Reproduction “Asexual and sexual” Reproduction • What is reproduction? • In a nutshell, reproduction is the creation of a new individual or individuals from previously existing individuals. In animals, this can occur in two primary ways: through asexual reproduction and through sexual reproduction. Let's look at asexual reproduction. Asexual Reproduction • In asexual reproduction, one individual produces offspring that are genetically identical to itself. These offspring are produced by mitosis. There are many invertebrates, including sea stars and sea anemones for example, that produce by asexual reproduction. Common forms of asexual reproduction include: Budding -In this form, an offspring grows out of the body of the parent. -Hydras exhibit this type of reproduction. Hydra with ovaries Hydra Budding Gemmules (Internal Buds) -In this form, a parent releases a specialized mass of cells that can develop into an offspring. -Sponges exhibit this type of reproduction. Sponge Gemmules Fragmentation • -In this form, the body of the parent breaks into distinct pieces, each of which can produce an offspring. -Planarians exhibit this type of reproduction. Planarian Anterior end of a planarian showing eyespots, auricles and digestive system. Regeneration -In this form, if a piece of a parent is detached, it can grow and develop into a completely new individual. -Echinoderms exhibit this type of reproduction. Oral surface of a star fish Water vascular system of a star fish injected with blue dye Asexual Reproduction in Plants • All plant organs have been used for asexual reproduction, but stems are the most common. • Stems • In some species, stems arch over and take root at their tips, forming new plants. • The horizontal above-ground stems (called stolons) of the strawberry (shown here) produce new daughter plants at alternate nodes. • Underground stems • rhizomes • bulbs • corms and • tubers • are used for asexual reproduction as well as for food storage. • Irises and day lilies, for example, spread rapidly by the growth of their rhizomes. • Leaves • This photo shows the leaves of the common ornamental plant Bryophyllum (also called Kalanchoë) . Mitosis at meristems along the leaf margins produce tiny plantlets that fall off and can take up an independent existence. • Roots Some plants use their roots for asexual reproduction. The dandelion is a common example. Trees, such as the poplar or aspen, send up new stems from their roots. In time, an entire grove of trees may form — all part of a clone of the original tree. • Plant Propagation • Commercially-important plants are often deliberately propagated by asexual means in order to keep particularly desirable traits (e.g., flower color, flavor, resistance to disease). • Cuttings may be taken from the parent and rooted [More]. • Grafting is widely used to propagate a desired variety of shrub or tree. All apple varieties, for example, are propagated this way. • Apple seeds are planted only for the root and stem system that grows from them. After a year's growth, most of the stem is removed and a twig (scion) taken from a mature plant of the desired variety is inserted in a notch in the cut stump (the stock). So long the cambiums of scion and stock are united and precautions are taken to prevent infection and drying out, the scion will grow. It will get all its water and minerals from the root system of the stock. However, the fruit that it will eventually produce with be identical (assuming that it is raised under similar environmental conditions) to the fruit of the tree from which the scion was taken. • Apomixis • Citrus trees and many other species of angiosperms use their seeds as a method of asexual reproduction; a process called apomixis. • In one form, the egg is formed with 2n chromosomes and develops without ever being fertilized. • In another version, the cells of the ovule (2n) develop into an embryo instead of — or in addition to — the fertilized egg. • Hybridization between different species often yields infertile offspring. [Link to a discussion of this postzygotic isolating mechanism.]s But in plants, this does not necessarily doom the offspring. Many such hybrids use apomixis to propagate themselves. • The many races of Kentucky bluegrass growing in lawns across North America and the many races of blackberries are two examples of sterile hybrids that propagate successfully by apomixis. • Recently, an example of apomixis in gymnosperms was discovered (see Pichot, C., et al, in the 5 July 2001 issue of Nature). In a rare cypress, the pollen grains are diploid, not haploid, and can develop into an embryo when they land on either • the female cones of their own species (rare) or • those of a much more common species of cypress. • Is this paternal apomixis in a surrogate mother a desperate attempt to avoid extinction? • Breeding apomictic crop plants • Many valuable crop plants (e.g., corn) cannot be propagated by asexual methods like grafting. • Agricultural scientists would dearly love to convert these plants to apomixis: making embryos that are genetic clones of themselves rather than the product of sexual reproduction with its inevitable gene reshuffling. After 20 years of work, an apomictic corn (maize) has been produced, but it does not yet produce enough viable kernels to be useful commercially. Sexual Reproduction • In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population. Gametes In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by a type of cell division called meiosis. The gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes). In most cases, the male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete. Types of Fertilization There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body); the second is internal (the eggs are fertilized within the female reproductive tract). External Fertilization External fertilization occurs mostly in wet environments and requires both the male and the female to release their gametes into their surroundings (usually water). An advantage of external fertilization is that it results in the production of a large number of offspring. One disadvantage is that environmental hazards such as predators greatly reduce the chance of surviving into adulthood. Internal Fertilization Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother. This extra protection increases the chances of survival because mom supplies everything that the embryo needs. In fact, most mammalian mothers continue to care for their young for several years after birth. Patterns and Cycles Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively. For example, many animals have estrous cycles that occur during certain parts of the year so that offspring can typically be born under favorable conditions. Likewise, these cycles and patterns can be controlled by hormonal cues as well as other seasonal cues like rainfall. All of these cycles and patterns allow organisms to manage the relative expenditure of energy for reproduction and maximize the chances of survival for the resulting offspring.
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