- 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.
- 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.
Image source: researchgate
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.
Image source: microbiologynote
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.
Image source: kullabs
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.
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