Male Gametophyte: Check Definition, Types, Structure & Development

Male Gametophyte: Plant reproduction is an important topic in Biology. Among everything a student learns regarding this topic, an essential term that they must always keep in mind is gametophyte. Plants show alternation of generation, which alternate between the diploid sporophyte and the haploid gametophyte. 

The diploid zygote is formed by the fusion of haploid male and female gametes. The sporophytic generation develops from the zygote. The development of gametophytes is different in the case of males and females. A mature male gametophyte in angiosperms is a pollen grain that comprises a 3-celled structure, while a female gametophyte in angiosperms is also known as an embryo sac 7-celled structure. Read this article to learn more about male gametophytes.

What is Gametophyte?

A gametophyte is the multicellular haploid phase in the life cycle of every plant and algae. In lower plants, the different types of gametophytes are archegonium and antheridium, which produce eggs and sperms, respectively. In higher plants, the male and female gametophytes are comprised of male and female gametes, respectively.

The gametophyte is the sexual phase, while the sporophyte is the non-sexual phase during the alternation of generation. The sporophyte is the diploid generation formed by the union of male and female sex cells to produce a zygote (2n). This sporophyte again produces haploid spores through meiosis, giving rise to a new gametophyte phase.

Type of Gametophyte

There are two types of sex organs or gametangia found in gametophytes:
1. Archegonium is a multicellular sex organ found in females that produces eggs.
2. Antheridium is a multicellular sex organ found in males that produce sperms.

Bisexual gametophytes are those which contain both archegonium and antheridium. Unisexual gametophytes contain only one type of gametangium, either archegonium or antheridium.

In higher plants, male and female, two types of gametophytes are found, which eventually give rise to the male and female gametes. The female gametophyte in angiosperms is also known as the embryo sac.

Male Gametophyte Structure

a) Flowers are the main site of sexual reproduction in plants. They are mainly composed of calyx, corolla, androecium, and gynoecium. 
b) The stamen represents the male reproductive organ of a flower. It contains two parts, the anther and the filament.
c) The anther is the knob-like fertile part of the stamen.
d) The filament is long and slender.
e) Each anther is tetrasporangiate because it consists of four lobes, microsporangia, or pollen sacs.
f) Microsporangia form pollen sacs, which become filled with pollen grains on maturity.
g) Male gametophyte in angiosperms produces pollen grains. The haploid, uninucleate pollen grains represent the male gametophyte.
h) They are generally spherical (25-50 micrometre in diameter).
i) Each pollen grain has two-layered walls (sporoderm).
j) The exine is a hard outer layer made up of sporopollenin (one of the most resistant organic materials).
k) Sporopollenin is not affected by high temperatures, enzymes, strong acids, and alkali. Thus it helps in the fossilization of pollen grains.
I) It has prominent apertures called germ pores where sporopollenin is absent. Pollen tubes come out from germ pores.
m) Intine is the inner wall that is thin and continuous and made up of cellulose and pectin.
n) A plasma membrane surrounds the cytoplasm of the pollen grain.
o) Each mature pollen in angiosperm contains two cells, the vegetative and generative cells.
p) The vegetative cell is larger and contains abundant food reserves and a large irregularly shaped nucleus.
q) The generative cell is the small one that floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and nucleus.
r) In the majority of the angiosperms, i.e., more than 60% of plants, the spleen grains are released at a 2-celled stage consisting of a vegetative nucleus and a generative nucleus. In the remaining species, the pollen grains are released at a 3-celled stage where the generative cell divides mitotically and gives rise to two male gametes.

 Fig: Structure of Pollen Grain or Male Gametophyte

Explain the Development of Male Gametophyte in Angiosperms

The development of male gametophytes in angiosperms is a complex process. It takes place mainly through the following events:

1. Development before pollination or pre-pollination development:

I. The development of male gametophytes starts inside the microsporangium or pollen sac before pollination. So this type of development is known as precocious development.
II. The microspore or the pollen grain is the first cell of the male gametophyte.
III. During germination, the nucleus of the microspore or pollen grains is pushed from the centre to one side near the wall.
IV. The dense cytoplasm becomes highly vacuolated.
V. The microspore undergoes first mitotic division to produce two unequal cells.
VI. The larger cell is the vegetative cell, and the smaller thin cell is the generative or germinative cell.
VII. The tube cell or vegetative cell has a vacuolated cytoplasm rich in food reserve and a large, irregular-shaped nucleus.
VIII. A callose layer develops around the generative cell, separating the cell from the pollen grain wall. Later the dissolution of callose occurs, and the naked generative cell comes to liely in the cytoplasm of the tube cell.
IX. The germination of the vegetative cell gives rise to pollen tubes.
X. The pollen grains are generally shed from the anther at this 2-celled stage in case of most angiosperms.
XI. The generative cell produces two non-motile male gametes by second mitotic divisions. This occurs either in the pollen grain or in the pollen tube.
XII. In some cases, the pollen grain is shed at the 3-celled stage. In this case, the formation of male gametes occurs before pollination.
XIII. Further development of the male gametophyte takes place on the stigma after pollination.

2. Development after pollination or post- pollination development:

I. On the stigma, the pollen grains get deposited after pollination and absorb water and sugary substances from the stigmatic secretion through its germ pores.
II. The tube cell or vegetative cell enlarges.
III. The volume of the cytoplasm increases; thus, it creates pressure on the intine.
IV. The intine comes out of the pollen grain through one of the germ pores in the form of a pollen tube.
V. The growth of the pollen tube is influenced by the sugary substance secreted by the stigma, whereby the pollen tube pierces down the stigma and makes its way down the style and the wall of the ovary carrying the two nuclei with it.
VI. The tube nucleus descends to the tip of the pollen tube, and the generative nucleus lies just behind it.
VII. The generative nucleus soon divides to form two non-motile male gametes.
VIII. Each male gamete contains a large nucleus surrounded by a thin sheath of cytoplasm.
IX. Pollen tube carries male gametes and tube nucleus towards the female gametophyte.
X. A pollen grain with a pollen tube carrying male gametes represents a mature male gametophyte. It is a 3-celled (1 tube cell+ two male gametes) and 3-nucleated structure formed by two mitotic divisions.
XI. The pollen tube next pierces through the nucellus tissue and comes in contact with the synergids.
XII. Due to the degeneration of synergids, the pollen tube enters the embryo sac.
XIII. Ultimately, the tip of the pollen tube bursts, liberating the two male gametes within the embryo sac.

Fig: Development of the Male Gametophyte

Male Gametophyte Function

The male gametophyte is necessary for sexual reproduction. In flowering plants, haploid male gametes are formed in the male gametophyte. The male gametophyte in most angiosperms is shed at the 3-nucleate stage.

Difference between Male and Female Gametophyte

Summary

A gametophyte is the multicellular haploid phase in the life cycle of every plant and algae. In lower plants, the different types of gametophytes are archegonium and antheridium, which produce eggs and sperms, respectively. In higher plants, the male and female gametophytes comprise male and female gametes, respectively. A mature male gametophyte in angiosperms is a pollen grain containing 3-celled structures. The male gametophyte develops mainly through two phases of growth, one pre-pollination and one post-pollination. 

FAQs Related to Male Gametophyte

Q.1. What is a gametophyte?
Ans: A gametophyte is the multicellular haploid phase in the life cycle of every plant and algae. 

Q.2. What are the two different types of gametophytes?
Ans: The different types of gametophytes in lower plants are archegonium and antheridium, which produce eggs and sperms. In higher plants, the male and female gametophytes comprise male and female gametes, respectively. 

Q.3. What contains the male gametophyte?
Ans: A mature male gametophyte in angiosperms is a pollen grain that comprises 3-celled structures. 

Q.4. Write a few examples of gametophytes.
Ans: The few examples of gametophytes are:
Mosses: Antheridium and archegonium
Angiosperms: Pollens and embryo sac.

Q.5. Is a sperm haploid or diploid?
Ans: Sperm is haploid.

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