Asexual Reproduction
Asexual Reproduction, the formation of a new individual from cells of the parent, without meiosis, gamete formation, or fertilization. There are several types of asexual reproduction. Fission is the simplest form and involves the division of a single organism into two complete organisms, each identical to the other and to the parent. Fission is common among unicellular organisms such as bacteria, many protists, some algae (such as Spirogyra and Euglena) as well as a few higher organisms such as flatworms and certain species of polychaete worms. A similar form of asexual reproduction is regeneration, in which an entire organism may be generated from part of its parent. The term regeneration normally refers to regrowth of missing or damaged body parts in higher organisms, but whole body regeneration occurs in hydroids (see hydra), starfish and many plants. Spores are another form of asexual reproduction and are common among bacteria, protists and fungi.
Conditions necessary for Asexual Reproduction
Asexual reproduction may provide a secondary means of multiplying in organisms that ordinarily reproduce sexually. Under certain conditions this may be the only way to reproduce. For example, if there are no other individuals with which to exchange gametes, or in plants, if pollinators are absent. Asexual reproduction demands less time and energy and may be the most efficient way for certain species to reproduce under harsh environmental conditions. Some species switch between sexual and asexual modes of reproduction in an annual cycle so that each takes place at the most favorable time. Examples of this are aphids that reproduce asexually in the summer but sexually in the fall, and water fleas, which have a similar cycle.
Types of asexual Reproduction
1. Spore Formation : Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who undergo gametic meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or ???phases??? of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red alga polysiphonia.
2. Budding : Budding is another method of asexual reproduction in which a group of self-supporting cells sprouts from and than detaches from the parent organism. Unlike eggs or spores, buds are multicellular and usually contain more than one cell layer. Hydroids and sea squirts reproduce by budding.
3. Binary fission :
