Alternation of generations
Say it: awl-tur-NAY-shun uv jen-uh-RAY-shunz
Alternation of generations is a life cycle in which a multicellular haploid gametophyte and a multicellular diploid sporophyte produce each other in turn.
Also: haplodiplontic life cycle
Definition
What is alternation of generations?
In alternation of generations, a life cycle includes two multicellular forms. The diploid sporophyte, with two sets of chromosomes, makes spores by meiosis. Each haploid spore, with one set, grows by mitosis into a gametophyte, which makes eggs or sperm, also by mitosis. When two gametes fuse, the diploid zygote grows into a new sporophyte and the cycle repeats. All land plants have this cycle, as do many algae such as kelps and sea lettuce. Animals do not: in an animal, eggs and sperm are the sole haploid cells.
Across the plant kingdom the balance between the two generations shifts. In mosses and liverworts the green plant you see is the gametophyte, and the sporophyte is a small stalked capsule that stays attached to it and depends on it for food. In ferns both stages live independently, but the leafy fern is the sporophyte and the gametophyte is a tiny heart-shaped plate on the soil. In seed plants the sporophyte dominates completely: the tree or herb is the sporophyte, the male gametophyte is reduced to a pollen grain of a few cells, and the female gametophyte develops hidden inside the ovule.
This trend toward a dominant sporophyte, which can shelter the embryo and the gametophytes, is a central theme in the evolution of land plants.
Word parts
Where does the term “alternation of generations” come from?
- altern-
- by turns, one after the otherLatin alternus
- gener-
- begetting, producing offspringLatin generare
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Terms that go with it
- HaploidHaploid describes a cell or organism that has a single set of chromosomes, such as a human egg or sperm cell with 23 chromosomes.
- DiploidDiploid describes a cell or organism with two complete sets of chromosomes, one inherited from each parent, as in most human body cells.
- MeiosisMeiosis is a two-stage cell division that turns one diploid cell into four genetically different haploid cells, the precursors of eggs and sperm.
- SporeA spore is a small, hardy reproductive cell made by fungi, plants and some microbes that can spread widely and grow into a new organism on its own.
- GymnospermA gymnosperm is a seed plant whose seeds develop exposed on cone scales or similar structures rather than enclosed in an ovary, as in pines and cycads.
- AngiospermAn angiosperm is a flowering plant, a seed plant whose ovules develop inside an ovary that ripens into a fruit enclosing the seeds.
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Checked against
Written for biofideX in plain words, after checking these references:
- OpenStax Biology 2e, 25.1 Early Plant Life
- Graham and Wilcox, The origin of alternation of generations in land plants (Phil Trans R Soc B, 2000)
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