Question

3. Compare and contrast megagametogenesis and embryo development in Pinus and angiosperms. You must use, but are not limited to, the following terms: functional megaspore, archegonia, eggs, free nuclear, suspensor, basal cell, developing embryo, megagametophyte, fertilization.

Answer 1

Megagametogenesis is the process of formation of female gametophyte i.e., embryo sac from megaspores. The process involves -

• Megaspores result from the meiotic division of the megaspore mother celll, megasporocyte.
• The megasporocyte undergo meiotic division resulting in the formation of a linear tetrad of haploid megaspores.
• Three of the four megaspores degenrates and one remain functional which lies towards chalza end
• Megaspores give rise to the embryo sac, the female gametophyte.
• the nuclei of megaspore undergo three mitotic division and gives an eight nucleated embryo sac.
• The eight nucleate cell is arranged into the seven celled embryo sac.
• The three cell at the micro[pylar end constitute the egg apparatus comprising of the egg and two synergids.
• At the opposite end of the embryo sa are the three antipodal cells.
• Between two groups of the cells is the large central cell containing the two polar nuclei derived one from each of two groups of four nuclei. The two polar nuclei fuse together to form a diploid secondary nucleus.
• Thus the mature embryo sac is a seven celled structure.

Embryo development -

• The pollen tube passing through the style reaches the ovarian cavity. The pollen tube comes in contact with the ovule and enters the embryo sac through the micropyle or through the chalzal end.
• The pollen tube enters one of the degenerating synergids and discharging its content within the cell.
• The synergid receiving the pollen tube partly degenerates before the pollen tube arrives
• The second synergids degerates later
• Out of the two sperms in the pollen tube, one sperm fertilizes the egg to form te zygote and the other sperm combines with the secondary nucleus to form a triploid 3n nucleus called the primary endosperm nucleus
• The fertilized egg becomes the zygote and develops into an embryo.
• The primary endosperm nucleus after division gives endosperm, that provides nutrients to the developing embryo.