In angiosperms the correct sequence of events in formation of female gametophyte in the ovule is (A) 3 successive free nuclear divisions in functional megaspores. (B) Degeneration of 3 megaspores. (C) Meiotic division in megaspore mother cell. (D) Migration of 3 nuclei towards each pole. (E) Formation of wall resulting in seven celled embryo sac. Choose the correct answer from the options given below. (NEET 2023 Phase-2)

Solution & Explanation

Answer: B. Correct order of female gametophyte formation: (C) meiotic division of the megaspore mother cell, producing four megaspores; (B) degeneration of three of the four megaspores leaving one functional; (A) three successive free-nuclear mitotic divisions of the functional megaspore giving the 2-, 4- and 8-nucleate stages; (D) migration of nuclei (synergids/egg toward micropylar pole, antipodals toward chalazal pole, polar nuclei to centre); (E) cell-wall formation organising the 7-celled, 8-nucleate embryo sac. This gives (C),(B),(A),(D),(E) = option (2). NCERT: Ch 1, p.11, lines 5-12 — "The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac... After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac."