Q.Embryo sacs of some apomictic species appear normal but contain diploid cells. Suggest a suitable explanation for the condition.

In apomictic species, the embryo sac develops without undergoing meiosis, a process called apomeiosis. This means the megaspore mother cell either skips the reduction division entirely or the meiotic divisions fail to reduce the chromosome number. As a result, the cells inside the embryo sac—such as the egg cell, synergids, and antipodals—remain diploid instead of becoming haploid. The embryo sac thus appears structurally normal (with the usual seven cells and eight nuclei) but contains diploid cells throughout. This condition allows the diploid egg cell to develop into an embryo without fertilisation, which is t …

Apomixis is a fascinating reproductive strategy where plants produce seeds without fertilisation. In many apomictic species, the embryo develops directly from the maternal tissues, bypassing the usual fusion of male and female gametes. The NCERT textbook explains that in some of these plants, the embryo sac — the female gametophyte — looks perfectly normal under the microscope. It has the usual seven cells and eight nuclei, just like a sexually reproducing plant. But here is the twist: the cells inside that embryo sac are diploid, not haploid.

How does this happen? The key lies in the formation of the embryo sac itself. In a typical sexual plant, the megaspore mother cell (MMC) undergoes meiosis to produce four haploid megaspores, three of which degenerate. The surviving one then undergoes three rounds of mitosis to form the eight-nucleate embryo sac. In the apomictic species you are asking about, the MMC either skips meiosis entirely or the reduction division fails. Instead of meiosis, the MMC undergoes mitosis directly. This means the resulting megaspore — and all the cells that come from it — remain diploid. The embryo sac thus looks structurally normal but is genetically identical to the mother plant.

Another mechanism, also described in the NCERT, involves the nucellus cells. In some apomicts, a diploid cell from the nucellus — the tissue surrounding the embryo sac — simply enlarges and takes over the role of the egg cell. This cell then develops into an embryo without any fertilisation. The original embryo sac may still form, but it is the nucellar cell that gives rise to the next generation. In such cases, the embryo sac itself might be haploid or degenerate, but the seed contains a diploid embryo from the nucellus. …