Sexual reproduction in yeast

March 17, 2019

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.

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.

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.

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.

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