Alteration of generations: conceptual and historical perspectives

Alternation of Generations

Alteration of Generations

"Alternation of Generations" is a term used to explain the life cycle and pattern of reproduction in certain vascular plants, protists and fungi. Usually generations of a species take one complete life cycle; therefore, in order to avoid confusion, this term is usually replaced with the term "Alternation of Phases of a single generation."

In "Alternation of Generations," the process of reproduction is different from other reproductions; it keeps moving back and forth from a sexual from of reproduction in one generation to an asexual form of reproduction in the other generation (Polunin, 1960). Each phase of the life cycle of organisms possessing "Alternation of Generations" contains one or two independent living organism gametophyte and a sporophyte. Sporophyte and gametophyte come from the concepts: spore bearing stage (sporophyte) and gamete bearing stage (gametophyte). Genetically, a gametophyte (which can be a tissue or plant) is a haploid with single set of chromosomes, while a sporophyte (which can also be a tissue or plant) is a diploid with double set (paired) of chromosomes.

In the alternation of generation process, a haploid belonging to the gametophyte generations uses the mitosis process to produce the gametes at its maturity. Then two gametes fuse to forms a zygote. These two gametes can be either from the same organism or from the different organism of the same species. This zygote than develops into a diploid plant of the sporophyte generation.

After development, this sporophyte, which is multi-cellular diploid with two paired chromosomes, uses the meiosis process to produce spores. These spores than grow and develop into a gametophyte of the next generation. This complete cycle of plants from gametophyte to gametophyte (or from sporophyte to sporophyte) is the process through which many plants and algae go through the sexual reproduction.

The land plant trees are divided into two branches; non-vascular plants (the bryophytes) and the vascular plants. These both have similar life cycles, as discussed above; the sporophyte generation and the gametophyte generation. However, despite these similarities of vascular plants and non-vascular plants, they also differ from each other.

In non-vascular plants, the gametophyte dominates the life cycle. For instance, in case of a moss, the sporophyte generation develops or grows on the gametophyte generation, which is termed as a parasitic relationship if they both are not from the same species. It is called parasite only due to the reason that a baby is parasite on its mother.

However, the situation is opposite in the vascular plants, as the sporophyte dominates the life cycle. Example can be any tree or flower or any other vascular plant in which the gametophyte is microscopic in size and grows like a parasite on the sporophyte tissue or plant. This gametophyte than becomes the part of the seed in the flowering plants.