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Asexual Reproduction Notes
Budding
Budding, in biology, a form of asexual reproduction in which a new individual develops
from some generative anatomical point of the parent organism. In some species buds may be
produced from almost any point of the body, but in many cases budding is restricted to
specialized areas. The initial protuberance of proliferating cytoplasm or cells, the bud,
eventually develops into an organism duplicating the parent. The new individual may
separate to exist independently, or the buds may remain attached, forming aggregates or
colonies. Budding is characteristic of a few unicellular organisms (e.g., certain bacteria,
yeasts, and protozoans); however, a number of metazoan animals (e.g., certain cnidarian species) regularly
reproduce by budding.
Budding, which is another
method of asexual reproduction,
occurs in most yeasts and in some
filamentous fungi. In this process,
a bud develops on the surface of
either the yeast cell or the hypha,
with the cytoplasm of the bud
being continuous with that of the
parent cell. The nucleus of the
parent cell then divides; one of the
daughter nuclei migrates into the
bud, and the other remains in the
parent cell. The parent cellis
capable of producing many buds over its surface by continuous synthesis of cytoplasm and repeated nuclear
divisions. After a bud develops to a certain point and even before it is severed from the parent cell, it is itself
capable of budding by the same process. In this way, a chain of cells may be produced. Eventually, the
individual buds pinch off the parent cell and become individual yeast cells. Buds that are pinched off a hypha of
a filamentous fungus behave as spores; that is, they germinate, each giving rise to a structure called a germ tube,
which develops into a new hypha.
Although fragmentation, fission, and budding are methods of asexual reproduction in a number of fungi, the
majority reproduce asexually by the formation of spores. Spores that are produced asexually are often termed
mitospores, and such spores are produced in a variety of ways.
http://www.youtube.com/
watch?v=jH4Y3Qh5Xqw
Fragmentation
Fragmentation in multicellular organisms is a form of asexual reproduction or cloning where an organism is
split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the
original organism.
The splitting may or may not be intentional- it may occur due to man made or natural damage by the
environment or predators or the organism may develop specific organs or zones that may be shed or easily
broken off. If the splitting occurs without the prior preparation of the organism, both fragments must be able to
regenerate the complete organism for it to function as reproduction.
Fragmentation is as a method of reproduction is seen in many organisms such as filamentous cyanobacteria,
molds, lichens, many plants, and animals like sponges, acoel flatworms, some annelid worms, and sea stars.
Molds, yeast, and mushrooms, all of which are part of the Fungi kingdom, produce tiny filaments called hyphae.
These hyphae obtain food and nutrients from the body of other organisms to grow and fertilize. Then a piece of
hyphae breaks off and grows into a new individual and the cycle continues.
Many lichens produce specialised structures that can
easily break away and disperse. These structures
contain both the hyphae of the mycobiont and the
algae(phycobiont) (see soredia and isidia. Larger
fragments of the thallus may break away when the
lichen dries or due to mechanical disturbances(see the
section on reproduction in lichens).
Animals like sponges and coral colonies naturally
fragment and reproduce. Many species of annelids and
flat worms reproduce by this method.
When the splitting occurs due to specific
developmental changes, the terms architomy, paratomy
and budding are used. In architomy the animal splits at
a particular point and the two fragments regenerate the
missing organs and tissues. The splitting is not
preceded by the development of the tissues to be lost. Prior to splitting, the animal may develop furrows at the
zone of splitting. The headless fragment has to regenerate a complete head.
In paratomy, the split occurs perpendicular to the antero-posterior axis and the split is preceded by the
"pregeneration" of the anterior structures in the posterior portion. The two organisms have their body axis
aligned i.e. they develop in a head to tail fashion. Budding can be considered to be similar to paratomy except
that the body axes need not be aligned: the new head may grow toward the side or even point backward (eg.
Convolutriloba retrogemma an acoel flat worm).
Corals
Many types of Coral colonies can increase in number by fragmentation that occurs naturally or artificially.
Within the reef aquarium hobby, enthusiasts regularly fragment corals for a multitude of purposes including
shape control; selling to, trading with, or sharing with others; regrowth experiments; and minimizing damage to
natural coral reefs. Both hard and soft corals can be fragmented, with the level of success depending on the skill
of the aquarist, method used, tolerance of the specific species, and conditions of care.
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of
embryos occur without fertilization by a male. In plants, parthenogenesis means development of an embryo
from an unfertilized egg cell, and is a component process of apomixis.
The word "parthenogenesis" comes from the Greek, parthenos, meaning "virgin", genesis, meaning "birth". The
term is sometimes used inaccurately to describe reproduction modes in hermaphroditic species which can
reproduce by themselves because they contain reproductive organs of both sexes in a single individual's body.
Parthenogenesis occurs naturally in some invertebrate animal species (e.g., water fleas, aphids, nematodes,
some bees, some Phasmida, some scorpion species, and parasitic wasps) and some vertebrates (e.g., some
reptiles, fish, and very rarely birds and sharks). This type of reproduction has been induced artificially in fish
and amphibians.
Normal egg cells form after meiosis and are haploid, with half as many chromosomes as their mother's body
cells. Haploid individuals, however, are usually non-viable, and parthenogenetic offspring usually have the
diploid chromosome number. If the chromosome number of the haploid egg cell is doubled during
development, the offspring is "half a clone" of its mother. If the egg cell was formed without meiosis, it is a
full clone of its mother.
The offspring produced by parthenogenesis in species that use the XY sex-determination system have two X
chromosomes and are female. In species that use the ZW sex-determination system they have either two Z
chromosomes (male) or two W chromosomes (non-viable or female), or (theoretically) if clonal parthenogenesis
was involved (also called apomixis), they could have one Z and one W chromosome (female).
Parthenogenesis is seen to occur naturally in aphids, Daphnia, rotifers, nematodes and some other invertebrates,
as well as in many plants and certain lizards. Komodo dragons and the hammerhead- and blacktip sharks have
recently been added to the list of vertebrates—along with several genera of fish, amphibians, and reptiles—that
exhibit differing forms of asexual reproduction, including true parthenogenesis, gynogenesis, and
hybridogenesis (an incomplete form of parthenogenesis). As with all types of asexual reproduction, there are
both costs (low genetic diversity and therefore susceptibility to adverse mutations that might occur) and benefits
(reproduction without the need for a male) associated with parthenogenesis.
The offspring of parthenogenesis will be all female if two like chromosomes determine the female sex (such as
the XY sex-determination system), but they will be male if two like chromosomes determine the male sex (such
as the ZW sex-determination system), because the process involves the inheritance and subsequent duplication
of only a single sex chromosome. The offspring may be capable of sexual reproduction, if this mode exists in
the species. In many cases, parthenogenesis occurs when one gender (typically the male) is unavailable in the
general vicinity. Once males are again available, the parthenogenesis-created females would be capable of
mating with the males and creating normal offspring.
Parthenogenesis is distinct from artificial animal cloning, a process where the new organism is necessarily
genetically identical to the cell donor. In cloning, the nucleus of a diploid cell from a donor organism is inserted
into an enucleated egg cell and the cell is then stimulated to undergo continued mitosis, resulting in an organism
that is genetically identical to the donor. Parthenogenesis is different, in that it originates from the genetic
material contained within an egg cell. Egg cells may be produced via meiosis or mitosis oogenesis. If produced
by mitosis, the egg that undergoes parthenogenesis can be either haploid or diploid, leading to a number of
possible outcomes in terms of the genetic fingerprint of the parthenogen. A diploid parent organism that
undergoes parthenogenesis via meiosis will create haploid offspring with a new genetic fingerprint due to
crossing over of the chromosomes in the parent. Because there are so many variables in parthenogenesis, there
is little that can be said for sure unless
the specific methods of the particular
parthenogenetic tendencies of an
organism are known.
A litter of offspring resulting from
parthenogenesis may contain genetically
identical siblings. In organisms
possessing an XY chromosome system,
parthenogenic offspring are always
female, but they are not necessarily
genetically identical to one another or to
their mother (some chromosome
segments may differ because of
meiosis).
Parthenogenesis may be achieved
through an artificial process as described
below under the discussion of mammals.
Alternation between parthenogenesis
and sexual reproduction is called
cyclical parthenogenesis or heterogamy.
A form of reproduction related to
parthenogenesis, but that only requires
the presence of sperm that do not
fertilize an egg, is known as
gynogenesis. In hybridogenesis the sperm fertilizes the egg, but its chromosomes are not carried to subsequent
generations.
Binary Fission
A method of asexual reproduction that is employed by
most prokaryotes. In binary fission, the living cell
divides into two equal, or nearly equal, parts. It begins
when the DNA of the cell is replicated. Each circular
strand of DNA then attaches to the plasma membrane.
The cell elongates, causing the two chromosomes to
separate. The plasma membrane then invaginates
(grows inward) and splits the cell into two daughter
cells through a process called cytokinesis.
Binary fission theoretically results in two identical
cells. However, the DNA of bacteria has a relatively
high mutation rate. This rapid rate of genetic change is
what makes bacteria capable of developing resistance
to antibiotics and helps them exploit invasion into a
wide range of environments.
Similar to more complex organisms, bacteria also have
mechanisms for exchanging genetic material. Although
not equivalent to sexual reproduction, the end result is
that a bacterium contains a combination of traits from
two different parental cells. Three different modes of
exchange have thus far been identified in bacteria. (See
gene transfer.)
Conjunction involves the direct joining of two bacteria,
which allows their circular DNAs to undergo
recombination. Bacteria can also undergo transformation by absorbing remnants of DNA from dead bacteria
and integrating these fragments into their own DNA. Lastly, bacteria can exchange genetic material through a
process called transduction, in which genes are transported into and out of the cell by bacterial viruses, called
bacteriophages, or by plasmids, an autonomous self-replicating extrachromosomal circular DNA.
Binary fission, asexual reproduction by a separation of the body into two new bodies. In the process of binary
fission, an organism duplicates its genetic material, or deoxyribonucleic acid (DNA), and then divides into two
parts (cytokinesis), with each new organism receiving one copy of DNA.
Binary fission is the primary method of reproduction of prokaryotic organisms. In protists, binary fission is
often differentiated into types, such as transverse or longitudinal, depending on the axis of cell separation.
Regular transverse fission in some organisms, such as tapeworms and scyphostome polyps, is called
strobilation. Commonly, this results in a chain, called a strobilus, of the fission products—the proglottids of
tapeworms and the ephyrae of scyphozoan jellyfish; each proglottid or ephyra matures in turn and separates
from the end of the strobilus. A few metazoan (multicellular) species regularly undergo a body division into
several units simultaneously, a process called fragmentation. Planarian fission and fragmentation generally
represent direct reproduction in which each portion regenerates missing parts to become a complete new animal.
Strobilation products, however, are only indirectly reproductive: proglottids are not regenerative but carry and
release great numbers of eggs and die; ephyrae do not produce new polyps but mature into sexually reproducing
medusae, the larvae of which become polyps.
Grafting
Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that
the two sets of vascular tissues may join together. This vascular joining is called inosculation. The technique is
most commonly used in asexual propagation of commercially grown plants for the horticultural and agricultural
trades.
In most cases, one plant is selected for its roots and this is called the stock or rootstock. The other plant is
selected for its stems, leaves, flowers, or fruits and is called the scion. The scion contains the desired genes to
be duplicated in future production by the stock/scion plant.
In stem grafting, a common grafting method, a shoot of a selected, desired plant cultivar is grafted onto the
stock of another type. In another common form called bud grafting, a dormant side bud is grafted onto the stem
of another stock plant, and when it has inosculated successfully, it is encouraged to grow by pruning off the
stem of the stock plant just above the newly grafted bud.
For successful grafting to take place,
the vascular cambium tissues of the
stock and scion plants must be placed
in contact with each other. Both tissues
must be kept alive until the graft has
'taken', usually a period of a few weeks.
Successful grafting only requires that a
vascular connection take place between
the grafted tissues. Joints formed by
grafting are not as strong as naturally
formed joints, so a physical weak point
often still occurs at the graft, because
only the newly formed tissues
inosculate with each other. The existing
structural tissue (or wood) of the stock
plant does not fuse.
Advantages of Grafting: Grafting fruit trees enables you to clone the commercial qualities of a particular fruit
variety on another tree - whereas the quality of the fruit from trees grown from seed can be highly variable.
Also, grafted trees come into production much earlier than trees grown from seeds - they usually bear fruit
within 2-3 years, whereas in the case of trees grown from seed you have to wait 5-10 years before harvesting.
Production ORE uses two major grafting strategies - nursery production and top-grafting trees in the
field:
Nursery production makes it possible to produce large quantities of seedlings in plastic bags and graft them
with commercial varieties. These seedlings are then distributed to farmers for planting in the field and around
the home.
Top-grafting is the technique used to transform existing low-quality fruit trees, by pruning them and then
grafting them with commercial varieties. This activity is implemented by teams of locally trained grafting
technicians who go from one locality to another, grafting the farmers' trees. Top-worked trees generally bear
fruit with the grafted varieties during the next one or two seasons.
Propagation
Vegetative propagation is a form of asexual reproduction of a plant. Only one plant is involved and the
offspring is the result of one parent. The new plant is genetically identical to the parent.
New plants grow from parts of the parent plant. They include:
Stems: Runners are stems that grow horizontally above
the ground. They have nodes where buds are formed.
These buds grow into a new plant.
Roots: New plants will grow out of swollen, modified
roots called tubers. Buds develop at the base of the stem
and then grow into new plants.
Leaves: Leaves of some plants will grow into a new plant if they become detached from the parent plant. Other
plants grow small plants called plantlets on the edge of their leaves.
Bulbs: A bulb contains an
underground stem. Leaves are
attached to the stem. These leaves
contain much stored food. At the centre
of the bulb is an apical bud. Also
attached are lateral buds. The apical
bud will produce leaves and a flower
while the lateral buds will produce new
shoots. As the plant grows and develops
it will form a new bulb underground.
Artificial Vegetative Reproduction
Horticulturists and farmers use artificial means to produce plants that are identical to the parent
plant. Some of the methods used
are:
Cuttings: Cuttings are part of the
plant that is cut off of the parent
plant. Shoots with leaves attached
are usually used. New roots and
leaves will grow from the cutting. The
shoot is cut at an angle. A growth
promoter may be used to help with
the growth of the roots.
Grafting: In grafting 2 plants are used to develop a new plant with combined traits from the 2 parent plants. In
grafting the scion is the above ground part of one plant. The scion is attached to the stock which is the rooted
part of the second plant.
Layering: In layering a shoot of a parent plant is bent until it can be covered by soil. The tip of the shoot
remains above ground. New roots and eventually a new plant will grow. These plants can then be separated.
Name(s): _______________________________________
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Date: _________________________
Asexual Reproduction Jigsaw
Each group will be assigned a type of asexual reproduction. You will read your article about that type of
asexual reproduction and become the experts about it. You will present your findings to the class. You will
create a poster or visual aid to help you do this. Your poster should include the following information:
 Definition of your type of asexual reproduction
 Explanation of how it happens
 Examples of organisms that perform this type of reproduction
 A diagram of this type of reproduction
 Any interesting details about this type of reproduction
Type of Asexual Reproduction: _________________________________________________________
Grade: ___________________
Name(s): _______________________________________
_________________________________________
_________________________________________
_________________________________________
Date: _________________________
Asexual Reproduction Jigsaw
Each group will be assigned a type of asexual reproduction. You will read your article about that type of
asexual reproduction and become the experts about it. You will present your findings to the class. You will
create a poster or visual aid to help you do this. Your poster should include the following information:
 Definition of your type of asexual reproduction
 Explanation of how it happens
 Examples of organisms that perform this type of reproduction
 A diagram of this type of reproduction
 Any interesting details about this type of reproduction
Type of Asexual Reproduction: _________________________________________________________
Grade: ___________________