3 of Life Cycles Found in Yeast | Biology

Some of the important types of life cycles found in yeast are as follows:
Guilliermond (1940) has recognized three main types of life cycles in the yeast. These life cycles are known as — 1. Haplobiontic, 2. Diplobiontic and 3. Haplo- diplobiontic.
1. Haplobiontic type:
This type of life cycle is found in Schizosaccharomyces octosporus and some other yeasts. In these yeasts the vegetaive stage is predominantly haploid and the diploid stage is very short. The diploid stage is represented by the zygote cell only which undergoes meiosis immediately after nuclear fusion.
Here each somatic cell acts as a potential gametangium. During sexual union two cells fuse (plasmogamy) and this is followed by the fusion of the two nuclei (karyogamy).

The fusion or zygote nucleus divides thrice of which the first division is meiotic one. Now the zygote cell becomes ascus containing eight ascospores. After their liberation from the ascus the ascospores behave as vegetative cells.
2. Diplobiontic type:
This type of life-cycle is found in Saccharomycodes ludwigii and some other yeasts. In these yeasts the vegetative stage is predominantly diploid and the haploid stage is very short. In this type the ascospores are not liberated from the ascus but they copulate within the ascus. The zygotes found within the ascus bud out diplobiontic vegetative cells.
3. Haplo-diplobiontic type:
This type of life cycle is found in Saccharomyces cerevisiae and some other yeasts. In these yeasts the vegetative cells consist of both haploid and diploid phases. The haploid cells copulate and form a dipolid cell (zygote). The zygote nucleus divides thrice of which the first division is meiotic one, producing eight nuclei which metamorphose into eight ascospores. Only the diploid cells produce the ascospores which give rise to haploid vegetative cells by budding.

Some more types of copulation methods are also found in yeasts. In certain yeasts there is copulation between two adjoining sister cells; this is known as adelphogamy. Here the cells involved in copulation do not separate after fusion and remain united to form short chains, e.g., Schizosaccharlnyces mellacei and S. pombe. Another type of copulation takes place between the mother and the daughter cells formed by budding; this method is known as pedogamy
In this type the daughter bud remains continuous with the mother cell and the nucleus from the daughter bud is being transferred into the mother cell. Both the nuclei fuse together forming diploid (2n) nucleus. The fusion nucleus divides meiotically forming 4 nuclei; three of which degenerate and remaining one develops into an ascospore. This process shows anisogamy and occurs in a few yeasts such as Nadsonia and Debaryomyces.
In certain yeasts the asci are formed without copulation of two cells, e.g., in species of Zygosaccharomyces, Saccharomyces, Schizosccharmoyces. This process is known as parthenogamy.
The number of ascospores per ascus varies from genus to genus, e.g., the asci of Monospora and Nadsonia are monospores; 2-4 ascospres per ascus in Debaryomyces and Hensenula; 4 in Saccharomyces and 8 in Schizosccharomyces. The ascus in Kluyveromyces polysporus contains several spores.
The shape and structures of the ascospores is also variable, e.g., the ascospores of Saccharmyces and Schizosaccharomyces are globose or ovoid; rough and warty of Debaryomyces; hat-shaped and flat on one side of Hansenula; needle-like and tapering at one end of Nematospora.