Prothallus

Prothallium which is more commonly known as the prothallus can be defined as the gametophyte of the fern and other pteridophytes species. The prothallium or prothallus is known to produce both male and female gametophytes of the plant. To understand the role of the prothallus it is important to understand what prothallus is, the structure of the prothallus. In this article, the discussion will be focused on the basic understanding to define prothallus, the reproductive phase of the fern or the sexual reproduction in the fern, and the significance of the fern prothallus in the reproductive cycle of the plant. Lastly, this article will also provide some examples of plants that have prothallium.

About Prothallus

It is very important to have a basic understanding of what prothallus is, to understand the role of the prothallium in the reproduction process of the plant. Prothallus is the gametophyte of the plant that is they are responsible for producing the gametes which are the male and female sex cells of the plants. These gametes fuse to form the zygote. The male and female gamete is fused allowing the cell to enter a diploid state, this process is known as fertilization. The prothallus of prothallus is produced during the sporophyte stages of the fern, that is they are produced by the spores of the plant. It is very important to note that the zygote formed after the fertilization is the sporophyte, this sporophyte grows and develops by uptaking the nutrition from the gametophytes of the plant, that is the prothallium generally assists in the growth of a spore.

The prothallium is generally a heart-shaped appendage, they are generally colorless, and have features like rhizoids which acts as the primary root, there is also the presence of the primary leaf, and the rudiment of a new stem. It is interesting to note that the plant prothallium provides nutrition to the sporophyte by an appendage named the foot.  They are generally small, that is their diameter is only about 2-5 millimeters long. They are a very short-lived phase of the plant, they generally can perform photosynthesis. Some of the prothallus reported are colorless while the others are generally of green color. 

It is very important to understand the alternation of generation in the fern and similar pteridophytes, it is the understanding of this that gives the proper answer to what is prothallus and its role in the life cycle of the fern prothallus. It is because of the alteration of a generation that structures like prothallium are evolutionarily developed.

Alternation of Generations

Alternation of generation can be defined as the distinct haploid and diploid phases of the life cycle of the plant. It is because of the alteration of a generation that a prothallus structure is formed. There are two different phases one is the haploid phase and the second is the diploid phase. The gametophytes are produced during the haploid state by the spores of the plant, that spores give rise to prothallus and the prothallus further produces the gametophyte by the mitotic cell division. The gametes produced by it are haploid when male and female gametes fertilize the plant and enter into the diploid state. The initial nutrition is provided by the prothallium, after which the plant germinates and develops all its features. 

The common example of alternation of generation in ferns, liverwort, moss. The class of pteridophyte shows it, among them fern prothallus is the most widely studied example.

Fundamentals of the Alternation of Generation

There are basic steps or the fundamentals of the alternation of generation seen in plants. Pants that produce sporophytes undergo alteration. The fundamental steps are as follows,

• The haploid stage of the life cycle is known as the sporophyte

• Sporophytes produce the spores

• Spores undergo the process of mitotic division to produce the gametophyte, these are known as prothallium. The common example is fern prothallus

• The prothallium essentially contains the gametophyte

• The gametophyte produces the sex cell, also known as gametes. They produce both the male and female gametes

• Male gametes are called the antheridia, they are flagellated sperm

• The female gametes are known as ova. They are also referred to as the archegonia.

• Fertilization gives rise to the zygote, which is diploid.

• Zygotes undergo development and the nutrition for it is provided by the prothallus.

• Zygote germinates and undergoes mitotic division to produce sporophyte

• Sporophytes at maturity produce sporangia, they can produce a single or multiple sporangia. Single sporangia are known as sporangium.

• Sporangia further produces a diploid mother cell. These are also known as the sporocytes

• Sporophytes undergo meiotic division to produce the spores

This is the complete summary of the alternation of generation, and the role of prothallus in it. This cycle continues to maintain the population of the plant and to ensure its survival even during harsh conditions. Alternation of generation is also known by the name of metagenesis and heterogenesis.

Examples of Prothallus

There are the following examples of an organism that gas prothallium

• Fern

• Moss

• Liverwort

• Algae 

• Club mosses

• Horsetails

• Psilopsida, 

• Lycopsida, 

• Sphenopsida

•  Pteropsida and all the trees that come under pteridophytes

Function of Prothallus

There is the following function of a prothallium

Assist in reproduction

Produce gametes, both the male and female gametes

Provide nutrition to the zygote

Protection of the zygote in adverse conditions and environmental stress

Prothallus also serves as the food source of many herbivores

Importance of Alternation of Generations

After understanding the answer to the question about what is prothallus and the importance of the role of prothallium in sexual reproduction it is important to understand the need for evolution of this life cycle. Some of the points of importance of alternation of generation are mentioned as follows-

Ensures the survival of plants even in stressful environmental conditions

Protection from a predator, prothallium sometimes serves as the food source for herbivores in the food chain, but a diploid structure formed due to alteration of generation are generally not consumed, thus providing the protection.

They ensure the maintenance of the seed in the dormant stage in unfavorable condition

It is because of alteration of generation a plant can maintain the balance and follow both the methods of reproduction that is asexual and sexual reproduction.

It is due to the alteration of a generation that multicellular gametophytes are produced rather than haploid gametes, this evolutionary importance is because it ensures that there is only a single allele for any genetic trait. It can be said that the alleles are not masked by a dominant counterpart.

Alternation of generation also preserves the strength of the gene pool as in case of lethal mutation because the presence of the single gametophytes is eliminated in that generation itself without passing it to the progeny generations. It also allows genetic diversity to persist and develop. 

Life Cycles of Various Groups of Plants 

The plants that we are going to discuss are the Archaeplastida which refers to the green and red algae, different land plants and glaucophytes. In these red and green algae, we can see that alternation of generations occurs. These red and green algae include both seaweeds and freshwater forms. In some species, the alteration of species is triphasic while most generations are isomorphic or homomorphic. On the other hand, the heteromorphic or an isomorphic type of alternation of generations occurs which means that both the gametophyte and sporophyte are different and can be easily distinguished. The bryophytes have the most conspicuous type of generation of the gametophyte. Some of these bryophytes can be hornworts, mosses or liverworts. 

How is a Prothallus Similar to a Thallus? 

Although there are not many similarities between a prothallus and a thallus, still these two share some things in common. Between both prothallus and thallus, Roots, stems, and leaves are not distinguished. Both of these are photosynthetic structures which means both of them help a plant to prepare food. Both of these are used in producing gametes, which are essential for sexual reproduction. One more thing that both prothallus and thallus have in common is that they are both gametophytes.

How is a Prothallus Different to a Thallus? 

Both thallus and prothallus are structures that are not differentiated. Prothallus, on one hand, is a heart-shaped thallus-like structure while thallus, on the other hand, is a filamentous structure. This can be the primary distinction between these two. Even though they both are gametophytes, but thallus reproduces both sexually and asexually, whereas prothallus can only reproduce sexually and this makes prothallus different to a thallus. In addition to that, a prothallus is known to contain specialised sex organs i.e., antheridia and archegonia. But in the case of the thallus, these specialised sex organs are absent. Another distinction between thallus and prothallus is that thallus can be unicellular or multicellular, but prothallus can only be multicellular.