Question

What is meant by the monosporic development of female gametophyte? Explain the development of female gametophyte with well-labelled diagrams.

Answer 1

Hint: It is considered monosporic growth if the female gametophyte (embryo sac) emerges from a single megaspore. The megaspore mother cell typically divides by meiosis in most angiosperms to form four haploid megaspores arranged in a linear manner.

Complete answer:
The development of the female gametophyte is considered monosporic growth from a single functional megaspore. The functional megaspore nucleus breaks mitotically to form two nuclei that shift to opposite poles. A two-nucleate embryo sac forms this. The development of the four-nucleate and eight-nucleate stages of the embryo sac results in two further consecutive mitotic nuclear divisions. These mitotic divisions are nuclear free forms, in which cytokinesis is not directly accompanied by karyokinesis. The walls are laid down after the eight-nucleate point, contributing to the organisation of the standard gametophyte female.

Six out of eight nuclei are surrounded and organized into cells by cell walls; the other two nuclei, known as polar nuclei, are found in the large central cell below the egg apparatus. 8 nuclei and seven cells are a standard angiosperm embryo sac at maturity.
At the end of the micropylar forming egg apparatus lie three cells. Three ends, called antipodal cells, at the chalazal end. A single wide central cell containing two polar nuclei.

    

     Development of female gametophyte

Note: Bisporic formation results from cytokinesis deficiency after meiosis II. As a consequence, two cells each containing two megaspore nuclei are composed of the megaspore tetrad. From one of these dyad cells, the embryo sac grows by two rounds of mitotic division and the other dyad cell is silenced.