PROPAGULE: WHAT IS IT, TYPES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

A propagule is a propagating structure from which a new individual can originate. It may or may not be the product of sexual reproduction and is generally produced by plants, fungi, bacteria and other microorganisms to colonize new spaces, increase its coverage area or pass from one stage of the life cycle to another.

Seen this way, a propagule can be both the sexual seed of a plant, the proglottid of a tapeworm (parasitic flatworm) or the spore of a fungus and the cyst of a bacterium.

Seeds are a type of sexual propagule (Source: No machine-readable author provided. Mdf assumed (based on copyright claims). Via Wikimedia Commons)

Some living beings use propagules as their main mechanism of asexual reproduction, such as the tubercles in some plants, the propagules of bryophytes, certain asexual fungal spores, and the endospores of many types of gram-positive bacteria.

These structures can be very different in size, shape and appearance to the organism that gives rise to them, they are usually very resistant and are designed to remain viable for quite long times.

So, propagules not only function in the dispersal and propagation of the species that produce them, but also ensure their survival when environmental conditions change or become adverse.

However, propagules produced by living organisms can only perform their propagation (dispersal) functions if they "find" favorable conditions to establish the new individual that they are destined to form.

Types of propagules

In nature there are many living beings that produce propagules, among these are plants, fungi, some parasitic protozoa and certain bacteria.

Propagules in plants

Different authors agree that plant propagules are of two types: seeds (sexual propagules) and some modified forms of stems, roots and leaves (asexual propagules). Their function is usually propagative and preservative, as they serve for the multiplication or for the preservation of the species.

The seeds are typical propagative structures of flowering plants and are produced after fertilization of the egg cell by the pollen grain. They can have very variable sizes and shapes and usually contain enough reserve substances for the sustenance of the embryo that they house inside.

Depending on the plant species, the seeds can have extremely thick and resistant or soft and labile coats. In addition, their germination depends on different external factors such as temperature, relative humidity, the wavelength of light they receive, the presence of substances with specific pH, etc. (also from endogenous factors).

As is true for seeds, asexual propagules of plants such as tubers, bulbs and rhizomes, for example, are characterized by a "suspension" of growth or a metabolic "lethargy" of the tissues that compose them, controlled endogenously by chemical and hormonal factors, but which are imposed by the environment.

Propagules in fungi

The most common propagules in fungi are spores. In these organisms, the spores can be of sexual or asexual origin, and they fulfill functions of vegetative propagation or resistance, since they are much "stronger" than the beings from which they originate.

Spores are common propagules for a wide variety of organisms including bacteria, protozoa, algae, and certain plants. Whatever their origin (sexual or asexual), when they germinate, they produce new individuals or cell masses.

Spores of Panaeolina foenisecii, a fungus (Source: Alan Rockefeller, via Wikimedia Commons)

In the Fungi kingdom, which includes fungi, yeasts and molds, reproduction is highly dependent on the production of desiccation resistant single-celled spores.

The asexual spores of many fungi are usually called "conidia" and are produced by special structures called sporangia, and it is interesting to mention that the phylogenetic grouping of fungi is based on the mechanisms by which they produce their sexual spores.

Propagules in parasites

Many animal parasites produce propagules for dissemination or transmission from one host to another. These are usually larvae or eggs that almost always depend on the intervention of some vector or mobile transmitter.

Most parasitic species spend at least part of their life cycle in the form of "free-living propagules" and, depending on the species and host type, many parasites have developed different strategies for dispersal and survival. of these propagules.

Egg of a species of Taenia, a mammalian intestinal parasite (Source: Andréatl, via Wikimedia Commons)

For example, the eggs of many parasites in the human intestine are released with the feces of their infected hosts and depend on mobile hosts or transmitters to make contact with the soil or bodies of water and thus ensure that their larvae continue their life cycle.

The propagules of parasitic species that have wide ranges of hosts and vectors show complex patterns of cyclical changes that are highly dependent on the environmental conditions to which they are exposed throughout the life cycle.

Often, the larvae (the typical propagules of some species of parasites) do not feed on their host, but acquire the energy necessary to maintain themselves from the degradation of internal reserve substances.

Propagules in bacteria

Some gram-positive bacteria of the genus Bacillus and Clostridium form propagules of resistance against signs of starvation (due to lack of food) that are characterized by a drastic decrease in metabolic activity and, therefore, growth.

Said propagules are often produced by a “sporulation” event, characterized by unequal cell division (they are asexual propagules), which results in the production of “pre-spores” that are smaller than the “mother” bacteria..

Diagram of the formation of a bacterial endospore (Source: Farah, Sophia, Alex via Wikimedia Commons)

Apparently, many sporulating bacteria engulf these "pre-spores", which is why they are known as "endospores", which are released once the cell that originated them is lysed.

Bacterial endospores are modified in the cytosol, they are covered by different layers and, in addition, they lose a large amount of moisture. They also slow down their metabolism and gain the ability to resist heat, radiation, and exposure to different chemicals.

Under appropriate stimulation, these resistance endospores can "germinate" and form new bacteria, genetically identical to the "mother" cell that gave rise to them.

Reference

1. Chrungoo, NK (1992). Concepts of dormancy regulation in vegetative plant propagules: a review. Environmental and experimental botany, 32 (4), 309-318.
2. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). New York: McGraw-Hill.
3. Lamberton, PH, Norton, AJ, & Webster, JP (2010). Propagule behavior and parasite transmission.
4. Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson.
5. Raven, PH, Evert, RF, & Eichhorn, SE (2005). Biology of plants. Macmillan.