View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Magazine R718 Primer tissues. These form multicellular The first gametophytic division bud-like structures, each of which is asymmetric, producing one develops into a leafy shoot. The large and one small cell. In C. mature gametophytes produce richardii, the large cell divides Gametophyte male and female sexual organs, again asymmetrically to produce development the antheridia and archegonia, another small cell, which later respectively. The gametophyte is develops into the rhizoid, a root- often sexually distinct, and plants like structure. The small cell from Wuxing Li and Hong Ma are either male or female. the first division becomes the Each antheridium has an outer protonemal initial, which divides Unlike animals, which produce layer that encloses and protects further to form a linear three- single-celled gametes directly thousands of motile sperm, which celled protonema. The middle cell from meiotic products, plants swim through available external then undergoes a transition in have generations which alternate water layer to the egg. Fertilization divisional plane, forming a two- between the diploid sporophyte at the base of the cylindrical dimensional structure. During and haploid gametophyte (Figure archegonium produces a diploid hermaphrodite development, a 1A). The diploid sporophytic zygote which develops into an meristem is formed in the two- generation develops from the unbranched sporophyte. The dimensional plane and gives rise zygote, the fusion product of sporophyte consists of a thin stalk to the male antheridia and the haploid gametes. Sporophytic attached to the gametophyte, and female archegonia. In the male cells undergo meiosis to produce a capsule that encloses the gametophyte, most cells, except haploid spores, which divide sporophytic meiotic cells. the apical and basal cells, will mitotically to form the In recent years, the mosses develop into antheridia. multicellular gametophyte. Certain Physcomitrella patens and In C. richardii, each antheridium cells in the gametophyte Funaria hygrometrica have contains an outer sterile tissue subsequently differentiate into emerged as attractive model and an inner spermatogeneous gametes. systems for studying gene cell, which undergoes five rounds Land plants are divided into function in non-vascular plants of mitosis to produce 32 small four groups (Figure 1B). The most because of the relative ease of spermatocytes. Further basal group, the bryophytes, molecular manipulation by differentiation of the including mosses and liverworts, homologous recombination. spermatocytes generates lack vascular tissues. Plants of Mutants affecting gametophyte functional sperm cells, each with the second group, including ferns development have been isolated a coil structure and a flagellum. In and horsetails, have a vasculature and their analysis should provide the flask-shaped archegonium, a but lack seeds. Gymnosperms insights into the molecular basis highly cytoplasmic egg is formed and angiosperms both produce of gametophyte development in near the interior base. During seeds. Angiosperms produce mosses. fertilization, the archegonium flowers and include most of the opens a channel allowing direct familiar plants. Although land Gametophyte development in access of the sperm to the egg. plants in all four groups share a ferns The differences between male common life cycle (Figure 1A), the Unlike bryophytes, in vascular and hermaphrodite gametophyte relative sizes of, and nutritional plants the sporophyte generation development allow screening for relationship between, the is macroscopic but the C. richardii mutants that have sporophyte and gametophyte gametophyte generation is altered sex determination. Studies vary greatly among different microscopic. Fern gametophytes of such mutants have led to a groups of land plants. are free living and they require model for sexual determination in Gametophyte development also moist conditions for reproduction. fern gametophyte development. varies among these groups. Most ferns, such as Ceratopteris richardii — an attractive genetic Gametophyte development in Gametophyte development in model system — are gymnosperms mosses homosporous, producing only one In seed plants, the sporophytic In bryophytes, the sporophyte is kind of spore. In isolation, a C. generation is dominant and free- minute and dependent on the richardii spore develops into a living, and the gametophytes are relatively prominent and hermaphrodite gametophyte, very small and dependent on the nutritionally independent producing both eggs and sperm. sporophyte for nutrients. Among gametophyte for resources. The The gametophyte becomes a gymnosperms, conifers have male moss gametophyte looks like a male if exposed to enough of the and female cones on the same miniature herb, with tiny leaf-like pheromone antheridiogen during tree. Other gymnosperms bear photosynthetic organs. The its early development. After this male and female reproductive gametophyte generation begins initial period, the gametophyte is structures on different individuals. as a dormant spore, which insensitive to antheridiogen and In all gymnosperms, the male germinates under appropriate develops into a hermaphrodite, gametophyte (pollen grain) reaches conditions to produce filamentous which produces and secretes the female reproductive structure and branching protonemal antheridiogen. helped by wind or animals. Magazine R719 The cones bear scale-like male A B Seedless vascular or female organs, called Bryophytes plants Gymnosperms Angiosperms microsporophylls and megasporophylls, respectively. In Sporophyte (2n) pines, the microsporophyll has Mitosis Fertilization Meiosis two microsporangia, sac-like LivewortsHornwortsMosses Club mossesHorsetailsFerns Cycads Ginkgo ConifersGnetales Angiosperms structures filled with many diploid Sperm cells + eggs Spores microspore mother cells, which undergo meiosis to produce Mitosis haploid microspores. The Gametophyte (1n) megasporophyll has two ovules, each containing a single Current Biology megaspore mother cell which produces four meiotic products, Figure 1. The life cycle and phylogenetic tree of land plants. one of which becomes the (A) The life cycle. From the diploid (2n; red) sporophyte generation, sporocytes undergo functional megaspore. The meiosis and produce haploid spores (1n; green). Mitoses then form gametophytes, microspore and megaspore then which produce gametes, sperm cells and eggs. The fertilization of an egg with a sperm develop into male and female cell produces a zygote. (B) A phylogenetic tree of major land plants, showing the rela- gametophytes, respectively. tionships of the groups of plants discussed here. In the west white pine Pinus monticola, male gametophyte Male gametophyte development in completely envelopes the development involves a series of angiosperms generative cells. In contrast, the asymmetric cell divisions. First, Angiosperms are defined by generative nucleus is more tightly the microspore divides to produce having seeds in the enclosing fruit organized and less active in a large central cell and a small derived from the ovary of a flower. transcription. The generative cell first primary prothallial cell. The The flower consists of primarily later undergoes a second mitosis central cell divides to produce a sporophytic tissues, with both to produce two sperm cells. second primary prothallial cell and male and female gametophytes Pollen development depends on an antheridial initial. Another which are highly reduced in size in the function of a surrounding unequal division of the antheridial comparison to all other land sporophytic tissue called tapetum. initial results in a large tube cell plants. Angiosperms also have the The pollen dehydrates during and a small antheridial cell. The unique property of double maturation; after pollination, the small antheridial cell divides to fertilization, producing a usually pollen grain rehydrates and form a sterile cell and a generative triploid endosperm in addition to germinates to produce a pollen cell, which divides later to form the embryo. The male tube. This tube grows towards the two sperm cells. gametophyte is formed in the ovule, providing a passageway for In the female cone of P. anthers of the stamens, and the the sperm cells to reach the monticola, the megaspore female gametophyte is located in female gametophyte. Because the undergoes multiple mitoses, the ovules within the pistil. female gametophyte is within the followed by cellularization, to In the anther, four pollen sacs female reproductive organ of the produce about 2000 primary (locules) contain numerous sporophyte, the pollen tube must prothallial cells. Three to five microspore mother cells, each of extend considerable distances — primary prothallial cells near the which undergoes meiosis to many times the size of the pollen micropyle, an opening of the produce four microspores in a grains — to reach the egg. ovule, enlarge and develop into tetrad (Figure 2A). The male archegonial initial cells. Each gametophyte generation begins Female gametophyte development initial cell divides and develops with the microspore. Initially, the in angiosperms into an archegonium containing microspore has a uniformly The female reproductive organ, an egg and other supportive distributed cytoplasm with a the ovary, completely encloses cells. centrally located haploid nucleus. one or more ovules, the