NDA (II) 2023 General Ability Test · Question 69 of 146

Correct — D, One gamete fuses with the egg cell and the other one fuses with a diploid secondary nucleus. This is double fertilisation, and it is unique to the flowering plants. The pollen tube discharges both male gametes into the embryo sac. One fuses with the egg — that is syngamy, and it gives the diploid zygote from which the embryo grows. The second fuses with the secondary nucleus of the central cell, itself diploid because two polar nuclei have already merged; that three-nucleus event is triple fusion and it gives the triploid primary endosperm nucleus, which develops into the food store of the seed. Both gametes are used, which is exactly why the process is called double.

• (a)Both the gametes fuse with a single egg cell — Two male gametes entering one egg would give a triploid zygote and a non-viable embryo. Only one male gamete fuses with the egg; the second has a separate destination inside the embryo sac.
• (b)Both the gametes fuse with two different egg cells — There is only one egg cell in a normal embryo sac. It sits at the micropylar end flanked by two synergids, so a second egg for the second gamete simply does not exist.
• (c)One gamete fuses with the egg cell and the other one eventually degenerate — That describes what happens in many gymnosperms, where only one male gamete is functional. In angiosperms the second gamete has a definite job — it forms the endosperm — and if it degenerated the seed would have no nutritive tissue.

Double fertilisation is the defining reproductive event of angiosperms. A pollen grain landing on the stigma germinates and sends a pollen tube down the style into the ovary and through the micropyle into the ovule. The tube releases two male gametes into the embryo sac. Syngamy fuses one with the egg to make the zygote; triple fusion fuses the other with the diploid secondary nucleus to make the primary endosperm nucleus. The zygote becomes the embryo, the endosperm feeds it, the ovule becomes the seed and the ovary becomes the fruit.

Count the parts before you choose. The embryo sac normally holds one egg, two synergids, three antipodal cells and one central cell whose two polar nuclei have fused into a secondary nucleus. With one egg available, options (a) and (b) are arithmetically impossible. That leaves whether the second gamete works or wastes away, and the phrase 'diploid secondary nucleus' in option (d) is the giveaway — it names precisely the structure the second gamete targets.

• The pollen tube delivers two male gametes into the embryo sac, and both are used — hence the name double fertilisation.
• Syngamy is the fusion of one male gamete with the egg, producing a diploid (2n) zygote.
• Triple fusion is the fusion of the second male gamete with the diploid secondary nucleus, producing a triploid (3n) primary endosperm nucleus.
• The endosperm formed from that nucleus is the nutritive tissue of the seed, and double fertilisation is found only in angiosperms.

• Assuming both gametes must go to the egg because both were produced together.
• Forgetting that the secondary nucleus is already diploid, so the endosperm ends up triploid rather than diploid.
• Carrying the gymnosperm pattern, where the spare gamete degenerates, into angiosperms.

Asked as a mechanism item like this one, as a ploidy item on the endosperm, or as a sequence item on the pollen tube's route.

• practice — not a real PYQ

  The endosperm of a typical angiosperm seed is

  • (a)haploid
  • (b)diploid
  • (c)triploid
  • (d)tetraploid

  Answer(c) triploid — it arises from the fusion of one male gamete with the diploid secondary nucleus.

• practice — not a real PYQ

  The fusion of the second male gamete with the secondary nucleus of the embryo sac is called

  • (a)syngamy
  • (b)triple fusion
  • (c)parthenogenesis
  • (d)apomixis

  Answer(b) triple fusion — three nuclei are involved, giving the primary endosperm nucleus.