WHAT IS THE ALTERNATION OF GENERATIONS? - BIOLOGY - 2025

The alternation of generations consists of the succession of two different generations of the same species. That is, the plants alternate between gametophyte and sporophyte generations, haploid and diploid generations. This phenomenon is characteristic of the plant kingdom, although it can occur in other kingdoms.

Gametophyte plants reproduce sexually through gametes (that is why they are called "gametophytes") and are haploid, which means that they have cells that have a single set of chromosomes.

Sporophytic plants reproduce asexually through spores (hence the name "sporophytes") and are diploid, which means that each chromosome in the cell nucleus is duplicated.

According to Hofmeister (1862, cited by Haig, David), the first generation, the gametophyte, is intended to create the sexual organs, while the second generation, the sporophyte, is aimed at the production of reproductive cells in massive quantities (spores), which will give rise to another generation of gametophytes.

Alternation cycle

Sporophytes have structures (sporangia) that produce spores through meiosis, a process that refers to cell division that results in four cells that have half the chromosomal load of the parent. Therefore, these spores are haploid.

Haploid spores go through a process of mitosis (cell division that produces two cells identical to the mother cell), giving rise to gametophyte organisms. When they reach maturity, they will produce haploid gametes: eggs and sperm.

Gametes are produced in the gametangia through the process of mitosis. The gametangia represent the reproductive apparatus of plants: the male is called antheridium while the female is called archegonium.

The union of the gametes produced by the gametophytes will give rise to a diploid zygote, which will become a sporophyte individual. Once it has reached maturity, this plant will produce spores, starting the cycle again.

One of the two generations is dominant while the other is recessive. Dominant individuals tend to live longer and grow longer than others. Recessive individuals are usually very small and, in some cases, live in the dominant organism.

In non-vascular or bryophyte plants, gametophytes are dominant; however, in vascular plants, sporophytes are dominant.

Alternation of generations in bryophytes

Bryophytes are non-vascular plants, which means that they do not have a circulatory system, therefore, they depend on the humidity of the environment to survive. They are generally small in size.

Gametophyte individuals are the dominant generation of this species. An example of these plants is moss.

Alternation of generations in the moss

Mosses are non-vascular plants, which means that the gametophyte generation is the dominant one. The most developed part of the moss is gametophyte and in the upper part of it you can see the sporophyte plant, which is small.

Some of these gametophytes have antheridia, which are sac-shaped, while others have archegonia, which are bottle-shaped. In the antheridia, large amounts of anterozoids are produced that can be displaced thanks to a flagella (filament located on the outer wall of the cell, similar to a whip).

On the other hand, in archegonia, a single oosphere is produced, which lacks flagella and is therefore immobile. The anterozoids are released to the outside and, aided by the humidity of the plant, move to the oosphere, fertilizing it to create a zygote.

These zygotes have chromosomes from both parents and give rise to the diploid or sporophyte generation. It should be noted that diploid zygotes are extremely vulnerable, this is why the oosphere-bearing plant keeps them within itself and, in this way, the sporophyte plant grows on the gametophyte plant.

The sporophyte individual is composed of a filament that culminates in a capsule that resembles a bell. Once the zygote has matured, the capsule opens and releases tiny haploid cells that will grow as gametophyte individuals.

Photos recovered from word-builders.org

Alternation of generations in tracheophytes

Tracheophytes are plants that have vascular tissues. They are divided into gymnosperms (coniferous plants) and angiosperms (flowering plants). Unlike bryophytes, these plants protect their embryos through seeds.

The diploid, or sporophyte, generation is the dominant one. An example of a tracheophyte plant is the fern.

Alternation of generations in ferns

The most visible part of ferns is a sporophyte. The sporophyte produces leaves called fronds, which produce sori, clusters of tiny spores. When sori mature, the spores are released and eventually make landfall.

These spores give rise to the gametophyte, called the prothalo, which is very small and grows separately from the mother plant.

Its life cycle is reduced to a couple of days. These organisms produce gametes that will later join others. The new plant, the sporophyte, grows on the gametophyte, which deteriorates and eventually dies, starting a new cycle.

Alternation of generations in angiosperms

As in ferns, the dominant plant in angiosperms is a sporophyte. On the other hand, the gametophyte is a tiny structure found in the flower of the plant. In angiosperms there are two fertilization processes.

In the first, a male cell fuses with a haploid cell, giving rise to a diploid zygote. In the second process, a male cell fuses with another diploid, creating endosperm, the reserve tissue from which the seeds of angiosperms are born.

References

1. Alternation of Generations. Retrieved on February 24, 2017, from libertyprepnc.com.
2. Alternation of Generation. Retrieved on February 24, 2017, from msu.edu.
3. Krempels, D. Workshop on Alternation of Generations. Retrieved on February 24, 2017, from bio.miami.edu.
4. Plant Biology. Retrieved on February 24, 2017, from shmoop.com
5. Alternation of Generations: How Mosses Play on Land. Retrieved on February 24, 2017, from world-builders.org.
6. Kent, L. (1997). Studying the Alternation of Generations of Bryophytes in the Laboratory. Retrieved on February 24, 2017, from saps.org.uk.
7. The life cycle of plants: Alternation of Generations. Retrieved on February 24, 2017, from sparknotes.com.