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).
Types of Gametophytes
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.
Female Gametophyte
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.
Development of Female Gametophyte
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.
Male Gametophyte
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.
Development of Male Gametophyte
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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Difference between Gametophytes and Sporophytes
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.
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The gametophyte is a haploid stage and the sporophyte is the diploid stage in the plant’s life cycle.
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Gamete formation takes place through mitosis whereas spore formation occurs through meiosis.
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The mother cell undergoes meiosis to form haploid spores. But the gametes fuse to form a diploid zygote.
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The meiospore develops into gametophytes and the zygote produces sporophytes.
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The sporophyte cells have two sets of chromosomes whereas gametophyte cells have one set of chromosomes.