Question

Polygonum type of embryo sac is?
A. 8-nucleate, 7-celled
B. 8-nucleate, 8-celled
C. 7-nucleate, 7-celled
D. 4-nucleate, 3-celled

Answer 1

From one functional megaspore, the female gametophyte i.e., embryo sac forms. To create nucleate embryo sacs, this megaspore undergoes three consecutive mitotic divisions.

Complete Answer:
- "The embryo sac or female gametophyte is an oval component located in the flowering plant ovule." As the haploid megaspore nucleus splits, an embryo sac is said to develop.
- It has two nuclei of haploids and six cells of haploids that don't have any cell walls. The haploid nuclei join to form a single (endosperm motor cell) cell in some situations.
- In the case of fertilisation, for the forming of zygote leading to embryo development, one male nucleus and one egg nucleus combine.
- By the fusion of the main endosperm nucleus and the second male nucleus, the endosperm nucleus develops.

Now, let us find the solution from the options-
Option A: Two nuclei are produced by the first mitotic division in the megaspore. One nucleus travels toward the end of the micropylar while the other nucleus travels toward the end of the chalazal.
- Then, at their respective ends, these nuclei divide and re-divide to form 8 nucleate phases. Consequently, at both ends, i.e. at the micropylar and chalazal end of the embryo sac, there are 4 nuclei each.
- Just three out of the four nuclei at the micropylar end divide into two synergids and one egg cell. Together, they are called the apparatus of the egg. Similarly, three out of four nuclei separate at the chalazal end as antipodal cells.
- At the micropylar and the chalazal end, the remaining two cells migrate toward the middle and are referred to as the polar nuclei, which are located in a large central cell. Thus, the female gametophyte occurs as a 7-celled form at maturity, and it has 8 nucleates. Therefore, the option A is correct.

Option B, C and D: The female gametophyte has 3 antipodal cells + 2 synergids + 1 egg cell + 1 central cell at the time of fertilisation, which is equivalent to the composition of 7 cells. But there are 2 polar nuclei in the central cell, which makes it 8-nucleated. At maturity, however, the gametophyte is 7-celled and 8-nucleated. Therefore, from this we came to know that options B, C. and D are incorrect.

Thus, the correct answer is option (A) 8-nucleate, 7-celled.

Note: The development of the embryo sac happens in two stages: megasporogenesis happens in the first stage, where the haploid megaspore tetrad develops due to a single diploid mother cell's meiosis.