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Sexual Sporulation in the Oomycota

Overview of the life cycle of fungi in the Oomycota:
Life cycle of fungi in the Oomycota.
  • SEXUAL SPORULATION involves the formation of specialised reproductive structures (ANTHERIDIA and OOGONIA) in which MEIOSIS results in the fomation of HAPLOID NUCLEI (GAMETES).
  • The FUSION of compatible gametes is a preliminary to the formation of DIPLOID OOSPORES, which upon germination return us to the DIPLOID SOMATIC PHASE
Formation of oogonia and antheridia:
Diagram illustrating formation of oogonia and antheridia.
  • OOGONIA are usually SPHERICAL and their entire contents differentiate into UNINUCLEATE HAPLOID OOSPHERES.
  • In contrast, ANTHERIDIA are ELONGATE, MULTINUCLEATE structures which originate either:
    • on the SAME hyphal branch as the oogonium (HOMOTHALLIC SPECIES)
    • on a DIFFERENT hyphal branch within the same colony (HOMOTHALLIC SPECIES)
    • or on a hypha from an entirely DIFFERENT COLONY (HETEROTHALLIC SPECIES).
  • In HETEROTHALLIC SPECIES each colony produces 99th oogonia and antheridia, but the oogonia can be fertilized only by antheridia from another colony - i.e. self-fertilization is impossible in heterothallic species due to incompatibility.
  • One or more antheridia may become attached to the oogonium.
  • The antheridia branch to form FERTILIZATION TUBES, sending one into each oosphere within the oogonium.
  • One nucleus from the antheridium passes through each fertilization tube into an oosphere where it fuses with the oosphere nucleus to form a DIPLOID ZYGOTE.
  • Each fertilized oosphere now develops a THICK WALL and becomes converted into an OOSPORE.
  • After a period of dormancy the oospore germinates and the resulting germ-tube eventually gives rise to a DIPLOID MYCELIUM.

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