A gametophyte is one of the two alternating phases in the life cycle of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte. The sporophyte can produce haploid spores by meiosis.
In some multicellular green algae (Ulva lactuca is one example), red algae and brown algae, sporophytes and gametophytes may be externally indistinguishable (isomorphic). In Ulva the gametes are isogamous, 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. In extant land plants, either the sporophyte or the gametophyte may be reduced (heteromorphic).
In bryophytes (mosses, liverworts, and hornworts), the gametophyte is the most visible stage of the life cycle. The bryophyte gametophyte is longer lived, nutritionally independent, and the sporophytes are typically attached to the gametophytes and dependent on them. When a moss spore germinates it grows to produce a filament of cells (called the protonema). The mature gametophyte of mosses develops into leafy shoots that produce sex organs (gametangia) that produce gametes. Eggs develop in archegonia and sperm in antheridia.
All vascular plants are sporophyte dominant, and a trend toward smaller and more sporophyte-dependent female gametophytes is evident as land plants evolved reproduction by seeds. Vascular plants such as ferns that produce only one type of spore are said to be homosporous. They have exosporic gametophytes--that is, the gametophyte is free-living and develops outside of the spore wall. Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus (monoicous), or specialized into separate male and female organisms (dioicous).
In heterosporous vascular plants (plants that produce both microspores and megaspores), the gametophyte develops endosporically (within the spore wall). These gametophytes are dioicous, producing either sperm or eggs but not both.
In most ferns, for example, in the leptosporangiate fern Dryopteris, the gametophyte is a photosynthetic free living autotrophic organism called a prothallus that produces gametes and maintains the sporophyte during its early multicellular development. However, in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi.
Extant lycophytes produce two different types of gametophytes. In the homosporous families Lycopodiaceae and Huperziaceae, spores germinate into bisexual free-living, subterranean and mycotrophic gametophytes that derive nutrients from symbiosis with fungi. In Isoetes and Selaginella, which are heterosporous, microspores and megaspores are dispersed from sporangia either passively or by active ejection. Microspores produce microgametophytes which the produce sperm. Megaspores produce reduced megagametophytes inside the spore wall. At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside. The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous arborescent lycophytes Lepidodendron and Lepidostrobus.
The seed plants (gymnosperms and angiosperms) are endosporic and heterosporous. The gametophytes develop into multicellular organisms while still enclosed within the spore wall, and the megaspores are retained within the sporangium.
In plants with heteromorphic gametophytes, there are two distinct kinds of gametophytes. Because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". The egg producing gametophyte is known as a megagametophyte, because it is typically larger, and the sperm producing gametophyte is known as a microgametophyte. Gametophytes which produce egg and sperm on separate plants are termed dioicous.
In heterosporous plants (water ferns, some lycophytes, as well as all gymnosperms and angiosperms), there are two distinct sporangia, each of which produces a single kind of spore and single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants. That is, some plants have distinct egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same kind of spore inside the same sporangium; Sphaerocarpos is an example of such a plant.
In seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of several (typically two to five) 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) 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 the endosperm, which becomes the food storage tissue in the seed.