Sporulation

Sporulation is a strategy used to overcome and adapt to unfavorable environmental conditions such as scarcity of nutrients, extreme heat and temperature, lack of moisture, and survival. Thus, sporulation is an adaptive response that allows the cell or the organism to survive in time and space until they find conditions suitable for vegetative growth.

Compared to vegetative cells, spores are multilayered structures that are dormant from the inside. Like an actively dividing cell, spores contain the genetic material, ribosomes, enzymes, and some acids that allow the spore to survive and germinate once favorable environmental conditions return.

Depending on the organism, the type of spores and sporulation varies.

Sporulation in Bacteria

The process of sporulation is most widely studied in bacteria. Sporulation in bacteria is ubiquitous and can be found in soil, water, and air, among other natural environments. Most spore-forming bacteria are gram-positive such as Bacillus and Clostridium, although some gram-negative bacteria also form spores.

Sporulation is a gradual process that starts in bacteria when they encounter unfavorable growth conditions. They initially try to move to a new place by chemotaxis to survive the stress. Some other modes of survival include changing their mode of reproduction. Thus, sporulation is the last survival strategy in bacteria.

Before the cells commit to sporulation, the cell must maintain chromosome integrity, and the Kreb Cycle must be active. Although the exact steps and the way the spores mature vary depending on the type of bacteria, they all follow the same basic steps as described for the Bacillus spp.

In Bacillus subtilis

Among others, bacterial sporulation is most widely studied in Bacillus subtilis. The different stages of sporulation in Bacillus subtilis are

1. End of Vegetative Stage: This stage is marked by phosphorylation of the master transcriptional regulator, Spo0A, by several histidine kinases.

2. Segregation of DNA, Asymmetric Cell Division, and Formation of Forespore: The activation of the master transcriptional regulator is followed by DNA replication, with each chromosome moving to the opposite ends of the cell. One chromosome is retained in the mother cell, and the other is found in the pre-spore, also called the forespore.

The septum forms, and the origin-proximal regions of the cell are tethered to the opposite poles. The exact mechanism of asymmetric septation is still not precise. On the other hand, the Chromosomal DNA stretches, forming axial filaments.

3. Engulfment: On division, almost 30% of the origin-proximal portion containing the chromosome is engulfed by a membrane. The peptidoglycan synthesizes around the spore.

4. Formation of Spore Protein Layers, Cortex, Membranes, and Spore Coat: The mature spore is enclosed in two different layers during this stage. The outer coat (spore coat) consists of different proteins, and the cortex consists of a special type of peptidoglycan.

5. Maturation of Spore, Lysis of the Mother Cell, and Release of the Spore: The mother cell then disintegrates, releasing the mature spores into the environment.

After favorable growth conditions, the spores germinate into a vegetative cell.

Types of Spores Formed in Bacteria

Accordingly, they can form four types of spores:

• Endospores are the most common type of asexual spore produced by Bacillus anthracis, Bacillus thuringiensis, Clostridium botulinum, and Clostridium tetani. They are reproductive cells that can resist extreme environmental conditions due to high dipicolinic acid, calcium content, and small acid-soluble DNA-binding protein.
• Cysts are a group of cells that join together to survive harsh environments. When conditions become favorable, all the cells come together, allowing them to survive. They are common in Azotobacter spp. Unlike endospores, cysts are non-reproductive cells far less resistant to environmental stress than endospores.
• Myxospores are formed in myxobacteria, Myxobacterateswithin their fruiting body. In contrast to other resting cells, they are formed from specialized spore-bearing structures. Myxospores can survive harsh environmental conditions such as high temperature, UV, and lack of moisture.
• Exospores are produced through the budding of mycelium and can survive in harsh environmental conditions better than vegetative cells. They are found in Actinobacteria.

Sporulation in Fungi

They are the most widely studied spore-producing organisms. Fungi produce a wide variety of spores that vary widely in size, shape, and other morphological features. 

Overall the spores produced sexually through meiosis (ascospores) remain dormant for an extended period. In contrast, spores produced asexually by mitosis (mitospores) are active and play an essential role in reproduction.  

Also, spores in fungi are classified based on the structure they produce, their function, their origin during the life cycle, and their ability to move.

Among all others, sporulation in the yeasts Saccharomyces cerevisiae, Aspergillus nidulans, and rhizopus are most widely studied.

In Saccharomyces cerevisiae

When nitrogen sources diminish, Saccharomyces cerevisiae responds by going into the stationary phase or changing their morphology. If the adverse situation persists, they form spores through meiosis through the following steps:

1. Early Phase: It is the first phase of sporulation. Cell division shifts from the mitotic cycle in the G1 phase and enters the S phase. Events such as DNA replication and homologous recombination occur in the early phase. They thus are characterized by RNA processing and changes in the machinery of the cell cycle.

2. Middle Phase: This phase is characterized by various cellular changes. Some of the events that happen during this phase are:

• The cells undergo meiotic division producing four haploid nuclei that form four daughter cells
• Individual cells then modify spindle pole bodies that develop into membrane compartments (prospore). 
• Post-Golgi secretory vesicles promote regeneration and expansion of the new compartments

• The prospores grow and engulf the nuclei
• Other organelles move to the cytoplasmic space between the prospores  and the nuclear membrane
• Form a nucleus that is separated from the cytoplasm

3. Late Phase: This is the last phase of sporulation in fungi and involves the following events:

• The prospore membrane closes completely
• A thick wall starts forming around the spores
• DNA compaction within the nucleus
• Regeneration of organelles

Sporulation in Protozoa

In protozoa, sporulation is most widely studied in amoeba. However, it is also found in slime molds and sporozoans, among others.

In Amoeba

During unfavorable environmental conditions, amoeba responds by forming a protective cyst wall. The process of forming cysts, known as encystment, is characterized by the following events:

• Reduced motility
• Withdrawal of the pseudopodia
• Changing into a spherical shape
• Cell shrinkage
• Forming a hard, protective layer
• Inducing dormancy

In addition to the morphological changes observed during encystation, several metabolic changes are also evident. These include:

• Binding of biogenic amines to specific receptors on the organism’s membrane, activating adenylate cyclase on the membrane
• Changing from aerobic to anaerobic metabolism and complete dormancy
• Increasing the formation of the phosphorylated compounds
• Significant loss of cellular RNA, DNA, and other proteins
• Synthesizing polysaccharides using glucose

In Slime Molds

They are similar to other protozoa and fungi in their life cycle. Both plasmodial and cellular slime molds produce spores sitting atop reproductive stalks or fruiting bodies (sporangia). Once placed in a suitable environment, the spores germinate, forming new slime molds.

In Sporozoans

They are parasites that form spores within the body of the host organisms they reside. Many sporozoans can alternate between sexual and asexual reproduction in their life cycles.

Sporulation in Plants

Like fungi, plants also exhibit alternation of generations. Plants without seeds like algae, mosses, and ferns produce spores as part of their life cycle.

In non-vascular plants such as mosses, the sexual phase (gametophyte stage) is dominantly expressed as green mossy vegetation. In contrast, the nonsexual phase (sporophyte stage) consists of elongated stalks with spores enclosed within sporangia located at the tip of the stalks.

In vascular plants without seeds, such as ferns, the sporophyte, and gametophyte generations are independent. The fern leaf (fronds) represents the mature diploid sporophyte. In contrast, the sporangia on the underside of the fronds produce spores that develop into the haploid gametophyte.

In flowering plants (angiosperms) and seed-bearing non-flowering plants, the gametophyte generation depends on the dominant sporophyte generation for survival. Here, the flower produces both male microspores within pollen and female megaspores within the flower ovary. Upon pollination, the microspores and megaspores unite to form seeds, while the ovary develops into a fruit.