What is alternation of generations?
Alternation of generations is a type of life cycle found in terrestrial plants, and some algae in which subsequent generations of individuals alternate between organisms haploid y diploids. This can be contrasted with the sexual reproduction in animals, in which both haploid and diploid cells are found in each generation. The alternation of generations has several distinct characteristics, and these characteristics can be slightly modified between species. In general, generations alternate between sporophytes capable of creating spores and the gametophytescapable of creating gametes.
Life cycle of alternating generations
Sporophyte
To form a sporophyte, two haploid gametes join together to form a zygote diploid. Normally, haploid organisms are defined as having an “n” number of chromosomes. When two gametes of the same species unite, each has n chromosomes. Therefore, the diploid zygote that forms is considered to have a value of 2n of genetic material, or exactly twice as much. Not only is there twice as much DNAbut rather represents codes for the same proteins in the same organism. The sporophyte is an organism multicellular consisting of multiple rounds of mitosis in the zygote. Therefore, the sporophyte individual remains an organism. 2n.
Then, when the sporophyte reaches maturity, a key point in the alternation of generations occurs. The sporophyte develops organs, known as sporangia. These specialised reproductive organs are used to create single-celled haploid spores. These cells will be released into the air or water and carried away. When they reach a suitable environment, they will start the process of development in the gametophyte.
Gametophyte
This represents the next generation in the alternation of generations, as the spore is created. haploid. The spore is technically a new organism and has only half the DNA as the parent organism. This spore will undergo successive rounds of mitosis to form a new multicellular individual, the gametophyte. Where the sporophyte generation creates spores, the gametophyte generation creates gametes. The gametes are produced by special organs in the gametophyte, the gametangia. These gametes are then transmitted to the environment or transferred between plants.
When they find a gamete opposite gamete, they begin the process of fusing to form another zygote. This zygote will eventually become a sporophyte, and the alternation of generations will continue to spin. While this is a simplistic version of the alternation of generations, there are many complexities, which will be discussed below. Because of these complexities, and because all plants experience some version of alternation of generations, scientists prefer to refer to other aspects of their reproductive cycles to define the species.
The simplest form of alternation of generations is found in the fern, as seen below. As seen in the diagram, the gametophyte and the sporophyte are distinctly different individuals. This is not always the case.
Complications with alternation of generations
Gender gametes
There are many additional factors that can complicate this basic issue of alternation of generations. The most notable complication is gender. Both sporophytes and gametophytes can have genders. Furthermore, the sporophyte or gametophyte may not be totally independent of the other generation. Consider flowering plants, for example. Some flowers have both male and female gametophytes housed within them. In other species of flowers, the male and female flowers are separate, but may be present in the same plant. In other flowers, individual sporophytes only give rise to a certain flower genus.
The terms monkey y dioic are used to describe gametophytes. If both genera are housed in the same individual, the species is monoecious. If they are separate, the alternation of generations is dioecious. This applies only to the gametophyte.
In the sporophyte, the terms used are. monoecious y dioic. Here, they describe whether the sporophyte will produce both genera, or whether the genera separate into different sporophytes. It is important to note that many plants are monoecious, but dioecious. This means that a single sporophyte can produce both male and female flowers. Many other combinations of these differences in the alternation of generations can be observed in various species of plants and algae.
Differences in the sporophyte and gametophyte
During the alternation of generations in some species, there is an unequal distribution in size or dominance of the sporophyte or gametophyte. In different species, this can be reversed. In the liver, the gametophyte is the dominant generation. The liverwort is a plant gametophyte. In ferns the opposite is true and the sporophyte is the larger, dominant individual. This would be called a plant sporophyte.
In other species, the size and shape of the sporophyte and gametophyte is almost indistinguishable. The only difference is the amount of DNA they carry and whether they will produce spores or gametes. While this may seem like they are not really doing anything, the alternation of generations still allows sexual reproduction to occur. This greatly mixes genes and increases the adaptability of the species.
Sporophytes and gametophytes of different species also produce different types of spores and gametes. If the gametes are the same, it is called isogamy. Some species of green algae produce only one type of gamete. Both are flagellated and swim in open water until they meet. This scheme is not particularly useful for terrestrial plants. Most terrestrial plants show anisogamy. or a difference in their gametes. This is most commonly seen as a difference between the motile male gamete and the larger immotile female gamete, or sperm and oocytes. Other times, it is seen only as a difference in size.
Below is a graphical representation of a dioecious, dioecious and anisogametic plant. This means that the plant has separate gametophyte and sporophyte individuals, and that the gametes it produces are of different sizes. This would be true for a holly or willow, which produces separate male and female flowers, in separate individual sporophytes.
Evolution of the alternation of generations
Fossil evidence and the existence of many algae with simplified alternation of generation life cycles hint that the evolutionary advantage of sexual reproduction through alternation of generations is what made land plants adaptable enough to colonise the land. Alternation of generations allows both the dynamic and volatile act of sexual reproduction and the constant and consistent act of asexual reproduction.
When the sporophyte creates spores, the cells experience meiosiswhich enables the generation of gametophytes to recombine the genetics present. This allows great diversity to emerge. As plants colonised the land, they were initially isomorphics, or both gametophytes and sporophytes looked and acted the same. As time went on, most plants found it beneficial to reduce one of these life cycles. Most flowering plants now have a much reduced gametophyte life cycle, while liverworts and mosses went in the opposite direction, preferring to decrease the sporophyte cycle. In the wide variety of living plants today there is almost every conceivable variation of alternation of generations.