Question

Which of the following is an outbreeding device?
$a$. If pollen release and stigma receptivity are not synchronised.
$b$. If the anther and stigma are placed at different positions so that pollen cannot come in contact with the stigma of the same flower.
$c$. Self-incompatibility which prevents self-pollen (from the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
$d$. Production of the unisexual flower.

• A. $a,b$ and $c$
• B. $b,c$ and $d$
• C. $a,c$ and $d$
• D. $a,b,c$ and $d$

Answer 1

Answer: (D)

Outbreeding devices
• Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower.
• Continued self-pollination result in inbreeding de pression.
• Flowering plants have developed many devices to discourage self-pollination and to encourage cross pollination.
• First Device: In some species, pollen release and stigma receptivity are not synchronised. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.
• This condition is called dichogamy in which stigma and anther matures at different time.
• Second device : In some species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower. This condition is called heterostyly.
• Both these devices prevent autogamy.
• Herkogamy: Non-transfer of pollen from anther to stigma of the same flower due to a mechanical barrier is present between anther and stigma. e.g.: Calotropis (Asclepiadaceae), Aristolochia, Gloriosa superba
• In Calotropis, pollen grains occur in pollinia and lifted by insect only.
• The third device to prevent inbreeding is self-incompatibility or self-sterlity. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
• Intraspecific incompatibility is overcome by mixed pollination.
• Fourth device : To prevent self-pollination plant produces unisexual flowers (Dicliny or unisexuality— Maize and cucurbitaceae).
• If both male and female flowers are present on the same plant such as castor and maize (monoecious), it prevents autogamy but not geitonogamy.
• In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male of female (dioecy). This condition prevent both autogamy and geitonogamy.