“Asexual and sexual”
• 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
• 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:
-In this form, an offspring grows out
of the body of the parent.
-Hydras exhibit this type of
Hydra with ovaries
Gemmules (Internal Buds)
-In this form, a parent releases
a specialized mass of cells
that can develop into an
-Sponges exhibit this type of
• -In this form, the body of the
parent breaks into distinct
pieces, each of which can
produce an offspring.
-Planarians exhibit this type of
Anterior end of a planarian showing
eyespots, auricles and digestive system.
-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.
• In some species, stems arch over and take root at their tips, forming
• The horizontal above-ground stems (called stolons) of the
strawberry (shown here) produce new daughter plants at alternate
• Underground stems
• corms and
• are used for asexual reproduction as well as for food storage.
• Irises and day lilies, for example, spread rapidly by the growth of
• 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.
• 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.
• In sexual reproduction,
two individuals produce
offspring that have
from both parents.
introduces new gene
combinations in a
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
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
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