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Alternation of Generations

by Ramneet Kaur
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Alteration of Generations

Alternation of Generations is also known as metagenesis. In Plants, there are 2 phases of the life-cycle:

  1. Gametophyte/sexual phase.
  2. Sporophyte/asexual phase
  • The zygote(2n) divides to form the sporophyte(2n) and spores(n) divide to form the gametophyte(n).
  • The sporophyte plant is diploid(2n), it produces spores(n) after meiosis.
  • The gametophyte plant is haploid(n), it produces gametes(n) that undergo fertilization to form the zygote(2n).
  • These 2 phases alternate each other in the life-cycle.

There are 3 types of life-cycles, depending upon the duration of gametophyte & sporophyte generations.

  • Haplontic.
  • Haplodiplontic/Diplohaplontic.
  • Diplontic.

Haplontic:

  • The plant body is gametophyte.
  • The main part of life-cycle is gametophyte.
  • Sporophyte generation is very reduced & is represented by a single cell i.e., the zygote that immediately undergoes meiosis.
  • Zygotic meiosis occurs.
  • Seen in most Algae.

HAPLONTIC

Haplodiplontic:

  • The plant body is the gametophyte.
  • Both sporophyte & gametophyte generations are well represented in the life-cycle.
  • Sporic meiosis occurs as meiosis occurs before spores formation.
  • Seen in Bryophytes, red Algae, green algae (Ulva), most brown algae.

haplodiplontic/diplohaplontic

Diplohaplontic:

  • The plant body is the sporophyte.
  • Both sporophyte & gametophyte generations are well represented in the life-cycle.
  • Sporic meiosis occurs.
  • Seen in Pteridophytes.

Diplontic:

  • The plant body is the sporophyte.
  • The main part of the life-cycle is sporophyte.
  • Have a reduced gametophytic generation.
  • Gametic meiosis is seen i.e., meiosis occurs before gametes formation.
  • Seen in Gymnosperms, Angiosperms & some green algae (Acetabularia, Vaucheria), brown algae (Fucus, Sargassum) & diatoms.

DIPLONTIC

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