WHAT IS GAMETOPHYTE? (EXAMPLES IN DIFFERENT SPECIES) - BIOLOGY - 2024

The gametophyte is a multicellular haploid organism that originates from a haploid spore that has a group of chromosomes. It corresponds to the haploid phase of a plant, destined to produce gametes, and to the sexual phase in one of the two alternate phases in the life cycle of land plants and marine algae.

This organism develops sex organs that give rise to gametes, which are haploid sex cells that are involved in fertilization. This gives rise to a diploid zygote, that is, it has two sets of chromosomes.

Gametophytes growing in a terrarium. Unknown species. Random Tree, from Wikimedia Commons

Of these two sets of chromosomes, one corresponds to the father and the other corresponds to the mother. Cell division of the zygote produces a new diploid multicellular organism.

In the second stage of the life cycle known as the sporophyte, its function is to produce haploid spores through cell division called meiosis.

The morphology of the gametophyte depends on the sex, that is, the female ones will have one shape and the male ones another different shape.

Gametophyte in Bryophytes

In this plant group (mosses, liverworts and horned) the gametophyte is the most prominent stage of the biological cycle.

This bryophyte gametophyte has a long life and is independent from a nutritional point of view. Sporophytes are generally attached to and need gametophytes.

In mosses, the spore begins to grow by germinating and makes a filament of cells called a protonema.

When maturing, the gametophyte evolves in the form of dense shoots that give rise to the sexual organs or gametangia that are the producers of gametes. Eggs are produced in archegonia and sperm in antheridia.

In groups such as liverworts belonging to the order Marchantiales, gametes originate from specialized organs called gametophores or gametangiophores.

Gametophytes in vascular plants

In all vascular plants (with stem, leaves and roots), sporophytes predominate with a tendency towards small, sporophyte-dependent female gametophytes. This was becoming more and more noticeable as plants evolved towards the form of reproduction by seeds.

Ferns produce only one type of spore called homospores. Its gametes are exosporic, which means that gametophytes are free-living and evolve outside the spore wall.

These exosporic gametophytes can be bisexual (monoecious), that is, with the ability to produce sperm and eggs in the same body. If they specialize in female and male organisms, separately, they are called dioecious.

Heterospore vascular plants (they produce both megaspores and microspores) have a gametophyte that develops endosporically, within the wall of the spore. In this case, the gametophytes are dioecious, producing eggs or sperm, but never both.

Gametophytes in ferns

In ferns, gametophytes are small, photosynthesize and have a free lifestyle, since they do not need the sporophyte for their food.

In the Leptosporangiate dryopteris species, of fern, the gametophyte is autotrophic (it makes its own food), it photosynthesizes and has a structure called the prothalo that produces the gametes. The prothalo maintains the sporophyte in its early multicellular stage of development.

In some groups, specifically in the genealogical branch (Psilotaceae and Ophioglossaceae), gametophytes are underground and survive establishing mycotrophic relationships with fungi.

Gametophytes in lycophytes

Two different types of gametophytes are produced in lycophytes. In the Huperziaceae and Lycopodiaceae families, spore-sprouted gametophytes are free-living, subterranean, and mycotrophic, meaning that they obtain their food through symbiotic relationships with fungi.

In the Selaginella and Isoetes families, the megaspores remain attached to the initial sporophyte and a megagametophyte develops within it.

As they mature, the fissures at the junction of the trilet open to facilitate the entry of male gametes into the archegonia, where they contact the ovules.

Gametophytes in seed plants

Plants that have seeds are called angiosperms and gymnosperms, all of them are endosporic and heterospores.

In these plants, the gametophytes transform into multicellular organisms when they are within the wall of the spores and the megaspores are retained in the sporangium.

In seed plants, the microgametophyte is known as pollen. The microgametophytes of the seed plant are made up of two or three cells when the pollen grains exit the sporangium.

All seed plants are heterospores and produce spores of different sizes: large female spores and small male spores.

The megagametophyte evolves within the megaspore in vascular seedless plants and within the megasporangium in a cone or flower of seed plants.

The microgametophyte of the seeds, which is the pollen grain, travels to where the egg cell is, carried by a physical or animal vector and there it produces two sperm by mitosis.

Gymnosperms

In gymnosperm plants, the megagametophyte is made up of several thousand cells and has one to several archegonia, each with a single egg cell. The gametophyte is transformed into tissue to store food in the seed.

Angiosperms

In angiosperm plants, the megagametophyte is reduced to a few nuclei and cells, and is called the embryo sac. The representative embryo sac has seven cells and eight nuclei, one of which is the egg cell.

Two nuclei unite with a sperm nucleus to form the endosperm, which is then transformed into tissue to store food in the seed.

Seed plants are characterized by the megaspore being retained within the sporophyte, by tissues called integuments. These have the function of enveloping and protecting the megasporangium.

In gymnosperm plants, the megaspores are surrounded by one integument, while in angiosperm plants, they are surrounded by two integuments.

The set formed by the megasporangium, the megaspore and the integuments-is called ovule. Inside each ovule, a female gametophyte evolves from a megaspore producing one or more female gametes.

When the pollen grains germinate, and growth begins, the appearance of a pollen tube begins, whose function is the introduction of the male gametes into the female gametophyte in the ovule.

In seed plants the female gametophyte remains in the sporophyte ovule. Male gametophytes are found in pollen grains and are travelers, therefore they can be carried long distances by wind or by pollinators, depending on the species.

Gametophytes in animals

In animals, evolutionary development begins with an egg or zygote, which goes through a series of mitoses to produce a diploid organism.

As it develops and matures, it forms haploid gametes based on certain diploid cell lines through meiosis. Meiosis is called gametogenic or gametic.

This cycle is present in all animals. Although there is no alternation of generations, there is an alternation of two nuclear phases, one haploid (the gametes) and the other diploid (development of an organism by mitosis starting from an egg or zygote).

Therefore, meiosis is gametic and this cycle is considered to be the most evolved in living organisms.

Heteromorphy of gametophytes

In plants that possess heteromorphic gametophytes, there are two types of gametophytes. Because they have different shapes and functions, they are called heteromorphs.

The gametophyte responsible for producing ovules is called megagametophyte, due to its large size, and the gametophyte responsible for the production of sperm is called microgametophyte. If gametophytes produce eggs and sperm on separate plants, they are called dioecious.

Heterosporous plants such as certain lycophytes, aquatic ferns, as well as all gymnosperms and angiosperms, have two different sporangia. Each of them produces a single spore and a single type of gametophyte.

But not all heteromorphic gametophytes come from heterosporic plants. This means that some plants have different egg and sperm-producing gametophytes.

But these gametophytes originate from the same type of spores within the same sporangium, an example of this: the Sphaerocarpos plant.

References

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