Sexual reproduction in yeast

  • Yeast is a unicellular fungus which varies in shape with the species and even within the species.
  • They may be oval, spherical and elliptical in shape.
  • They are microscopic and appear as a pinhead under the high power of microscope.
  • They mostly grow on media rich in sugar such as juice of grapes, sugarcane, toddy, decaying vegetables, ripe fruits and nectar of flowers.
  • They are also found in air and humus of soil.
  • Some yeast grows as parasites on plant and animal bodies.
  • Yeast mostly reproduces by asexual method. Sometimes it also reproduces by sexual method.

Sexual reproduction

  • This reproduction occurs at the end of growing season under unfavorable conditions.
  • It takes place by conjugation between two haploid somatic cells or ascospores.
  • As a result, formation of diploid zygote nucleus takes place.
  • This diploid nucleus either gives rise to diploid somatic cells by budding or it may behave as an ascus to form 4 or 8 haploid ascospores by meiotic division.
  • Depending upon the mode of sexual reproduction, three types of life cycles are recognized in yeast.

3 : Sexual and vegetative reproduction of the yeast Saccharomyces... |  Download Scientific Diagram

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A) Haplobiontic life cycle

  • In Schizosaccharomyces octosporus, the diploid (2n) phase is very short.
  • It is represented by zygote only whereas the haploid phase is prolonged.
  • During sexual reproduction two cells come in contact which develops short protuberances at the point of contact.
  • The partition wall between the protuberances gets dissolved to form a conjugation tube.
  • The nuclei of two cells gradually come into this conjugation tube and fuse to form a zygote (2n).
  • The diploid zygote thus formed grows and behaves as an ascus.
  • The zygote nucleus divides meiotically to form 4 haploid nuclei.
  • Each of these again divides mitotically to form 8 nuclei.
  • Thus, eight ascospores are formed in each ascus which are set free by rupturing ascus wall and become independent yeast cell.

Saccharomyces Life cycle, Morphology, Economic Importance.

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B)Diplobiontic life cycle

  • In certain species like Saccharomycodes ludwigii, the haploid phase is short which is represented by ascospores.
  • Here the diploid phase is prolonged and represented by somatic cell.
  • The diploid somatic cell directly behaves as ascus.
  • The nucleus divides meiotically to form four haploid ascospores which conjugate in pairs within the ascus to form 2 diploid zygotes.
  • The zygote develops a gem tube which cuts off diploid yeast cells.

Reproduction in Yeast | Notes, Videos, QA and Tests | Grade 11>Biology>Yeast  | Kullabs

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C) Haplo-diplobiontic life cycle

  • It occurs in Saccharomyces cerevisiae, where both the phases haploid and diploid are more or less equally represented.
  • Both the phases possess somatic cells which reproduce by budding.
  • During sexual reproduction two haploid somatic cells conjugate to produce a diploid zygote.

Saccharomyces Life cycle, Morphology, Economic Importance.

                                                                           Image source: microbiologynote

  • It multiplies by budding forming diploid somatic cells.
  • Lastly, each diploid cell behaves as an ascus and divides meiotically to form four haploid ascospores.
  • After liberation, each ascospore behaves as haploid somatic cell and multiplies by budding.





Sexual reproduction in yeast