Yeast: morphology and life cycle

They are single celled fungi
Size: generally larger than most bacteria; (1-5) um wide and (5-30)um length
Shape: cell is egg shaped, some are elongated or spherical
Size and shape varies among species
Yeast cell lacks flagella and other organ of locomotion.

Morphology of yeast cell

Cell wall: composed of thin chitinous cell wall
The protoplasm is surrounded by cell membrane which contains all the usual cell organelles like ribosomes, mitochondria, ER, nucleus and other granules
Vacuole is single, large and centrally located.

i. Haplodiplobiontic life cycle

ii. Haplobiontic life cycle

iii. Diplobiontic life cycle

This cycle occurs in Saccharomyces cerevisiae
In this life cycle somatic cell of yeast exists in two form ( Haploid dwarf cell and Diploid large cell)
Haploid cell contains two mating types “a” and “α”
During favorable condition each of the haploid cell multiply by budding only
If these mating types comes in contact with each other, they form gametangia and starts sexual reproduction
Fusion of these two haploid cell form large fusion cell called Plasmogamy
Nucleus also fused by karyogamy to form zygote
Zygote multiply by budding and forms several diploid cells
these diploid cells are larger than haploid cells
Like haploid cell, these large diploid cells also live independent life and reproduce by budding
Under unfavourable condition, diploid large cell become spherical and directly behaves as ascus mother cell
Nucleus of ascus mother cell divides by meiosis to form 4- haploid nuclei
Out of 4-nuclei, 2 belongs to mating type “a” and t2 belongs to “α”
Each nuclei gather some cytoplasm and becomes ascospore
Each ascospore is globular and thick walled structure and relased by rapture of ascus wall.
Ascospore germinates to give haploid dwarf cell.

This life cycle occur in Schizosaccharomyces.
In haplobiontic cycle, somatic haploid cell multiply by fission producing number of haploid cell.
During sexual reproduction, two sister haploid cell behaves as gamatangia and produce beak like structure
The beaks of two gamatangia fused and wall at the point of contac dissolve to form canal called conjugation tube.
Nucleus of two cell moves and fused to form diploid zygote
The zygote directly function as ascus mother cell
Nucleus of ascus mother cell divides by meiosis forming 4-haploid nuclei which themselves divides by mitosis to form 8- haploid cells
Each 8- nuclei gather some cytoplasm and become ascospore
Ascospore released by rapture of ascus
Ascospore germinates to give haploid cell and continue life cycle

This life cycle is found in Saccharomyces ludwigii
In diplobiontic life cyle, diploid somatic cell multiply by budding and fission
Under certain condition, these somatic cell directly functions as ascus mother cell and nucleus of ascus mother cell divides by meiosis to form 4-haploid nuclei
These haploid nulceii gather cytoplasm and transform into ascospore4 ascospore,
Out of 4-ascospore, 2 are mating type “A1” and 2 mating type “A2”
These two mating ascospore fused within ascus to form diploid zygote.
The diploid zygote gerinates within ascus producing a germ tube
The germ tube breaks the ascus wall and function as diploid sprout mycelium.
The sprout mycelium on budding gives sprout diploid cell which give diploid somatic cells.