Alternation of generations definition, Life cycle
Alternation of generations can be defined as the reproductive cycle of certain vascular plants, fungi, and protists, in which each phase consists of one of two distinct, free-living organisms: a gametophyte, which is often but not always genetically haploid, and a sporophyte, which is often but is not always genetically diploid.
All plants alternate between sexual and asexual reproduction and between the diploid sporophyte and the haploid gametophyte. Therefore, the life cycle of plants is called the alternation of generations. The ability of plants to reproduce both sexually and asexually contributes to their ability to adapt to the environment.
Embryonic development can only be observed in the diploid generation. However, only the haploid generation can produce gametes that are fused to form the embryo. Therefore, it is important to understand how the two generations interact while studying plant development.
Alternation of Generations Life Cycle
It comprises two different generations, namely:
- Sporophyte generation
- Gametophyte generation
It is an asexual phase in which a diploid plant (with two sets of chromosomes) develops and eventually produces spores. Through mitosis, this sporophyte produces haploid (having a single set of chromosomes) spores that germinate and develop into gametophytes for the next generation. A diploid zygote is formed when two gametes join during fertilization. This zygote then divides by mitosis to create a new sporophyte. Once spores are produced, the only way they can affect the next phase of alternation is if they are released, carried away by air and water, and end up in an environment that supports germination and gametophyte growth.
It is a sexual phase in which haploid male and female organs (gametangia) develop and produce eggs and sperm (gametes) for sexual reproduction through simple mitosis. A zygote formed from two gametes, either from the same organism or from different members of the same species, develops into the diploid (having two sets of chromosomes) sporophyte phase, which produces unicellular spores by meiotic division. This cycle continues.
While sporophyte generation predominates in angiosperms and gymnosperms, the gametophyte generation is the primary life stage in mosses such as mosses and liverworts. The haploid phase also predominates in fungi. In many species of algae, the sexual and asexual phases also alternate.
The haploid gametophyte and the diploid sporophyte are the two phases (ontogenesis) between which all land plants alternate, as recognized by the German botanists Hofmeister (1851, 1862) and Strasburger (1894). Despite significant physical differences and variances between the major groups, Hofmeister’s research has shown that the life cycle and technique of reproduction of land plants are fundamentally identical. This discovery had a significant impact on systematic botany. It has sparked much debate about the evolution of life cycles and the causes of the morphological variations between gametophytes and sporophytes.
Implications for Understanding the Origin of Land Plant Life Cycles
Two evolutionary ideas have been put out regarding the life cycles of putative “green algal” progenitors to explain the genesis of the alternation of generations in terrestrial plants.
- Antithetic theory
- Alternative homologous theory
According to the antithetical theory, land plants descended from green algae, with haploid, haplobiontic life cycles and anisomorphic alternation of generations, resemble Coleochaete, a “charophycean alga”. In a haploid, haplobiontic life cycle, the sporophyte (zygote) is unicellular and the initial sporophytic cell division is meiotic. However, the gametophyte generation is multicellular. According to this theory, sporophyte generation of land plants derives from somatic cell divisions that were crosslinked in the zygote before meiosis.
Alternative homologous theory
According to the alternative homologous theory, the first green algae had an isomorphic alternation of generations and a diplobionic life cycle. According to this theory, several features of the sporophyte and gametophyte generations of land plants may have previously existed in the algal ancestor.
The gametophytes and sporophytes of non-flowering plants are easy to monitor and regulate. They have therefore often been used to study the developmental processes underlying the alternation of generations. Moss, like Physcomitrella patens, is one of them and is an early divergent land plant, making it a suitable model to determine the origin and evolution of developmental pathways. Recent advances in plant systematics have had a significant impact on current theories of alternation evolution based on life cycles in existing taxa.
Advantages of Alternation of Generations
It allows species to adapt their reproductive strategies to shifting environmental conditions. An organism can reproduce asexually if it lives in a hostile environment and struggles to attract a partner.
The gametophyte is a dominant phase in the life cycle of a bryophyte. To explain.
In mosses, there is an alternation of generations, i.e. both gametophytic (haploid) and sporophytic (diploid) generations are present. However, the dominant phase is represented by the gametophytic plant. The gametophyte is an independent photosynthetic thalloid phase. Antheridia and archegonia are formed on the gametophytes. These organs are multicellular. These organs bear the respective gametes, viz. male and female. The antheridium produces two-lobed antherozoids. The archegonium is flask-shaped. It produces a single egg. Transmission of antherozoids to the archegonium occurs through the water. Flagella in the antherozoids facilitate motility. The zygote is formed after fertilization. But the zygote does not undergo meiosis immediately after fertilization. It develops into a multicellular body called a sporophyte.
The sporophyte in bryophytes is much smaller and is not independent but attached to the gametophyte. It is nourished by the gametophyte. It is clear that the gametophyte is the dominant stage in a moss life cycle in every respect. The gametophyte is independent and photosynthetic. It is much larger than the sporophyte. On the other hand, the sporophyte does not live freely but remains attached to the gametophyte so that it can feed.
When and where does reduction division occur in the life cycle of a liverwort, a moss, a fern, a nudibranch, and an angiosperm?
Hepatica – In hepatica, the main plant body is haploid (gametophytic). It carries the male and female sex organs that produce gametes. These gametes fuse to form a zygote. The zygote develops into a sporophyte on the gametophytic plant body. The sporophyte is divided into a foot, seta and capsule. As a result of the reduction division that takes place inside the capsule, many haploid spores are produced.
Moss – In mosses, the primary protonema (created in the first stage) develops into the secondary protonema. Both stages are haploid or gametophytic. The secondary protonema carries the sex organs that produce gametes. These gametes fuse to form a zygote. The zygote develops into a sporophyte. Many spores are formed as a result of the reduction division occurring in the capsule of this sporophyte.
Fern – In ferns, the main plant body is sporophytic. Its leaves are known as sporophylls and these bear the sporangia. Reduction division occurs in these sporangia, producing many spores.
Gymnosperm – In gymnosperms, the main plant body is sporophytic. They bear two types of leaves: microsporophylls and megasporophylls. The reduction division takes place in the microsporangia (which produce pollen grains) present on the microsporophyllene and the megasporangia (which produce megaspores) present on the megasporophyllene.
Angiosperm – In angiosperms, the main plant body is sporophytic and bears flowers. The flower’s male sex organ is the stamen, while the female sex organ is the pistil. Reduction division occurs in the anthers of the stamens (production of haploid pollen grains) and in the ovary of the pistil (production of eggs).
Name three groups of plants that bear archegonia. Briefly describe the life cycle of one of them.
Archegonium is the female sex organ that produces the female gamete or egg. It is present in the life cycles of mosses, pteridophytes, and gymnosperms.
The life cycle of a fern (Dryopteris) :
Dryopteris is a common fern with pinnate compound leaves. The main plant body is sporophytic. Many sporangia are borne on the undersides of its mature leaves. Each sporangium has spore mother cells that undergo meiosis to produce haploid spores. Upon maturation, these spores dehisce and germinate to form a heart-shaped gametophyte called a prothallus.
The prothallus carries the male and female sex organs called the antheridia and archegonia, respectively. The antheridia produce sperm that swim in the water to reach the archegonia. The Egg is produced by Archegonia. As a result of fertilization, a zygote is formed. The zygote forms an embryo, which in turn develops into a new sporophyte. The young plant originates from the archegonium of the parent gametophyte.
Alternation of generations definition, Life cycle