Alteration of Generation
Alteration of generation is a reproductive cycle and method of growth of certain species. It is found in both the animal and plant kingdoms although it is more common with certain plant species. It is characterized by the haploid phase (which reproduces sexually) alternating with the diploid phase (which reproduces asexually).
In mosses and vascular plants there is a sexual and an asexual phase. The haploid (sexual) phase involves gametophyte (gamete or sperm cell plant) generation; the diploid (asexual) phase involves sporophyte (spore plant) generation.
The cycle begins with a sexually mature plant which is capable of producing egg and sperm cells. Both egg and sperm, as well as the parent plant, are haploid. The egg and sperm fuse to produce a diploid zygote. The zygote (a cell formed by the union of male and female gametes) does not immediately grow into a new gametophyte, instead it grows by cell division to produce a sporophyte. The sporophyte plant is a separate individual to the gametophyte, although in some cases it may remain dependent upon the gametophyte for nourishment, giving the appearance of a parasitic organism. The function of the sporophyte is to undergo meiosis to produce spores.
These haploid spores are powder-like in nature and are readily dispersed by the wind. Once they land on a substrate capable of supporting their growth they germinate and grow by mitosis (in which a cell divides into two so that the nucleus of each new cell has the full number of chromosomes) to produce a new gametophyte, and the whole process is repeated.
The sexually and asexually reproducing phases are often very different from each other and are easily recognized. In plants, certain phases consist of only a few partially parasitic cells reaching a maximum of several millimetres across, while other phases comprise the plant, shrub, or tree. One disadvantage of this system is the heavy reliance on water, which limits where the plant may grow. Within the thallophyte plants (i.e., bacteria, algae, fungi and lichens) spores are not produced by the diploid generation, although clear differences are observable between the two generations.
The ascomycete fungi are haploid at all times other than when the gametes fuse to produce the zygote. As the first division of the zygote occurs it simultaneously undergoes meiosis, immediately returning to the haploid phase.
In basidiomycete fungi, each cell usually contains two nuclei, a condition referred to as the dikaryotic state, which is generally held to be the equivalent of the diploid situation. Once a spore has germinated it can join with the mycelium of another spore and a swapping of nuclei and subsequent migration occurs to produce the dikaryotic state. This can also occur with hyphae from the same spore.
Within the mosses and liverworts the part of the plant that we are familiar with is the gametophyte. The sporophyte is a small capsule that is nutritionally dependent upon the larger plant. Within the flowering plants the gametophyte generation is vastly reduced in size. The male gametophyte is the pollen grain and the female gametophyte is the embryo sac that is contained within the ovule. The sporophyte generation is the plant itself.
If the sporophyte and gametophyte generation are markedly different in appearance the life cycle is termed heteromorphic, whereas if they are very similar the life cycle is said to be an isomorphic one.
The situation is very similar in animals such as jellyfish but one striking difference is that both phases are diploid in nature. In the case of the jellyfish, the asexual part of the reproductive life cycle is carried out in a planktonic phase, the polyp form, by an animal only one or two millimetres across. The corresponding sexually reproducing part of the life cycle is the medusa form, such as the Portugese man-of-war, which can have tentacles trailing for 10m (30 feet).
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