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. Lets 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 outof the body of the parent.-Hydras exhibit this type ofreproduction. Hydra with ovaries Hydra Budding
Gemmules (Internal Buds)-In this form, a parent releasesa specialized mass of cellsthat can develop into anoffspring.-Sponges exhibit this type ofreproduction. 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 fishWater 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 years 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.
GametesIn animals, sexual reproductionencompasses the fusion of twodistinct gametes to form a zygote.Gametes are produced by a typeof cell division called meiosis.The gametes are haploid(containing only one set ofchromosomes) while the zygoteis diploid (containing two sets ofchromosomes). In most cases,the male gamete, called thespermatozoan, is relatively motileand usually has a flagellum. Onthe other hand, the femalegamete, called the ovum, isnonmotile and relatively large incomparison to the male gamete.
Types of FertilizationThere are two mechanisms by whichfertilization can take place. The first isexternal (the eggs are fertilized outside ofthe body); the second is internal (the eggsare fertilized within the femalereproductive tract).
External FertilizationExternal fertilization occurs mostly in wetenvironments and requires both the maleand the female to release their gametesinto their surroundings (usually water). Anadvantage of external fertilization is that itresults in the production of a large numberof offspring. One disadvantage is thatenvironmental hazards such as predatorsgreatly reduce the chance of surviving intoadulthood.
Internal FertilizationAnimals that use internal fertilization specialize in theprotection of the developing egg. For example, reptilesand birds secrete eggs that are covered by a protectiveshell that is resistant to water loss and damage.Mammals, with the exception of monotremes, take thisidea of protection a step further by allowing the embryoto develop within the mother. This extra protectionincreases the chances of survival because mom supplieseverything that the embryo needs. In fact, mostmammalian mothers continue to care for their young forseveral years after birth.
Patterns and CyclesReproduction is not a continuous activity and is subjectto certain patterns and cycles. Oftentimes these patternsand cycles may be linked to environmental conditionswhich allow organisms to reproduce effectively. Forexample, many animals have estrous cycles that occurduring certain parts of the year so that offspring cantypically be born under favorable conditions. Likewise,these cycles and patterns can be controlled by hormonalcues as well as other seasonal cues like rainfall. All ofthese cycles and patterns allow organisms to managethe relative expenditure of energy for reproduction andmaximize the chances of survival for the resultingoffspring.