Plants are multifarious organisms which show various degrees of evolution which ranges from plants that are not differentiated into leaves stems and roots like Thallophytes to those that are fully differentiated, like the Angiosperms. Some plants like the Cryptogams (Thallophytes, Bryophytes and Pteridophytes) are non-flowering and do not produce seeds, whereas the Phenerogams are flowering and produce seeds. Even among the Phenerogams, Gymnosperms have naked seeds however in angiosperms; the seeds are well protected inside a fruit. The life cycle of plants is as complex and diversified as the morphology or anatomy or any other aspect of the same. Irrespective of the level of hierarchy, all plants show alternation of generation in reproduction. Using a multicellular gametophyte alternately with a multicellular sporophyte in for the purpose of reproduction is called Alternation of Generation. Depending on a plant’s grade on the evolutionary ladder, one stage is more dominant than the other. These two stages are different in various aspects, as listed below.
Ploidy: The number of sets of chromosomes (ploidy) is different for both these stages. Gametophytes are haploid (n) and have a single set of chromosomes, whereas Sporophytes are diploid (2n), i.e., they have two sets of chromosomes.
How they are important in Alternation of Generation: Gametophytes produces male and female gametes, by mitosis which fuse to form a zygote, which in turn gives rise to a diploid sporophyte which generates haploid spores, each of which again gives rise to a gametophyte. This process helps alternating haploidy with diploidy. A sporophyte reproduces asexually and a gametophyte sexually.
Significance: For a diploid (2n) sporophyte to produce haploid (n) spores, the cells have to undergo meiosis. While a superficial glance at this phenomenon shows it as a kind of biological phenomenon that halves the number of chromosomal sets, it actually involves a process of much deeper importance. During this amitotic division, the internal repair mechanism of a cell restores the damaged parts of DNA to normalcy and when the damage is irreparable kills the cell thus preventing abnormalities from being carried to the next generations, thus giving adaptive advantage in favour of meiosis(1). There is also a selective advantage in producing haploid spores. When an aberrant part of genetic material slips through the scrutiny of the repair mechanism and forms a spore, it is eliminated by the environment when the trait expressed causes a disadvantage to the gametophyte that has germinated from it.
Fusion of male and female gametes produced by the gametophytes gives the advantage of genetic variation, and is known to induce vigour in resulting progeny. Many species ensure that the male and female gametes are not released at the same time to ensure cross fertilization.
Sporophytes and Gametophytes in different groups of plants: Although both of these stages are common through all the plant groups their status and level of complexity is different across the same.
- Algae: The sporophytes and the gametophytes in algae may be isomorphic (similar appearing) or anisomosphic. In this group of organisms gametophyte is dominant while the sporophyte is restricted to the zygote (2).
- Bryophytes: Gametophyte is long lived among this group of plants. The sporophyte is nutritionally dependent on the former. The spore bearing capsule, sporangium obtains its nutrition through a small stalk called seta.
- Pteridophytes: These share similarity with the preceding groups, when it comes the dominant phase. However sporophyte (Prothallus) is independent, though it not so well differentiated as its haploid counterpart. It is considerably small in size. Leaves or sporophylls bear spores on the ventral side in their sporangia.
- Gymnosperms: Sporophyte is dominant and heterosporous. Separate male and female gametophytes are common which develop in micro and megaspores respectively.
- Angiosperms: Just like its fellow seed bearing group member, Angiosperms have sporophyte as dominant form. The only difference is that the gametophyte is more intricately developed in this group when compared to the former one.