Every plant, as well as some algae, has a gametophyte in its life cycle. It is one of the generation alternation phases. Sporophyte, or multicellular diploid generation, and Gametophyte, or haploid generation, are the two alternating phases.
Bisexual gametophytes are gametophytes that have both male and female organs. Unisexual gametophytes are gametophytes with only one type of sex organ or gametangium. Male and female reproductive organs, on the other hand, are microscopic structures.
Gametophyte
All plants and some algal species have a life cycle called alternation of generations. A sporophyte, a multicellular diploid generation, and a gametophyte, a multicellular haploid generation, are involved in this process. Diploid indicates your cells have two sets of chromosomes and is commonly denoted as ‘2n’. The letter ‘n’ stands for haploid, which indicates your cells have only one set of chromosomes.
Gametophytes Functions
A gametophyte’s primary function is to produce gametes. Gametes, or haploid reproductive cells like eggs and sperm, are haploid reproductive cells. Mitosis, or cellular division without chromosomal reduction, produces them in sex organs on the haploid gametophyte. In gametophytes, there are two sex organs or gametangia.
Archegonium
Female sex organ with multicellular cells that generate eggs.
Antheridium
Sperm-producing multicellular male sex organ.
As both organs generate haploid egg cells, an archegonium can be compared to a human female’s ovaries. Because both generate haploid sperm cells, an antheridium is similar to testis in human males. Archegonia and antheridia, on the other hand, are usually small structures.
Archegonia and antheridia are found in some gametophytes. Because they have both female and male structures, they are known as bisexual gametophytes. A unisexual gametophyte possesses just one type of gametangium (either an archegonium or an antheridium, but not both).
Sexual reproduction happens when a sperm cell fertilises an egg cell, and gametophytes are required for this to happen. A diploid unicellular zygote is produced during sexual reproduction and will undergo mitosis to become multicellular. The sporophyte is a multicellular diploid creature.
Production of Gametophytes
When spores are produced by the sporophyte generation, a gametophyte is formed. Meiosis, or cell division, produces spores by halving the number of chromosomes.
As sporophytes are diploid, a diploid cell that goes through meiosis produces haploid cells. Sporophytes produce spores, which are haploid cells. After that, the spores will go through mitosis and develop into a multicellular haploid gametophyte.
Gametophytes in Plants
The gametophyte generation is the major generation in nonvascular plants like mosses. The gametophyte generation is bigger and lives longer than the sporophyte generation. This is the gametophyte generation, which may be seen in the green leafy section of a moss.
The moss sporophyte is a short-lived moss spore that emerges from the gametophyte. It is also entirely nutrient-dependent on the gametophyte. The gametophyte is tiny and not the major generation in vascular plants like ferns and fronds.
Gametophyte Development in Mosses
In bryophytes, the sporophyte is small and reliant on the gametophyte, which is more conspicuous and nutritionally independent. With small leaf-like photosynthetic organs, the moss gametophyte resembles a microscopic herb.
The gametophyte is formed from a latent spore that germinates under ideal conditions to produce filamentous and branching protonemal tissues. Each of these grows into a leafy stalk after forming multicellular bud-like structures. Male and female sexual organs, antheridia and archegonia, are produced by mature gametophytes. Plants are either male or female, and the gametophyte is generally sexually different.
Gametophyte development in gymnosperms
The sporophytic generation is dominant and free-living in seed plants, while the gametophytes are small and depend on the sporophyte for nourishment. Conifers are gymnosperms with male and female cones on the same tree. On different individuals, other gymnosperms have male and female reproductive structures. Wind or animals assist the male gametophyte (pollen grain) in reaching the female reproductive structure in all gymnosperms.
Microsporophylls and megasporophylls, respectively, are scale-like male or female organs on the cones. The microsporophyll in pines has two microsporangia, sac-like structures that contain many diploid microspore mother cells that conduct meiosis to make haploid microspores.
Each ovule in the megasporophyll contains a single megaspore mother cell, which produces four meiotic products, one of which is the functioning megaspore. Male and female gametophytes originate from the microspore and megaspore, respectively.
Male Gametophyte Development in Angiosperms
Angiosperms are distinguished by the presence of seeds in the enclosing fruit, which is derived from a flower’s ovary. The flower is predominantly made up of sporophytic tissues, with male and female gametophytes that are much smaller than those found in other terrestrial plants.
In addition to the embryo, angiosperms have the rare property of multiple fertilization, forming a triploid endosperm. The male gametophyte is found in the stamens’ anthers, whereas the female gametophyte is found in the pistil’s ovules.
Four pollen sacs (locules) in the anther contain many microspore mother cells, each of which goes through meiosis to create four microspores in a tetrad. The microspore is the starting point for male gametophyte production. The cytoplasm of the microspore is uniformly dispersed at first, with a haploid nucleus in the center.
Later on, a huge vacuole grows in the center, pushing the nucleus to the side. During pollen development, many flowering plants, including the model species Arabidopsis and maize, undergo two mitotic divisions. A huge vegetative cell and a much smaller generative cell result from the first division.
Conclusion
In terrestrial plants, the gametophyte generation has evolved from a free-living, somewhat complex organism to microscopic structures that depend on the sporophyte for survival. Seed plants’ success is likely due to their strong vasculature, sporophyte-supported gametophytes, and drought-resistant pollen.
Flowering plants’ reproductive fitness was likely improved even more by the protective ovary/fruit and triploid endosperm, making them the most successful group of plants.
Understanding the evolution of gametophytic sexual dimorphism is crucial to gametophyte study. Mosses produce sexually dimorphic gametophytes from a single type of spore, whereas ferns produce morphologically uniform gametophytes that differ only in sexual organs.
From the first mitosis, the gametophytes in seed plants, including gymnosperms and angiosperms, are genetically predetermined and physically dimorphic.