Reproduction (I) reproductive strategies
P.1
Reproduction (I) -- Reproductive Strategies
An individual organism has limited life___.
Con_________ of the species depends upon
reproduction.
a.
Asexual reproduction - Rapidly produces large numbers of individuals, usually with identical genetic makeup with one another and to the single parent from which they are derived; gametes are never involved.
b.
Sexual reproduction - Less rapid, often involves two parents and produces offspring that are genetically
different. The fusion of haploid gametes is always involved.
In addition, reproduction may also associates with:
*
* formation of numerous spores, seeds or larvae and so reduces
1. Development of
resistant stages
intraspecific competition as well as capitalizing on any genetic variety
among the offspring
2. Dispersal
*
*
adapt to changing environmental conditions
3. Increasing
genetic variety
*
*
in a life cycle which are capable of withstanding periods of drought, cold
or other adverse conditions
I)
Sexual Reproduction
For sexual reproduction to occur, four main events must take place:
1.
Sexually mature


Production of sex cells by meiosis during which the chromosome number is
halved
2.
Gametogenesis


The coming together of two gametes of different types (the male and female) and
the production of a zygote.
3.
Liberation / transfer
of gamete


All species require a period of development from the first moment of the
existence of a new organism until sexual reproduction can occur.
4.
Fertilisation


At least one type of gamete (generally referred to as the male garnete).
Sometimes both gametes are released
Hermaphroditism and the Monoecious state
Hermaphroditism in animals and the *monoecious state in plants are terms used to describe the
situation when both male and female reproductive organs are present in the same individual.
Among animals, hermaphrodites occur mainly in the coelenterates, molluscs, annelids and some
crustaceans and fish. They are also common in endoparasites. Most hermaphrodite animals are
sedentary or slow-moving.
Q. Guess whether the following animals are hermaphroditic or have separate sexes :
Phylum Molluscs:
Phylum Arthropoda:
1. snail___________________________
1. barnacles_______________________
2. squid__________________________ ;
2. crabs__________________________
Q. Explain the advantages of hermaphroditism to sedentary or slow-moving animals.
Q. Suggest a reason why most flowering plants are monoecious.
Monoecious plants may either have both male and female organs in the same flower (bisexual flowers) or
have separate male and female flowers (unisexual flowers). Plants that have separate male and female
individuals are said to be dioecious. Most hermaphrodite animals and monoecious plants do not selffertilize. Many possess special features that ensure cross-fertilization.
Q Give a reason why self-fertilization might not benefit a species.
Reproduction (I) reproductive strategies
II)
P.2
Asexual reproduction
Asexual reproduction requires only a s
parent and haploid g
are not involved. There are
five major forms of asexual reproduction:
A)
Binary or multiple fission
This occurs in s
-celled organisms. DNA re
first and the nu
then divides, followed by
the division of the c
as a whole. The organism divides into two or more parts each of which
leads a separate life. e.g. bacteria and protozoans
B
fission in bacteria. The organism divides into t__. Under favourable conditions (temperature around
20°C and an abundant supply of food) the daughter cells grow rapidly and may themselves divide within 20 min.
Under unfavourable conditions some species develop a thick resistant wall around each daughter cell.
Multiple fission occurs when a cell divides into m___ parts rather than just two. In Plasmodium, the malarial
parasite (a protozoan), the process occurs at a number of stages in the life cycle. Such prolific powers of
reproduction compensate for the large l_____ associated with the difficulties of successful transfer
between hosts, namely, Man and the vector organism, the mosquito.
Reproduction (I) reproductive strategies
B)
P.3
Budding
An out
develops on the
parent and this later becomes
detached and is then an
in__________ organism.
It differs from binary fission in
that the two resultant cells are not
of equal s
but comprise a
smaller bud cell, becoming
detached from the larger parent
cell..
This occurs in yeast and in
Coelenterates e.g. Hydra.
C)
Fragmentation
In one sense fragmentation is no more than a form of reg
. If certain organisms are divided
into sections, each portion will regenerate the missing parts thus giving rise to new individuals. If
the division occurs as a result of injury then the process is regeneration.
If however an organism regularly and spontaneously divides itself
up in this way the process is fragmentation.
Organisms exhibiting fragmentation must have relatively
undifferentiated tissues and it is therefore limited to certain algae,
Coelenterates and flatworms.
In Spirogyra, for example, portions of the filamentous alga break away
when the filament reaches a certain length. These drift away, attach
themselves elsewhere and begin vegetative growth again.
D)
Sporulation
Sporulation is the formation of small unicellular bodies called
s
, which detach from the parent and, given favourable
conditions, grow into new organisms.
Spores are usually s
,l
and easily dis
. They are
produced in vast numbers - a single mushroom may produce 500
000 spores a minute. e.g. in algae, fungi, mosses and ferns (in one
sense, all plants produce spores).
Some organisms enclose themselves into res
unfavourable conditions. e.g. bacteria, protozoans
spores under
Reproduction (I) reproductive strategies
E)
P.4
Vegetative Propagation
In general, vegetative propagation involves the separation of a part of the parent plant which then
develops into a new individual.
Almost any part of the plant body may serve the purpose. e.g. root, stem, leaf or bud.

It ensures the continued survival of f___________ genotypes,

Organs of vegetative propagation also often act as per________ organs which lie in the soil over the
winter.

They are frequently swollen with excess f____ from the previous summer which is used to produce the
new off_____ in the following year.

One advantage is that growth can begin e____ in the spring using the stored food and the plant is
therefore able to start photo_________ earlier when there is little com______ for light from other species.
Organs of vegetative propagation and perennation
Reproduction (I) reproductive strategies
Name
Example
Description of organ
Mechanism of action
Bulb
Onion
Garlic
Apical and axillary b___ among the
leaves each give rise to more new
plants
Corm
Crocus
Gladiolus
Underground, swollen,
fleshy l___ - bases closely
packed on a short stem, i.e.
a bud
Underground, ver____
sw_____ s___
Buds develop in the base of the scale
leaves surrounding the corm. Each
may develop into a new plant
Yes
Rhizome
Ginger
Underground
hor_________ branching
s___
Stem grows and branches. At the tip
of each branch a bud produces new
vertical growth which gives rise to a
new plant.
Yes
certain
grasses
Perennating
organ
Yes
Stem
tuber
Potato
Sw____ tip of slender s___
/ rhizome
The tips sw___ to form a stem tuber,
each gives rise to a new plant
Yes
Tap root
Carrot
A swollen, vertical r____
Not an organ of vegetative propagation
- only a perennating organ
Yes
F)
P.5
Artificial Vegetative Propagation
Certain plants may have particularly desirable traits ( e.g. flower colour, flavour, fruit quality and
size, resistance to disease, etc ), com
important plants are propagated by asexual means.
Offspring produced from asexual reproduction are cl
, they have the same genotype as the
original plant, they are also highly uni
phenotypically. Therefore, in artificial propagation,
plants with superior q_____ are selected and then clones are being produced by different asexual
means. Many clones of horti______ interest have been discovered and perpetuated by humans.
Cutting
This involves the c
of a small section of a stem root or leaf from desirable plants (stock
plant ) and induce the cuttings to form r___ ( or shoots ) and then develop into indep________
plants. Each of the cuttings should possess one or more b .
i)






ii)




ii)

Procedure
Obtain cuttings from the s____ plant;
Remove larger l_______ from the cuttings to reduce water loss by transpiration.
Treat the cutting bases with auxins to initiate formation of adventitious r____.
Place in a rooting med___ which holds the cutting in place and to provide moisture, air and nutrients.
Cuttings are kept in shelter where the temperature, humidity and light intensity are adjusted.
After rooting, the cuttings are potted up or grown in field.
Advantages of cutting:
M____ new plants can be started in a limited space from a few stock plants;
inexpensive, rap___ and sim___ and NO special technique necessary as in grafting and cloning.
there is no problem of incomp_______ with rootstocks or of poor graft unions;
all the ben_______ characteristics of the stock plants can be passes on to the new plants.
Disadvantages:
New plants obtained by this method are more susceptible to diseases and pests because they are derived
from the same clone, with no genetic d
.
Reproduction (I) reproductive strategies
b)
P.6
Grafting
Grafting is done by connecting two pieces of
living plant tissue together

so that they will un
and subsequently grow
and develop into one plant.

The scion forms the upper part of the graft. It
is a short piece of cut shoot containing
dormant b
which will give rise to new
branches, leaves and flowers & fruits.

The rootstock form the lower portion of the
graft, which develops into the r
system of
the grafted plant.
i)

Importance of Grafting to agriculture:
It can comb_____
advantageous features
from both the scion and the
stock.
The stock provides a
strong r
system which
can withstand adv_____
conditions and resist soilborne infection and
pathogens; the scion
provides the required
f
,f
etc.

It is able to allow r_____
production of large
numbers of the new
scion/stock combinations
for marketing; it also
sh_____ the growth
period of the plants.

N
kind of fruits from
grafting of related species
can be produced.
G)
Cloning & Tissue Culture
A group of genetically i
offspring is called a clone.
Cloning has the advantage that all the offspring behave sim
(for example uniform ripening of
crops making harvesting easier) but should disease break out, no r________ strains of plants are
present to lessen the impact.
It has been established that differentiated (specialised) cells still contain all the information needed
to make the whole organism and that cells become sp__________ as a result of the s__________ on
and off of g____ rather than by the loss of certain genes.
Reproduction (I) reproductive strategies
a)
P.7
Tissue culture in plants
The nucleus of every cell of an individual contains all the g
information needed to develop
the entire organism. It is therefore possible under suitable conditions to produce a w
organism
from a s
cell.
If a cell divides m_________ it will produce a clone. If each cell of the clone is separated and
allowed to develop into the complete organism, a group of genetically identical offspring is formed.
This is known as tissue culture -- a kind of cloning.
E.g. Cloning of Carrot plants from cambial tissues

Cambial tissue excised from plants (eg from a carrot root)

Excised tissues are stimulated to divide and growth by the addition of
nutrients, IAA and cytokinins.

These new cells could be subcultured to generate a population of identical
cells -- a clone.

Individual cells grown in a medium containing suitable nutrients and
hormones, could be induced to start dividing again and to produce new
carrot plants.
Tissue culture is now widely used for the rapid propagation of desired varieties
(such as date oil palms) or for varieties difficult to propagate by cuttings.
b)
Cloning in animals
Taking a cell from the intestine of a frog
and introducing its nucleus into an egg
cell whose own nucleus had been
destroyed by ultra-violet radiation, it is
able to grow a tadpole, and thence a
frog, identical to the parent from which
the nucleus was donated
Similar techniques are successfully used
in cloning more advanced vertebrates.
The cloning of desirable animals such as
prize bulls, racehorses and so on.
Theoretically any number of
genetically identical copies of the same
man or woman might be made.
Although superficially it might seem that,
for example, brilliant individual person
might be perpetuated in this way, it has
to be remembered that the degree to
which en
influences
development is not fully known, and
any cloned cell would have to go through
all the phases of development once again
including, in the case of a human
embryo, fetus, baby and childhood.
Reference Reading : Cloning the oil palm
Reproduction (I) reproductive strategies
P.8
Oil palm oil is used in large quantities in the manufacture of margarine and detergents. When grown under
ideal conditions some oil palms (Elaeis guineensis) will yield 6 tonnes of oil per hectare. When seeds of high
yielding plants are sown not all the plants are high yielding - there is a great deal of variation. The obvious
solution is to clone the high yielding oil palm plants.
There is nothing new about cloning plants. Indeed, it is an ancient art which was practised by the earliest
horticulturists. It is usually quite easy and amateur gardeners do it when they want to increase the numbers of
a valued plant, for example by taking cuttings.
There are now laboratory techniques by which parent plant tissue can be induced to form large numbers of
new plants. It relies on the fact that certain cells in the plant remain unspecialised and are capable of further
development. It is possible to stimulate these cells to undergo mitosis and form clumps of cells called a
callus. All the cells in a callus are identical and each one can be grown by the technique of tissue culture
into a new plant identical to the parent. In this way thousands of identical plants can be created all
belonging to the same clone.
Oil palms reared in this way are now growing in plantations in Malaysia and all the plants are capable of a
producing a high yield.
Other food plants such as pineapples and bananas are being cloned, and attempts are being made to clone
forest trees such as the Norway spruce which is resistant to acid rain. For the gardener, orchids and roses
and many other ornamental plants are being cloned, using tissue culture.
Q. Search for environmental issues related to the extensive cultivation of Oil palm in the tropics.
III)
Genetic Variability and reproduction
Genetic Variation and Sexual Reproduction
Sex provides the var________ that is needed for diploid organisms to e_______ to suit new
conditions. The two fundamental processes that generate variability in sexual reproduction are
m______ and f___________.
Independent a_________ of chromosomes and c_______-over resh
the genes in an individual
into new combinations. This genetic recombination through meiosis is very important in providing
an im_______ source of variation in gametes on which natural s
can act.
Fertilisation brings together new com________ of genome from different individuals (cross
fertilization). The fusion of gametes from two separate parents is called cross-fertilization. Provided
the two parents are genetically d_______, greater variability in offspring results.
In some species, self-fertilization involves a single parent occurs; the amount of variability
generated is therefore l
--but not none. The process is nonetheless sexual as fusion of gametes is
involved and as gametes are produced by meiosis, some variability is generated.
Mutation is important because it add n__ genes to the gene p___. Mutant genes that arose in
separate individuals can be assembled in one organism through s_____ reproduction
Ten mutations in an asexually reproducing population could result in only ten novelties, even if the effects
were phenotypically expressed. Ten mutations in a sexually-reproducing diploid population could give 2 10 =
1024 novelties. This creates a much greater v
of genotypes than mitosis and mutation alone would
produce, and thus enhances the potential for adaptation and future ev__________ change.
Reproduction (I) reproductive strategies
B)
P.9
Inbreeding & Outbreeding in Flowering Plants
If transfer occurs between two plants of different genetic make-up / constitution the process is
cross-pollination. If this takes place between flowers of identical genetic make-up, the process is
self-pollination.
Inbreeding
Inbreeding is the breeding between c_____-related individuals.
Self-pollination leads to s___-fertilisation, it is the extreme form of i________; Self-fertilization
allows the recombination of genes belonging to that one plant only.

Inbreeding produces a population with highly homozygous conditions and thus whose individuals have
very similar genotypes, highly suited to the environment.

They may survive extremely well, yet lack the ability to evolve rapidly to suit new conditions.

Highly homozygous condition allows appearance of recessive phenotypes, which might be undesirable
or even harmful.
Q. Inbreeding is very effective in producing uniformity. Consider a pair of alleles A and a, starting with a pair of
heterozygous parents, calculate how many generation of inbreeding would it take to achieve a more than 90%
homozygosity in the descendents.
Outbreeding
Outbreeding is breeding between only distantly related / un
individuals. Two advantages of
outbreeding are
i)
Immediate Hybrid Vigour
Two different strains each of which was previously inbred, if brought together for outbreeding,
usually produce a generation which is unusually vigorous-- having sup
quality compared to
their parents..
This ‘hybrid vigour’ is exploited in agriculture when F1 seeds of plants such as sweetcorn and
tomatoes are commonly used to give productive cr . The reason for the exceptional vigour of the
F1 plants has been explained as follows:

The two parental inbred lines have accumulated different favourable alleles promoting good growth.
These may include genes for extensive root systems, more chlorophyll and earlier germination.

In the inbred lines, these genes are in the homozygous condition and each line has a different
collection.

On crossing, the F1 will be het
for each pair of alleles and so gain benefit from all of them, since
most advantageous alleles express dominance.
ii)
Long Term Evolutionary Adaptability
Outbreeding favours heter
maintains more v ______ in the gene-pool than does inbreeding.
It promotes evolutionary flex______ at the expense of producing progeny not equally well-suited to
the prevailing conditions.
Reproduction (I) reproductive strategies
P.10
Sexual reproduction is expensive in terms of the organism's energy resources.
While sexual
reproduction is an important mechanism in long-term evolutionary change, there will be no such
evolution unless sexual organisms are competitively successful now and in the immediate future.
a.
In what ways are sexual reproduction more ‘expensive’ than asexual reproduction ?
b
What might be the ‘immediate’ benefits of sexual reproduction to the chance of survival of a species?
C)
Asexual reproduction and its Significance
Asexual reproduction allows a successful individual to propagate rapidly to form a cl____
population. This enables successful or selected genotypes to fu
e.g. in favourable conditions
exploit an available resources.
Asexual reproduction also provides a temporary relief for sexually st
hybrids and poly_____
which would otherwise not survive naturally beyond the life span of one individual.
Asexually reproduced individuals may be very successful in a particular environment, since they
can spread ra
when conditions are favourable without the need to obtain a partner for mating.
Constant high-yielding strains of plants such as potatoes and fruit-trees are maintained by
horticulturists using techniques of asexual propagation. If the plants were allowed to reproduce
sexually, the offspring would not be of uni_______ good quality. Asexual reproduction has the
advantage of pre
valuable gene combinations.
Multicellular organisms which rely exclusively on asexual reproduction are genetically rig . Should
environmental conditions change, there is no reservoir of variety in a genotypically identical
population to provide dif
__ -adapted forms.
D)
Conserving Favourable genotypes / Successful gene combination
a
Sometime variability is not advantageous. Under what conditions could little or no genetic variation be advantageous?
b
By what means is the variation generated by recombination at meiosis restricted in all eukaryotic organisms?
The recombination of alleles made possible by sexual reproduction is the basis of variation and
adaptation in later generations. However, wholly unrestricted recombination would not be
advantageous. This is because,

Firstly, unrestricted recombination would disassemble any successful combination at the next
generation rather than conserving them in the gene pool of the population.

Secondly, some of the unsuccessful phenotypes would have resulted from new combinations of alleles.
So conservation of much of the genotype must be necessary to maintain su____. The ch_________,
each of which contains a large number of genes, provide the means of preserving many
advantageous com _____
of alleles at related gene loci. The maximum amount of recombination
made possible by the events of sexual reproduction is, therefore, strictly limited.
Reproduction (I) reproductive strategies
P.11
The optimum amount of recombination, is therefore a balance between conservation and change
which allows for conservation of suc________ genotypes, reviving vig___ and evolutionary
adap_________.
Where conservation of the genotype is favoured, there are ways in which generation of variation in
offspring can be severely restricted, or even eliminated in reproduction:

A________ reproduction gives clones which remain identical--apart from the accumulating mutations. On
the other hand, asexual reproduction preserves any existing heterozygosity, with its attendant vigour.

In sexual reproduction, reduction of variation can be achieved by the fact that functionally related genes
are l_____ and inherited as a group. It is also reduced when the total number of chromosomes is
reduced, thus cutting down the number of bivalents assorting randomly at meiosis.

In_______, that is breeding between related individuals reduces the level of heterozygosity. The low level
of heterozygosity then, in turn, limits the amount of variation which can be generated at meiosis.
As most higher plants are monoecious, a balance of asexual, self- & cross-fertilization in sexual
reproduction will allow for both the generation of new genotypes and the conservation of the most
successful ones. The relative contribution of self- and cross-fertilization varies between species. It
may favour outbreeding, even to the point of completely excluding self-fertilization (or selfing).
Q. Arrange the following in descending order in which amount of genetic variability in offspring is generated:
1. Self fertilization in an individual which is wholly genetically homogeneous / pure-breeding in all gene
loci
2.
Cross fertilization between two genetically-distant individuals
3. Self fertilization in an individual which is genetically highly homogeneous
4. asexual reproduction through mitosis
5. Cross fertilization between two genetically-close individuals
6. Self fertilization in an individual which is genetically highly heterogeneous
7. asexual reproduction through meiosis (spore production in some plants e.g. moss, fern)
Is it true that no genetic variability is possible in the last case?
2, 5, 6, 7, 3, 4, 1
Reproduction (I) reproductive strategies
C)
P.12
Summary of Sexual and Asexual reproduction
Asexual reproduction
Sexual reproduction
(excluding bacteria)
No. of parent
involved
Involvement of
gametes
One parent only
Usually t__ parents
Asexual reproduction never does
Gametes are always produced. These are
usually h______ and nuclei of two gametes
f___ (fertilization) to form a di_____ zygote
Involvement of
meiosis and
production of
genetic variability
Rarely involves mei____, the
offspring are usually i_______ to each
other and to their parents
If these g_______ are produced by meiosis
they will show considerable genetic variety.
Adaptability of
offspring and
evolutionary
consequence
Complexity and
efficiency
It would be inaccurate to say that
asexual reproduction always produces
identical offspring. Mei______ are
involved in certain cases (e.g. spore
formation in ferns and mosses) and
mutations, although rare,
nevertheless occur and so might
create some variability.
Lack of v_______ is a disadvantage in
adapting to environmental change
Asexual reproduction is normally
more s_____ and straightforward.
It is f___ and efficient in producing
large number of offspring within short
periods of time.
The offspring resulting from the f_____ of
gametes will likewise show genetic
v________ due to genetic recombination.
Mut______ also arise during sexual
reproduction, indeed the greater complexity
of the process means they arise more
frequently.
Better able to adapt to environmental
change. In other words, they have the
capacity to ev____ to suit new conditions
More complex and l____ efficient.
Meiosis are more complex than mitosis,
and the processes of producing and
transferring gametes are often complicated.
Elaborate court____ sometimes needed
before mating
Ability to preserve
favourable
genotypes
If an individual has a genetic make-up
which is suited to a particular set of
conditions, asexual reproduction is a
means by which large numbers of this
successful type may be built up.
They might rapidly colonize the area
and exclude/reduce co________ from
other plants.
Plants do not exhibit locomotion
and individuals must remain where
they are. For this reason most have
retained the as_______ process as
part of their life cycle.
Occurrence
Commonly occurs in plants, less
differentiated animals and microorganisms
Sexual reproduction makes it less likely to
maintain a f____________ genotype
combination since sexual reproduction
tends to produce offspring with
vari________. These may not be as welladapted as identical copies of the parents
would be.
Animals, with their ability to move from place
to place, can spread and search out
conditions that suit new variants.
Occurs in majority of plant and animal
species
Absent in higher animals
Note :
In mosses and ferns, gametes are produced by a haploid gametophyte generation. Being haploid, its
gametes can only be produced mitotically. Mitosis does not introduce genetic variety; and these gametes are
therefore identical.
During asexual reproduction, mosses and ferns produce spores by meiosis. These spores show genetic
variability.
Reproduction (I) reproductive strategies
P.13
In plants, it is common for asexual and sexual methods of reproduction to alternate:
Q. Which of the following statements is/are true ? If you think a statement is false, give an example of
exception and explain briefly.
Statement
T/F
Exception and explanation
a.
Asexual reproduction
always results in the
production of identical
offspring.
F
In mosses and ferns, spores are produced by meiosis which
generates variability through random assortment
b.
Gametes are always
haploid.
F
Sometime gametes are diploid or even triploid, as in
production of polyploids.
c.
Gametes are produced by
meiosis.
F
In mosses and ferns, gametes are produced by mitosis as the
parent plant body –the gametophyte is haploid.
d.
Meiosis always produces
haploid cells and mitosis
always produces diploid
cells.
F
Mostly true, but mitosis of haploid mother cell will produce
haploid daughter cells only –as in gamete formation in ferns
and mosses
e.
Mitosis occurs only in
diploid cells
F
Mitosis can occur in both haploid, diploid cells or polyploidy
cells.
Reproduction (I) reproductive strategies
P.14
Extension: Parthenogenesis
It is the development of a female gamete in the without fertilization to give rise to a new individual.
As a gamete is produced the process is a modified form of sexual reproduction, even though only a single
parent is involved. This process occurs commonly among the arthropods.
In Aphids, female in summer produce successive generations of diploid parthenogenetic female. The egg
cells develop within the mother and young females are released viviparously, that is the parents do not lay eggs
but bear live young. The last generations of the season- (autumn) contains males also. Sexual reproduction
then occurs to produce an over-wintering generation of eggs that hatch in spring.
End