Gametophyte

What is Gametophyte?

The gametophyte is a stage in the life cycle that is found in all plants and certain species of algae. This process includes both multicellular diploid generation known as Sporophyte and a multicellular haploid generation known as Gametophyte.

The word Diploid refers to two sets of chromosomes in the cells, and normally written as ‘2n’. Haploid to only one set of chromosomes in the cells and written as ‘n’.

The primary job of gametophyte is the production of Gametes. The produced gametes are the haploid reproductive cells, such as sperm and eggs.

Reproductive Organs of Gametophyte

There are two types of reproductive organs found in a gametophyte:

Archegonium: A multicellular sex organ in the female that creates eggs. It is like an ovary in females because both structures form haploid cells of the egg.

Antheridium: A Multicellular sex organ in the male that produces sperm.  An antheridium is just like a testis in human males because both generate haploid sperm cells.

 Both antheridia and archegonia are normally microscopic structures.

Some of the gametophytes have both antheridia and archegonia. These are known as bisexual gametophytes since they consist of both female and male structures. A gametophyte that has only one kind of gametangium is known as a unisexual gametophyte.

Haploid

A cell is said to be haploid if it possesses only one set of chromosomes. Haploid can also be used to refer to the number of chromosomes in the gametes which can either be eggs in females or sperm cells in males.

The gametes in humans are haploid as they contain 23 chromosomes. Each of this chromosome exists in a pair which are present in diploid cells. The chromosome number that is present in a single set can be represented as ‘n’, where ‘n’ is also known as the haploid number. In the case of humans, n=23.

Gametes possess half of the chromosomes that are present in normal diploid cells of the body. These diploid cells are referred to as somatic cells.

During meiosis, haploid gametes are produced. It is a kind of cell division wherein the number of chromosomes is reduced in the parent diploid cell by half in number. Some entities such as algae contain haploid sections in their life cycle. Other entities such as male ants exist as haploid entities all through their life span.

Female Gametophyte: Embryo sac

The female gametophyte of gymnosperms is a large and multicellular structure that serves the double function of supporting the gametes as well as nurturing the growing embryo which is in contrast to the state in angiosperms, wherein female gametophyte is minute and typically eight-nucleated with a single operational gamete.

The female gametophytes are responsible for the formation of female gametes and produce the molecular and physical basis for fertilization and origination of the seed development. These minuscule formations make up for the central hub for plant reproduction that depends on the molecular processes for its growth and development. It is not only sustained by gene functions but by cellular functions as well that are encoded by accessory cells which can turn active genetically if there is a failure in the gametes.

Development of Female Gametophyte

The whole process of female gametophyte development occurs in two different phases. The first phase involves the megasporogenesis, where a single diploid mother cell undergoes meiosis to form haploid megaspore tetrad out of which only one will survive and other three disintegrate. The functional megaspore grows into an embryo sac.

Megagametogenesis is the second phase where the functional haploid megaspore undergoes mitosis to generate 7-celled, 8-nucleate gametophyte known as embryo sac. Out of the eight nuclei, polar nuclei move to the centre and fuse to produce a single diploid cell at the centre. This is the cell which fuses with the sperm to produce triploid endosperm. Three of the nuclei develop into antipodal cells and two will transform into synergid cells which eventually disintegrate.

Male Gametophyte: Pollen Grain

The first gametophytic structure is the modification of microspore mother cell into the pollen grain. The pollen grain grows into the male gametophyte on germination which is initiated before pollination takes place.

Inside the microsporangium, Pollen mother cell (PMC) undergoes meiosis and results in four microspores which eventually mature into pollen grains. The inner layer called tapetum nourishes the developing microspores.

The pollen grains consist of two cells- a vegetative cell and a generative cell. Once the microsporangium is matured it bursts and releases the pollen from the anther.

The pollen grain consists of two layers.

• the outer thick layer is called the exine,
• the inner thin layer is called intine which protects the pollen from damage.

Male gametophyte development can be classified into two stages:

Pre-pollination development and post-pollination development.

Development of Male Gametophyte

Pollen grains display germination that is initiated in pollen sac which is referred to as precocious germination wherein a large central vacuole formation causes the nucleus to be pushed to one side post which the nucleus goes through mitosis that gives rise to two daughter nuclei.

The small cell that is formed is known as the generative cell and the larger cell is known as the vegetative cell which has cytoplasm in sufficient amounts acting as a food reserve for the male gametophyte to develop, while the generative cell assembles at the mid-section of the pollen grain. At this phase, the pollen grains fall on the stigma wherein further development takes place.

The falling of the pollen grains on the stigma causes the nutrient absorption through the germ pore from the stigma which causes the vegetative cell to enlarge. The enlargement causes the intine to move out via the germ pore for the formation of the pollen tube.

The nuclei of the vegetative and generative cell travel to the pollen tube. The generative cell at this stage divides to produce two haploids, the non-motile and unicellular male gametes. For the deeper insertion of the tube, the size of the developed male gametophyte is diminished and derives the nutrition from the style’s tissues.

Examples of Gametophytes

Following are the important examples of gametophytes:

Mosses

The gametophyte stage of moss is long-lived while sporophytes appear as long stalks that release spores by the wind. The sporophytes are formed by mixing the gametophyte sex cells. Therefore, they have twice the number of chromosomes compared to gametophytes.

Hornwort

The hornwort gametophyte is green, long-lived and low to the ground, while the sporophyte is a thin, long stalk from which the spores are released.

Hornwort gametophytes and sporophytes are independent organisms with different chromosome numbers.

Gametophyte And Sporophyte

All plants reproduce both sexually and asexually due to variation in generations.

Gametophytes and sporophytes consisting of genetic elements of the plant species. Some of the complex vascular plants spend more time as sporophytes, while mosses are not the same. The noticeable part of the plant is visible and is the gametophyte stage of these species.

There is only one copy of every chromosome in Haploid gametophytes in their every cell; this is the reason why this life phase must produce asexually. Eventually, gametophytes create gametes that combine to produce a diploid zygote and grows into the sporophyte.

Diploid sporophytes consist of two copies of every chromosome, hence it is able to support meiosis to produce haploid spores that grow into gametophytes, and brings the alternation of generations.

Difference Between Gametophyte And Sporophyte

Though the stages are common in different plant groups, their level of complexity and status across various groups is different.

Following are the differences between both stages:

Ø  The gametophyte is the haploid(n) state in the life cycle whereas sporophyte is the diploid(2n) state in the life cycle.

Ø  Spore formation takes place post meiosis whereas gamete formation either occurs directly or through mitosis.

Ø  For the formation of spores, the mother cell goes through meiosis to produce haploids whereas gametes undergo fusion for the formation of diploid(2n), the zygote.

Ø  The meiospore grows to develop into the gametophyte. The zygote grows to produce sporophytes

Ø  The cells in sporophytes have two genomes or possess two sets of chromosomes. The cells of the gametophyte have a single genome or produce one set of chromosomes.