Reproduction

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					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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posted:11/25/2011
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