Biology OUTBREEDING DEVICES AND ARTIFICIAL FERTILISATION

### KEY TOPICS

star Outbreeding devices
star Artificial Fertilisation
star Double Fertilisation

### OUTBREEDING DEVICES

● Majority of flowering plants produce color{Violet}"hermaphrodite flowers" and pollen grains are likely to come in contact with the stigma of the same flower.

● Continued color{Violet}"self-pollination" result in color{Violet}"inbreeding depression".

● Flowering plants have developed many devices to color{Violet}"discourage self pollination" and to color{Violet}"encourage cross-pollination".

● In some species, pollen color{Violet}"release" and stigma color{Violet}"receptivity" are not color{Violet}"synchronised".

● Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.

● In some other species, the anther and stigma are placed at color{Violet}"different positions" so that the pollen cannot come in contact with the color{Violet}"stigma of the same flower."

● Both these devices color{Violet}"prevent autogamy".

● The third device to prevent inbreeding is color{Violet}"self-incompatibility".

● This is a color{Violet}"genetic mechanism" and prevents color{Violet}"self-pollen" (from the same flower or other flowers of the same plant) from fertilising the ovules by color{Violet}"inhibiting pollen germination" or pollen tube growth in the pistil.

● Device to prevent self-pollination is the production of color{Violet}"unisexual flowers".

● If both male and female flowers are present on the same plant such as color{Violet}"castor and maize"( color{Violet}"monoecious"), it prevents autogamy but color{Violet}"not geitonogamy".

● In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female ( color{Violet}"dioecy").

● This condition prevents both autogamy and geitonogamy.

### ARTIFICIAL HYBRIDISATION

● Artificial hybridisation is one of the color{Violet}"major approaches" of color{Violet}"crop improvement" programme.

● In such crossing experiments it is important to make sure that only the color{Violet}"desired pollen grains" are used for pollination and the stigma is protected from color{Violet}"contamination" (from unwanted pollen).

● It is achieved by color{Violet}"emasculation and bagging" techniques.

● If the female parent bears color{Violet}"bisexual flowers", color{Violet}"removal of anthers" from the flower bud before the anther dehisces using a pair of forceps is necessary. This step is referred to as color{Violet}"emasculation".

● Emasculated flowers have to be covered with a color{Violet}"bag of suitable size", generally made up of color{Violet}"butter paper", to prevent contamination of its stigma with unwanted pollen.

● This process is called color{Violet}"bagging".

● When the stigma of bagged flower attains color{Violet}"receptivity", mature pollen grains collected from anthers of the male parent are color{Violet}"dusted on the stigma", and the flowers are color{Violet}"rebagged", and the fruits allowed to develop.

● If the female parent produces color{Violet}"unisexual flowers", there is color{Violet}"no need" for emasculation.

● The female flower buds are color{Violet}"bagged before the flowers open".

● When the stigma becomes color{Violet}"receptive", pollination is carried out using the color{Violet}"desired pollen" and the flower color{Violet}"rebagged".

### DOUBLE FERTILISATION

● After entering one of the color{Violet}"synergids", the pollen tube color{Violet}"releases" the two color{Violet}"male gametes" into the cytoplasm of the synergid.

●One of the male gametes moves towards the color{Violet}"egg cell" and fuses with its color{Violet}"nucleus" thus completing the color{Violet}"syngamy".

● This results in the formation of a color{Violet}"diploid cell", the color{Violet}"zygote".

● The other male gamete moves towards the color{Violet}"two polar nuclei"located in the color{Violet}"central cell
" and fuses with them to produce a color{Violet}"triploid primary endosperm nucleus" (color{Violet}"PEN").

● As this involves the fusion of three haploid nuclei it is termed color{Violet}"triple fusion".

● Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed color{Violet}"double fertilisation", an event color{Violet}"unique to flowering plants".

● The central cell after triple fusion becomes the color{Violet}"primary endosperm cell (PEC)" and develops into the color{Violet}"endosperm" while the zygote develops into an color{Violet}"embryo".