Question

In a case if polyembryony, if an embryo develops from the synergid and another from the nucellus, then the synergid embryo is (i) and nucellar embryo is (ii)
A. (i) triploid, (ii) haploid
B. (i) diploid, (ii) haploid
C. (i) haploid, (ii) diploid
D. (i) diploid, (ii) triploid

Answer 1

Hint: Polyembryony is the condition where more than one embryo develops within a seed. This phenomenon can be seen in animals, plants, and humans, although in the case of humans, it is rarer.

Complete answer:
Generally, embryos are formed when a male gamete with the ploidy of n, fertilizes the female gamete (egg), which too has a ploidy of n, to form a zygote with ploidy of 2n. When this zygote develops into an embryo, its ploidy remains 2n.
However, when an embryo forms from cells other than the egg cell, it occurs without fertilization with a male gamete. Due to the lack of fertilization, the ploidy of embryos formed from such cells remains the same as that of the cell from which it has been formed.
In this case, the embryos are formed from-
> Synergid cell: Synergid cells are haploid, i.e. they have a ploidy of n. Therefore, an embryo formed from a synergid cell will be haploid as well since it doesn’t undergo fertilization with a male gamete.
> Nucellus: Nucellus is diploid in nature, i.e. it has a ploidy of 2n. Therefore, an embryo formed from a nucellus will be diploid in nature too as it doesn’t undergo fertilization either.
In a case of polyembryony, if an embryo develops from the synergid and another from the nucellus, then the synergid embryo is (i) haploid, and nucellar embryo is (ii) diploid.

Thus, the correct answer to this question is option ( C ) (i) haploid, (ii) diploid.

Additional Information: Important terms related to asexual reproduction in plants-
> Apomixis: Replacement of the usual sexual reproduction in plants with asexual reproduction.
Apomixis can be of two types-
> Vegetative reproduction: Reproduction is through parts of the plant other than its seed.
> Agamospermy: Reproduction is through the seed, although the formation of embryo occurs without the normal meiosis and syngamy.

Note: Some other important points to remember-
> Polyembryony was discovered by Antonie van Leeuwenhoek in 1719. He first observed this phenomenon in citrus plants when their seeds showed to have two germinating embryos.
> Besides nucellus and synergids, embryos can also be formed from integument, which is a protective layer of cells surrounding the ovule.
> All cells inside the ovary sac are haploid (n), except for the secondary nucleus which is diploid (2n). > Secondary nucleus is formed by the fusion of the two haploid polar cells.
> On the other hand, all saprophytic cells like nucellus and integument are diploid (2n) in nature. Therefore, embryos formed by these cells will be diploid too.