Alternation of generations describes a unique life cycle common in plants and some algae. This biological pattern involves an organism switching between two distinct forms across successive generations. The life cycle includes two separate, multicellular stages that produce each other in turn. This specialized reproductive strategy allows for both sexual and asexual phases to contribute to propagation. It represents a fundamental difference from animal life cycles, which typically involve only one multicellular adult stage.
The Two Distinct Life Stages
The life cycle features two multicellular stages: the sporophyte and the gametophyte. Each stage possesses a different number of chromosome sets. The sporophyte represents the diploid (2n) stage, meaning its cells contain two sets of chromosomes, one inherited from each parent. This form is often the more conspicuous structure in many plants.
The sporophyte produces reproductive cells called spores. These spores are haploid (n), meaning they contain a single set of chromosomes. In contrast, the gametophyte is the haploid (n) stage, developing as a multicellular structure from a spore.
The gametophyte produces gametes, which are reproductive cells like sperm and eggs. These gametes are also haploid. Both the sporophyte and gametophyte are distinct, multicellular forms, but their appearance and prominence vary significantly among different species.
The Cycle of Transition
The transition between the sporophyte and gametophyte stages involves cellular processes. The diploid sporophyte initiates the cycle by producing haploid spores through meiosis. Meiosis is a cell division that reduces the chromosome number by half, ensuring spores carry one set of chromosomes. These spores can develop into a new organism without fertilization.
Once released, a haploid spore germinates and grows into a gametophyte. This growth occurs through mitosis, a cell division process that creates genetically identical cells and maintains the haploid chromosome number. The developing gametophyte continues to enlarge, forming the distinct haploid plant body.
Upon reaching maturity, the gametophyte produces haploid gametes, sperm and eggs. These gametes are also formed through mitosis, as the gametophyte is already haploid and does not require chromosome reduction. For fertilization to occur, two gametes, from different individuals or parts of the same individual, fuse together.
The fusion of a haploid sperm and a haploid egg forms a diploid zygote. This zygote contains two sets of chromosomes, one from each gamete. The diploid zygote then undergoes mitotic cell divisions and development, growing into a new multicellular sporophyte, completing the cycle. This continuous alternation between these two distinct generations ensures the propagation of the species.
Where We See It in Nature
Alternation of generations is a widespread life cycle, observed in plants and various algae. Among different plant groups, the prominence of the sporophyte and gametophyte stages varies significantly. In mosses and liverworts (bryophytes), the gametophyte is the more dominant and visible stage, often forming the lush green carpet. The sporophyte in these plants is smaller and remains dependent on the gametophyte for nutrients and support.
Ferns (pteridophytes) showcase a shift in dominance where the sporophyte is the larger, more recognizable plant, like a fronded fern. Their gametophyte is an independent, free-living, albeit small, heart-shaped structure. This gametophyte produces the gametes that lead to a new sporophyte.
In plant groups like flowering plants (angiosperms) and conifers (gymnosperms), the sporophyte becomes dominant and represents the entire visible plant, such as a tree or a flower. Their gametophytes are reduced in size and dependent on the sporophyte for survival, existing as microscopic structures such as pollen grains (male gametophyte) and embryo sacs (female gametophyte) within the reproductive organs. Some algae also exhibit alternation of generations, with some species having similar sporophyte and gametophyte forms.