anthers are placed at different levels (heterostyly and heteroanthy).

• This prevents the pollens from reaching the stigma and pollinating it.

• In heteromorphic flowers, pollen grains produced from anther pollinate stigmas produced at the same level.

• It is a mechanical device to prevent self pollination in a bisexual flower.

• In plants, natural physical barrier is present between two sex organs and avoid contact of pollen with stigma of same flower.

• e.g. Calotropis-pentangular stigma is positioned above the level of anthers (pollinia).

• This is a genetic mechanism due to which the germination of pollen on stigma of the same flower is inhibited,

• e.g. Tobacco, Thea.

5. Herkogamy :

6. Self incompatibility (self sterility):