Gametophyte

Gametophyte is a haploid stage of the plant life cycle. It is a multicellular, often small, and hardly noticeable entity that undergoes mitosis to produce gametes. The Gametophyte phase of the life cycle is followed by the sporophyte phase. Gametophyte and sporophyte phases form an alternation of generation in the plant life cycle. 

In this article, we will learn about the definition of gametophytes, the different types of gametophytes with their examples, male and female gametophyte structure and development, their importance, and the difference between the gametophytic and sporophytic stages.

What is Gametophyte?

Definition of Gametophyte: A multicellular, haploid, sexually reproducing stage in plants’ and algae’s life cycles is called a gametophyte. Gametes (eggs and sperm) are produced by gametophytes, which originate from spores.

Gametophyte is a haploid stage in the plant life cycle having only one pair of chromosomes. It is a multicellular, often small, and hardly noticeable entity that undergoes mitosis in order to produce gametes. The gametophyte phase comes before the sporophyte phase, which is diploid in nature, in the alternation of generations, a typical reproductive mechanism in plants. The diploid sporophyte has two sets of chromosomes. Gametes are the result of mitotic divisions from the gametophyte, which produces sperm and egg cells.

Male and Female Gametophytes in Plants

Male Gametophyte : The haploid male gametophyte, often referred to as the microgametophyte, is a stage in the life cycle of plants that exhibits generational alternation. It is essential to the process of sexual reproduction, notably to the creation and distribution of male gametes, or sperm cells. Male gametophytes usually begin with spore germination and end with the discharge of mature sperm.

Female Gametophyte: The female gametophyte is also known as the megagametophyte. Its development results in the creation of female gametes, also known as eggs or ova, which fertilise to produce the following generation of sporophytes.

Diagram of Gametophyte

The diagram of gametophyte is shown below:

Gametophytes Examples

There are three examples of gametophytes which are as follows:

• Mosses: The gametophyte stage of the life cycle has several leaves and looks like a little green plant.
• Ferns: The prothallus, or gametophyte fern, is a tiny, flat structure that is frequently seen on the forest floor among ferns.
• Seed Plants
  • Angiosperm: Pollen grains are a representation of the male gametophyte; they grow from microspores that are formed in flower anthers.
  • Gymnosperms: The male gametophyte of gymnosperms, like conifers, is located inside pollen cones.

Types of Gametophytes

Adaptations to certain ecological niches and reproductive strategies are reflected in the major differences in gametophyte morphology across various plant groupings. Plant species have different evolutionary pathways that have affected gametophytes’ size, structure, and reproductive organs.

Morphology

The vast variations in gametophyte form throughout plant species are indicative of reproductive strategies and environmental adaptations. Gametophytes are frequently less noticeable and smaller than the sporophyte phase that follows. Depending on the type of plant, they can be tiny structures or take on larger, more noticeable shapes. Following are the different plant categories with their specific gametophytes:

• Gametophyte of Bryophytes: The gametophyte phase is usually prominent in these non-vascular plants. In damp environments, moss gametophytes, for instance, may create thick carpets. Moss gametophytes resemble little plants since they are usually leafy and smaller structures. Their components include reproductive organs and rhizoids, which serve as the gametophyte’s anchor on the substrate. Different growth types are displayed by the gametophytes of liverwort and hornwort; the former often have flattened structures, while the latter have elongated, horn-like structures.
• Gametophyte of Pteridophytes: Prothalli, or fern gametophytes, are frequently found in moist, shady environments. Different species have different prothallus structures; some have bigger, more intricate prothalli than others. A common characteristic is the existence of antheridia, which are male reproductive structures, and archegonia, which are female reproductive structures. Though they have rhizoids for anchoring, and lack real roots.
• Gametophyte of Gymnosperms: Gymnosperms are heterosporous plants that have a distinct gametophyte shape. Male gametophytes are formed by microspores within the pollen grains of male cones. These wind-borne megaspores eventually land on female cones, where they mature into female gametophytes inside ovules. Gymnosperms do not depend on insects for pollination, in contrast to angiosperms. Their capacity to adapt to different conditions can be attributed to their dependency on the sporophyte and their smaller gametophyte production.
• Gametophyte of Angiosperms: Angiosperms have a much smaller gametophyte phase than bryophytes and pteridophytes. The female gametophyte is found within the ovule, whereas the male gametophyte is found within the pollen grain. Gametophyte size has decreased as a result of the evolution of flowers and specialised reproductive structures in angiosperms. For flowering plants, the reproductive organs of the flower typically house the female gametophyte (embryo sac) and the male gametophyte (pollen grain).

Reproductive Organs of Gametophyte

The reproductive organs are necessary for the mechanism of sexual reproduction in plants with a dominant gametophyte phase, supporting the creation, protection, and interaction of female and male gametes. There are two types of reproductive organs found in a gametophyte –

• Archegonium (female gametophyte)
• Antheridia (male gametophyte)

Archegonia

Female gametophyte is also known as Archegonia. Specialised structures called archegonia are present on the female gametophyte and are essential to the sexual reproduction of some vascular plants like ferns as well as non-vascular plants like mosses, liverworts, and hornworts. The key characteristics of archegonia are as follows:

• Structure: The structure of archegonia is usually flask-shaped, with a long neck and a bulging base. There is a channel in the neck that allows the sperm to enter and reach the egg.
• Egg Production: Inside of its enlarged base, the archegonium generates a single egg cell. Until the time comes for fertilization, the egg is kept inside the protective barrier.
• Fertilization: Plants that have a dominant gametophyte phase fertilise at locations called archegonia. In order to reach the archegonia, sperm cells, which are frequently generated in adjacent antheridia must pass through water (in mosses and liverworts) or damp areas (ferns). The sporophyte begins to develop when fertilization takes place and the diploid zygote is developed.

Antheridia

Male gametophytes, also known as sperm cells, have structures called antheridia that are responsible for producing and releasing male gametes. The following are the main characteristics of antheridia:

• Structure: Small, multicellular structures called antheridia are usually responsible for producing and releasing sperm cells. They could lie in clusters or be dispersed over the male gametophyte’s surface.
• Sperm Production: Several haploid cells in the antheridium divide mitotically to generate a large number of sperm cells. With the help of their flagella, these sperm cells may swim to the archegonia to fertilise.
• Fertilization: To get to the archegonia, where fertilization occurs, the sperm must travel through a layer of water (in mosses and liverworts) or through moist spaces (in ferns).

Development of Gametophytes

Different mechanisms and structures are involved in the formation of male and female gametophytes in plants. This method of development is most noticeable in plants that display generational alternation, in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte. The development of male and female gametophytes will be covered individually in this section:

Male Gametophyte Development

The spores that the sporophyte releases are the source of male gametophytes, also referred to as microgametophyte. In the presence of favourable environmental conditions, such as moisture, temperature, and light, spores germinate. The male gametophyte, also known as the prothallus, is a multicellular structure that develops from the germinated spore.

Some cells develop into antheridia within the prothallus; these structures are in charge of creating and releasing male gametes (sperm). Many haploid cells divide mitotically within each antheridium, producing a large number of sperm cells. Because of their flagellation, these sperm cells may more easily go to the female gametophyte (archegonium) via water where fertilisation will take place.

Female Gametophyte Development

Sporophyte-released spores give rise to megagametophytes. When the right circumstances are fulfilled, spores germinate and the female gametophyte begins to grow. The female gametes, or eggs, are housed and shielded by structures called archegonia, which resemble flasks. A single haploid cell gives rise to one ovum or egg cell within each archegonium. Until it is fertilised, the egg is protected by the archegonium.

When the gametophyte reaches maturity, it emits a chemical attractant that directs the sperm cells towards the archegonium. When a sperm cell and an egg cell effectively fuse, fertilisation takes place.The shift from the fertilised egg to the diploid zygote signifies the start of the sporophyte phase.

Importance of Gametophytes

A number of factors make the gametophyte phase of a plant’s life cycle vital. Some of them are:

• Gametes produced by the male and female gametophytes, join during fertilisation resulting in a zygote that matures into a new sporophyte. Genetic diversity is ensured within plant populations through this method.
• Plants have a unique reproductive system that involves alternating between the gametophyte and sporophyte stages. This life cycle enables plants to adapt to changing environmental conditions and assures survival through both sexual and asexual reproduction.
• The multicellular gametophyte phase provides an ideal habitat for gamete generation and dissemination since it is frequently suited for living in water or other wet environments.

Difference between Gametophyte and Sporophyte

Plants go through two separate life stages called gametophyte and sporophyte, during which they alternate between generations. These stages have distinct functions in the reproductive process and reflect various ploidy levels.

Conclusion: Gametophyte

Gametophyte is a reproductive stage seen in case of plants where the gametes are produced. The gametes are haploid in nature. The male reproductive gametophyte is called Antheridia and the female reproductive gametophyte is called Archegonia. Both of these structures produce gametes which when fuse result in the formation a zygote. The zygote then grows into an adult stage known as the sporophytic stage where it is diploid in nature. Different plants have different kinds of gametophyte structure and morphology.

FAQs – Gametophyte

What is a Gametophyte and Sporophyte?

Plants undergo a generation alternation with distinct haploid and diploid phases. The gametophyte, representing the haploid phase, produces gametes, while the diploid sporophyte generates spores in the plant life cycle.

What is the Gametophyte of a Human?

In humans, the gametophyte stage is represented by the haploid gametes – sperm (male) produced in the testes and eggs (female) in the ovaries. .

What is the Difference between Gametangia and Gametophyte?

Gametangia are structures in the gametophyte of plants that produce and protect gametes.The gametophyte is the entire independent, haploid phase generating gametes.

What is a Male Gametophyte?

In flowering plants, the pollen grain serves as the male gametophyte, while the female gametophyte is represented by the embryo sac.

Is Gametophyte Asexual?

Gametophytes are reproductive plants that produce eggs, sperm, or both, while sporophytes are non-reproductive plants that produce spores, capable of generating new gametophytes.