Life cycles, such as those of animals, in which there is only a diploid multicellular stage are referred to as diplontic. Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as diplohaplontic (the equivalent terms haplodiplontic, diplobiontic and dibiontic are also in use, as is describing such an organism as having a diphasic ontogeny ). (Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs however females produced from fertilized eggs are diploid.) No haploid spores capable of dividing are produced, so generally there is no multicellular haploid phase. In flowering plants, the reduction of the gametophyte is much more extreme it consists of just a few cells which grow entirely inside the sporophyte.Īnimals develop differently. In ferns the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is briefly dependent for its nutrition. By contrast, in all modern vascular plants the gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen. In liverworts, mosses and hornworts, the sporophyte is less well developed than the gametophyte and is largely dependent on it. In those algae which have alternation of generations, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. The relationship between the sporophyte and gametophyte varies among different groups of plants. This cycle, from gametophyte to sporophyte (or equally from sporophyte to gametophyte), is the way in which all land plants and many algae undergo sexual reproduction.
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Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a diploid zygote, which develops into a diploid sporophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. The haploid spores germinate and grow into a haploid gametophyte. A mature sporophyte produces haploid spores by meiosis, a process which reduces the number of chromosomes to half, from 2 n to n. In these groups, a multicellular haploid gametophyte with n chromosomes alternates with a multicellular diploid sporophyte with 2 n chromosomes, made up of n pairs. Diagram showing the alternation of generations between a diploid sporophyte (bottom) and a haploid gametophyte (top)Īlternation of generations (also known as metagenesis or heterogenesis) is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct haploid sexual and diploid asexual stages.