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A gametophyte is the haploid multicellular stage in the alternation of generations life cycle of plants and algae. It develops from a spore by mitotic cell division. Spores are the products of meiosis in sporophytes. Gametophytes produce haploid gametes by mitosis.
In some types of plants and algae (e.g. Ulva) gametes are isogamous--that is, the gametes are all of one size, shape and general morphology. In land plants, anisogamy is universal. As in animals, female and male gametes are called, respectively, eggs and sperm. The fusion of male and female gametes produces a diploid zygote, which develops by repeated mitotic cell divisions into a multicellular sporophyte.
In bryophytes (mosses, liverworts, and hornworts), the gametophyte represents the longer lived, nutritionally independent and more visible stage of the life cycle. Sporophytes are typically attached to and dependent on gametophytes. Moss gametophytes originate from the germination of a spore. The initial phase of growth leads to a filament of cells (called the protonema). The mature gametophyte of mosses produces gametangia (gamete-producing structures). Eggs develop in archegonia and sperm in antheridia. In gametophyte dominant organisms, such as bryophytes, specialized structures or even whole individual plants that produce gametes are sometimes called gametophores (or gametangiophores).
Among vascular plants, an interesting pattern exists with regard to gametophytes. Homosporous vascular plants (plants that produce only one type of spore) have exosporic gametophytes—that is, the gametophyte develops outside of the spore wall. In addition, these exosporic gametophytes are normally bisexual (produce both sperm and eggs). In heterosporous vascular plants (plants that produce both microspores and megaspores), the gametophyte develops endosporically—within the spore wall. These gametophytes are unisex, producing either sperm or eggs but not both. All vascular plants are sporophyte dominant, and a trend toward smaller and shorter-lived gametophytes is evident as land plants evolved in a terrestrial environment.
Extant lycophytes produce several different types of gametophytes. In the families Lycopodiaceae and Huperziaceae, gametophytes are subterranean and mycotrophic, deriving nutrients from symbiosis with fungi. Isoetes and Selaginella, which are heterosporous, megagametophytes develop inside the megaspores, which crack open to allow access to internal archegonia.
In most ferns, gametophytes are photosynthetic free living organism called a prothallus. However in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. In leptosporangiate ferns such as Dryopteris, the gametophyte is a free-living, autotrophic prothallus that maintains the sporophyte during its early multicellular development. By contrast, in seed plants (gymnosperms and angiosperms), gametophytes develop into multicellular organisms while still enclosed within the sporangium.
In heterosporous plants (water ferns, some lycophytes, gymnosperms and angiosperms) the egg producing gametophyte is known as a megagametophyte and the sperm producing gametophyte as a microgametophyte. In the seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of two or three cells when the pollen grains exit the sporangium. The megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte (pollen grain) travels to the vicinity of the egg cell (carried by a physical or animal vector), and produces two sperm by mitosis.
In gymnosperms the megagametophyte consists of several thousand cells and produces one to several archegonia, each with a single egg cell. The gametophyte becomes a food storage tissue in the seed.
In angiosperms, the megagametophyte is reduced to only a few nuclei and cells, and is sometimes called the embryo sac. A typical embryo sac contains seven cells and eight nuclei, one of which is the egg cell. Two nuclei fuse with a sperm nucleus to form endosperm, which becomes the food storage tissue in the seed.
In some multicellular green algae, red algae, or brown algae (Ulva lactuca is one example) sporophytes and gametophytes may be externally indistinguishable (isomorphic) or either the sporophyte or the gametophyte may be reduced (heteromorphic).
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