Gametophyte is a phenomenon found in the life-cycle of every plant, and some algae. It is one of the phases of alternation of generation. These alternate phases include Sporophyte or the multicellular diploid generation and Gametophyte or haploid generation.
In a diploid generation, the participating cells have two sets of chromosomes, which is referred to as “2n”. Haploid involves one set of chromosomes in cells and refers to “n”. The basic objective of the gametophyte process is to produce gametes, haploid reproductive cells like eggs and sperm.
During the haploid gametophyte phase, male and female gametophyte organs produce gametes by simple mitosis (Mitosis is a process of cell-division without reducing the chromosome sets).
Two kinds of sex organs (that are male and female) are identified in gametophyte process-
Archegonium is a multicellular female reproductive organ that produces eggs. This organ is similar to the ovary as it creates haploid egg cells.
Antheridium is a multicellular testis-like male reproductive organ that helps in creating haploid sperm cells.
Gametophytes with both male and female organs are called bisexual gametophytes. And gametophytes having one type of sex organ or gametangium are known as unisexual gametophytes. However, both male and female reproductive organs are microscopic structures.
The female gametophyte organ is widely known as Embryo sac. In gymnosperms, the female reproductive organ is relatively large and multicellular as the structure not only supports the gametes but also helps to develop the embryo.
Contrarily, in angiosperms, the female gametophyte is a small and eight-nucleated structure that only operates the gametes. Female gametophytes form female gametes that are a molecular basis of fertilization and help in seed development. This process is nursed by both gene and cellular functions, so in case of gametes failures, the accessory cells can be activated genetically.
The development of female gametophytes happens in two notable phases. Megasporogenesis is the first phase where tetrad haploid megaspores originate from a single diploid cell through meiosis. However, only one megaspore survives and starts developing inside an embryo sac; the other three break down.
In the second phase, megagametogenesis, the operative haploid megaspore produces the female gametophyte – embryo sac. The structure of the female gametophyte is 7-celled and 8-nucleate. Only polar nuclei of these 8 nuclei shift and merge to create a diploid cell right at the center. After merging with the sperm, this diploid cell creates a triploid endosperm.
Three nuclei create antipodal cells, and the other two develop synergid cells. But, all these cells eventually disintegrate.
The basic gametophytic structure is a pollen grain, which is modified from the microspore mother cell. Before the pollination process, pollen grain turns into male gametophyte through germination. Through meiosis cell-division, the pollen mother cell creates four microspores inside the microsporangium, and these microspores gradually develop the pollen grain.
The inner layer of microsporangium is called tapetum that nurses the growing microspores. Once the microsporangium reaches maturity, it breaks and discharges the pollen.
The structure of male gametophyte - pollen grain consists of two cells – one generative and one vegetative cell, along with two layers. The exine is the thick outer layer and inline is the thin inner layer that safeguards the pollen.
The germination in the pollen sac forms a central vacuole structure by pushing the nucleus to one side. Later, the nucleus divides into two nuclei by mitosis. The large cell acts as a vegetative cell and the small cell refers to a generative cell.
The vegetative cell with cytoplasm acts as the preserved food for male gametophyte and the generative cell corners to a smaller part of the pollen grain. During this phase, the developing pollen grain lands on the stigma and absorbs nutrients through the germ pore.
This absorption enlarges the vegetative cell and eventually forms the pollen tube by moving out the intine through the germ pore. During this stage, the nuclei move to the pollen tube, and the generative cell is divided into two haploid cells – non-motile and one-celled male gametes.
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Both the stages are common in different plants. However, the level of complexity and status is different for different phases.
Here are the differences between both phases.
The gametophyte is a haploid stage and the sporophyte is the diploid stage in the plant’s life cycle.
Gamete formation takes place through mitosis whereas spore formation occurs through meiosis.
The mother cell undergoes meiosis to form haploid spores. But the gametes fuse to form a diploid zygote.
The meiospore develops into gametophytes and the zygote produces sporophytes.
The sporophyte cells have two sets of chromosomes whereas gametophyte cells have one set of chromosomes.
1. What is gametophyte generation?
Gametophyte generation is a stage within the life-cycle of plants and some algae species that initiates with a haploid spore till the time gametophytes are created by several mitotic divisions. A gametophyte is a haploid multicellular plant form. Therefore, it has only a single set of chromosomes. This phase is the sexual phase in the plant’s life cycle and they develop sex organs that produce gametes which are also haploid. These gametes further enter into the sporophyte stage after fertilisation. The sporophyte is characterized by a diploid form of the plant.
2. What are the examples of gametophyte?
In plants, the gametophyte generation is one of the two phases of the plant’s life cycle that begins with a haploid spore (n). This spore undergoes subsequent mitotic divisions to produce a gametophyte. A gametophyte has only one chromosome set and is characterized as a haploid multicellular form of the plant. In the gametophyte phase, plants develop sex organs that further produce haploid gametes. After fertilisation, gametes enter into the diploid sporophyte stage. Hornworts, ferns, and mosses are some common examples of gametophytes.
3. What are the types of gametophyte?
There are two different types of gametophyte found in plants or algae – female gametophytes and male gametophytes. The embryo sac is the female gametophyte, whereas the male gametophytes are called pollen grains. The female gametophyte develops in the ovule and consists of one central cell, one egg cell, three antipodal cells, and two synergid cells. Whereas, the pollen grain changes into the male gametophyte after germination. The pollen mother cell inside microsporangium undergoes meiosis to produce four pollen grains. Pollen grains consist of two cells, one vegetative and one generative cell.
4. Describe the development of male gametophytes.
The pollen sac is responsible for giving rise to the pollen grains. In the sac, a large vacuole develops that pushes the nucleus into one side and then it undergoes mitosis and gives rise to two daughter nuclei. The small cell formed is known as a generative cell and the larger cell is known as a vegetative cell that has a large number of nutrients for nourishing the developing pollen grains. After pollen dispersion, pollen grains fall on the stigma where further development takes place.
5. Describe the development of female gametophytes.
The process of development takes place in two different phases. The first phase is megasporogenesis where a single megaspore mother cell gives rise to four megaspores through meiosis out of which three degenerate and one survives. This functional megaspore gives rise to the embryo sac.
The second phase is megagametogenesis where the functional haploid megaspore forms 7 cells 8 nucleate gametophyte or embryo sac through mitosis. Out of the eight nuclei, polar nuclei move at the centre and form a single diploid cell. The three nuclei develop into antipodal cells and two nuclei form synergid cells.