Sporulation: Sporulation in Bacteria, Amoeba, Fungi and Yeast

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What is Sporulation?

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Spore formation or sporulation is a process where organisms form spores for their survival and adaptation. The spores produced have a thick protective layer that enables them to tolerate extreme environments. These reproductive cells help bacteria from harsh conditions like heat, dryness, and radiation.

• Not only in prokaryotes, spore formation can occur in different organisms.
• Genus Methylosinus forms exospores, which are a type of spore formed outside the organism or cell.
• Bacteria form endospores, which are a type of spore that is produced inside the organism or cell.
• Some plants produce megaspores (female gametophytes) and microspores (male gametophytes).
• Lower plants that reproduce via sexual reproduction form spores which are generally visible and present outside the female reproductive organ.

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Sporulation in Bacteria

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Usually, aerobic Bacillus and anaerobic Clostridium species of bacteria show spore formation and these are all gram-negative rod-shaped (bacillus) bacteria species. But certain filamentous cocci also show sporulation and form endospores. The sporulation in bacteria starts soon after the vegetative growth phase ends.

• In the vegetative growth phase, the phosphorylation system, where all the kinases are present including the histidine kinase, sends the message of unfavorable condition of the environment to the master transcriptional regulator (Spo0A).
• It is very important to determine the phosphorylation system of the intercellular pool of the master transcriptional regulator (Spo0A), this stage determines the system. The master transcriptional regulator regulates the expression of many genes and some of these participate in the sporulation process.

There are various steps involved in sporulation, which are discussed below:

Stage I

Once this relator is activated it follows the next step of chromosome replication and formation of an axial filament, where each chromatid move to the opposite poles of the cell, one chromatid moves to the mother cell while the other one participates in forespore formation.

Stage II

In this step, asymmetrical septation occurs, where the initial/original proximal regions are being hitched to the opposite poles of the cell which results in the stretching of the chromosomal DNA and form axial filaments.

Stage III

As the forespore formation has been done, now this proximal portion is engulfed by the cell membrane by the enzyme peptidoglycan (it is being synthesized in large amounts in the previous step).

Stage IV

 Here the mature layer is enclosed within a bilayer capsule, where the outer coat is made up of different proteins and the inner layer is known as the cortex which is made up of a special type of peptidoglycan.

Lysis of Mother Cell

In this next step, the mother cell gets disintegrated and the material content of the mother cell gets used up in the coat formation. This is a prolonged stage, the spore remains in this step for a long time and comes out when the condition favors.

Spore release

Sometimes the mature cell releases out of the coat and is capable of surviving in tough conditions like extremely high temperatures, extreme cold temperatures, and radiation, under the influence of chemicals. These cells remain in a non-generative stage, once the conditions favor, the spore germinates into a new organism.

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Sporulation in Fungi

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Fungi are eukaryotic organisms that reproduce sexually as well as asexually via sporulation. These organisms produce a huge variety of spores according to size, shape, and other features that are suitable for dispersion.

• The sexual spores of fungi remain dormant in order to survive while the asexual spores are produced by mitosis and get spread in the environment.
• Asexual spores are produced in large quantities and are all genetically identical.
• Examples of sexual spores produced by fungi are ascospores, basidiospores, oospores, and zygospores.
• Chlamydospores, arthrospores, sporangiospores, and conidia are examples of asexual spores.

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Sporulation in Yeast

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When nitrogen content reduces in yeast, sporulation in yeast takes place via meiosis and involves the following three phases:

1. Early Phase
2. Middle Phase
3. Later Phase

Before the start of early phase, cells prepare themselves to disintegrate into spores.

Early Phase

In this phase, the cell division process shifts from the mitotic cycle to meiosis. In other words, it enters S phase from G₁.

• In this phase, DNA replication and homologous recombination or pairing take place.
• This phase is also known as RNA processing as it is characterized by changes in the machinery of cell cycle.

Middle Phase

In this phase, various cytological changes occur. Here the meiosis produces four haploid daughter cells, modifies the spindle fibers at the opposite ends, and contributes to prospore formation.

• Post-Golgi vesicles are being redirected to regeneration and expansion of new compartments.
• The prospores engulf the nuclei and all the other cell organelles move to cytoplasmic space which lies between the nuclear membrane and the prospore and the nuclei gets separated from the cytoplasm.

Late Phase

This step begins after the prospore membrane closes, a thick wall coat starts to surround the spore only when the membrane closes.

• The development of a thick wall or spore wall is important for maturation of spore.
• The chromosome compacts the nucleus of the spore and the rest of the organelles start to regenerate.

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Sporulation in Amoeba

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Sporulation in amoeba is known as Encystment. Generally, Dictyostelium Discoideum species show sporulation.

• Amoeba forms a protective cyst wall in unfavorable conditions.
• The process of encystment is characterized by the removal of pseudopodia, formation of cyst wall, and dormancy.
• Some of the chemical changes take place during sporulation in amoeba, such as the amoeba changes itself from aerobic metabolism to anaerobic metabolism followed by complete dormancy, the phosphorylated compounds, loss of cellular RNA, DNA, and proteins and incorporation of glucose compounds into large polysaccharides.
• During the lack of nutrition condition, the amoeba aggregates together and forms an elongated structure to ultimately form a migratory slug of large amounts of cells.
• The slug becomes upright as a stalk which is located at the tip of the cellulose tube.
• Then the cell enters the tube and is vacuolized to contribute to its strength.
• The cells climb from the bottom of the substrate then the stalk, prestalk followed by the prescore
• The prescore then encapsulates and turns into dormant spore which is being protected by various protein coats.

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Things to Remember

• Clostridium and Botulinum are the common producers of spores.
• The thick walls of spores constitutes enzymes, so that they can tolerate harsh environments.
• In bacteria, 4 types of spores are formed i.e. cysts, endospores, exospores, and myxospores.
• Zygomycetes, Ascomycetes, and Basidiomycetes are the three types of fungal spores.
• Both amoeba and yeast undergo sporulation and have different stages of sporulation.
• Spores are produced in a sac-like structure called sporangia.