Gametophyte development

Unlike animals, which produce single-celled gametes directly from meiotic products, plants have generations which alternate between the diploid sporophyte and haploid gametophyte (Figure 1A). The diploid sporophytic generation develops from the zygote, the fusion product of haploid gametes. Sporophytic cells undergo meiosis to produce haploid spores, which divide mitotically to form the multicellular gametophyte. Certain cells in the gametophyte subsequently differentiate into gametes. Land plants are divided into four groups (Figure 1B). The most basal group, the bryophytes, including mosses and liverworts, lack vascular tissues. Plants of the second group, including ferns and horsetails, have a vasculature but lack seeds. Gymnosperms and angiosperms both produce seeds. Angiosperms produce flowers and include most of the familiar plants. Although land plants in all four groups share a common life cycle (Figure 1A), the relative sizes of, and nutritional relationship between, the sporophyte and gametophyte vary greatly among different groups of land plants. Gametophyte development also varies among these groups. Gametophyte development in mosses In bryophytes, the sporophyte is minute and dependent on the relatively prominent and nutritionally independent gametophyte for resources. The moss gametophyte looks like a miniature herb, with tiny leaf-like photosynthetic organs. The gametophyte generation begins as a dormant spore, which germinates under appropriate conditions to produce filamentous and branching protonemal tissues. These form multicellular bud-like structures, each of which develops into a leafy shoot. The mature gametophytes produce male and female sexual organs, the antheridia and archegonia, respectively. The gametophyte is often sexually distinct, and plants are either male or female. Each antheridium has an outer layer that encloses and protects thousands of motile sperm, which swim through available external water layer to the egg. Fertilization at the base of the cylindrical archegonium produces a diploid zygote which develops into an unbranched sporophyte. The sporophyte consists of a thin stalk attached to the gametophyte, and a capsule that encloses the sporophytic meiotic cells. In recent years, the mosses Physcomitrella patens and Funaria hygrometrica have emerged as attractive model systems for studying gene function in non-vascular plants because of the relative ease of molecular manipulation by homologous recombination. Mutants affecting gametophyte development have been isolated and their analysis should provide insights into the molecular basis of gametophyte development in mosses. Gametophyte development in ferns Unlike bryophytes, in vascular plants the sporophyte generation is macroscopic but the gametophyte generation is microscopic. Fern gametophytes are free living and they require moist conditions …