Double fertilization and triple fusion

B.Sc. II Semester III
Unit VI

1. B.Sc.- II, Sem- III Unit – VI Embryology of Angiosperm Double Fertilization and triple fusion Dr. Swati Pundkar Assistant Professor Departmnet of Botany Shri Shivaji Science College, Amravati

2. DOUBLE FERTILIZATION AND TRIPLE FUSION Double fertilization was discovered by Nawaschin in 1898 in liliaceous plants, Lilium martagon and Fritillaria tenella .

3. Double fertilization is the fusion of the embryo sac with two male gametes. It occurs in angiosperms. Pollination is the deposition of pollen grains from anthers on the stigma of a flower, facilitating the fertilization, which is one of the major events of sexual reproduction of plants. Pollen contains two cells, a tube cell, which produces the pollen tube, and a generative cell, which produces two sperm cells. Pollen grains are germinated on the stigma, producing the pollen tube, which goes down the style until it reaches the micropyle in the ovule. Once it meets the micropyle, pollen tube bursts, releasing the two sperm cells which are carried through the pollen tube. One of the haploid sperm cells is fertilized with the haploid egg cell in the embryo sac, while the other haploid sperm cell fertilizes the central cell, which is also found in the embryo sac of the female gametophyte.

4. Fertilization of the two gametes is called syngamy. The fertilization of a sperm cell with the central cell is called triple fusion. The embryo sac is fertilized twice, the fertilization of gametes in angiosperms is called double fertilization. After fertilization, the ovule is developed into the seed. The fusion of egg cell with the sperm cell produces the diploid zygote, which is developed into the embryo. The zygote is divide into two cells called terminal cell and basal cell by mitosis. The basal cell divides to produce the suspensor, which provides nutrients from mother plant to the growing embryo. The division of the terminal cell produces the proembryo. Further development of the embryo is arrested until the germination of the seed.

5. Significance of Double Fertilization Two products are obtained as a result of double fertilization. There are chances of polyembryony, and the plant has better chances of survival. Double fertilization gives rise to an endosperm that provides nourishment to the developing embryo. It increases the viability of the seeds of angiosperms. It utilizes both the male gametes produced by the pollen grains.

6. Triple fusion is the fertilization of the two polar nuclei in the embryo sac. It occurs during the double fertilization of angiosperms. While one sperm cell fertilizes the egg cell in the embryo sac, the other sperm cell fertilizes the two polar nuclei found in the central cell. Since the fusion of three haploid nuclei forms a triploid nucleus, the process is called triple fusion. The triploid nucleus serves as the primary endosperm nucleus. It gives rise to the endosperm of the seed, which stores nutrients of the seed. During triple fusion, three types of endosperm development are observed: 1- Nuclear endosperm , 2-Cellular endosperm , and 3-Helobial endosperm Liquid endosperm is developed from the nuclear endosperm development as in coconut water. Coconut meat is an example for the cellular endosperm development and monocots have helobial endosperm development.

7. ( i )Nuclear type:   It is the most common type of endosperm found. Here PEN divides repeatedly (mitotic division) without cytokinesis. It results in the formation of a large number of free nuclei in the cell. A large central vacuole is formed and nuclei get arranged at the periphery. There are more nuclei at the chalazal and micropylar end, compared to the sides. At this stage, cell wall formation takes place from the periphery towards the centre and multicellular endosperm is formed. Examples: Maize, Rice, Wheat, Cotton, Sunflower . In coconut, the cell wall formation is incomplete resulting in the outer multicellular solid endosperm and inner multinucleated (having free nuclei) liquid endosperm (coconut milk).

8. (ii) Cellular type: It is not very common. In this type of development, division of PEN (karyokinesis) is followed by cytokinesis and two cells are formed due to transverse division, giving rise to the chalazal and micropylar chamber. Further division is similar, which leads to the formation of the cellular endosperm. Examples: Petunia, Balsam, Datura. (iii) Helobial type: This type of endosperm development is common in monocotyledons. The first division is similar to cellular endosperm and results in a large micropylar cell and small chalazal cell. The chalazal cell mostly does not divide further and functions as a base cell. The micropylar cell divides further, similar to nuclear endosperm. So it is an intermediate type, a combination of both nuclear and cellular endosperm. Examples:  Eremurus

9. Significance of Endosperm Endosperm is important for the growth of an embryo and stores the food reserve They supply nutrients and provide protection to the developing embryo Endospermic tissues have shown to regulate gene expression and seed germination Endosperm induces signals according to environmental conditions and regulates embryonic growth The endosperm contains cytokinin, which regulates cellular differentiation It may induce abortion of seeds from the genetically mismatched cross