Reproductive biology
1.1
Asexual reproduction
Learning outcomes
On completion of this section, you
should be able to:
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define the terms asexual
reproduction and clone
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describe binary fission in bacteria,
budding in yeast and Aiptasia,
fragmentation in Spirogyra and
spore formation in Rhizopus.
Asexual reproduction is the production ofnew individuals--from.a single
organisn1 without the fusion of gametes. All the individuals formed in
this way form a clone of genetically__i_denticaLorganisms. There may be
some limited variation amongst the organisms in a clone that is caused
by random mutation. In prokaryotes and eukaryotes replication of DNA
occurs before cells divide and replication is usually free from errors. In
eukaryotes, mitosis occurs so that daughter cells receive the same
number and types of chromosomes. This maintains genetic stability.
Examples of asexual reproduction are:
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binary fission in bacteria
budding in yeast and Aiptasia (a sea anemone)
fragmentation in Spi.rogyra (a filamentous alga)
spore production in Rl1izopus nigricans (a mould fungus) .
Asexual reproduction in prokaryotes
Binary fission in bacteria
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Asexual reproduction in prokaryotes, such as bacteria, is simpler than in
eukaryotes but shares some similarities. This type of reproduction is
known as binary fission as each cell splits into two. Bacterial cells absorb
food and grow larger. When a cell reaches a certain size it starts to divide:
Figure 1.1.1 These bacteria are dividing
by binary fission
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Remember that prokaryotes do not
have a nucleus or linear
chromosomes like those of
eukaryotes, see page 32.
Didyou know?
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the circular chromosome replicates
the two chromosomes separate while held on to the cell surface
membrane
two new cell membranes form across the middle of the cell
cell wall material is formed between the new membranes
the cells remain attached for a while and then split apart.
Some bacteria, such as Escherichia coli, divide every 20 minutes under
optimum conditions.
Asexual reproduction in eukaryotes
Budding in yeast
Multiple fission occurs in some
organisms; while in red blood cells
the malarial parasite divides into
many tiny infective stages that are
picked up by a female Anopheles
mosquito when it feeds on blood.
Yeast cells grow in size but do not divide equally in half. At a certain size:
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~ Studyfocus
Remember that yeast does not
divide by binary fission as the bud is
much smaller than the parent cell.
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the 16 linear chromosomes replicate
the nucleus begins to divide by mitosis, but the nuclear membrane
remains intact and does not break up as shown in the diagram on
page 79
a small swelling appears at the side of the yeast cell to form a bud
one of the daughter nuclei enters the bud
the bud remains attached for a while and then breaks off leaving a bud
scar on the parent yeast cell.
Module 3 Reproductive biology ■
Budding in Aiptasia - p-. .. -.\. '-•c.-..- •'---•Aiptasia is a sea anemone, which reproduces asexually by a form of
budding called pedal laceration. Small groups of cells from the base of the
animal grow into small buds. After about a week these separate from the
animal and each then develops a mouth surrounded by tentacles so it can
feed itself.
Fragmentation in Spirogyra
Spirogyra is one of many types of multicellular, filamentous alga that
grows on the surfaces of ponds. A short single chain\)£ cells grows in
length. Any celLin the chain,._can enlarge, divide by 1nitosis t0-lonn two
cells and make the chain longer. Growth does not happen:just at the ends
of the filaments. New filaments form when lines of weakness develop
between cells. Any disturbance to the filan1ents causes them to break.
When conditions are perfect for growth, algae like Spirogyra can cover the
surfaces of freshwaters in a thick growth called blanket weed. Many
flowering plants also reproduce by fragmentation, for example the
Mexican Hat plant, Kalanchoe daigremontiana grows tiny plantlets
around its leaves. After a while these break off and fall to the ground.
Spore formation in Rhizopus
Leave some damp bread exposed to the air for 30 minutes, place in a Petri
dish and look at it every day for the next few days. You will see a growth
of mould over the surface of the bread. This is bread mould, a type of
fungus. The common bread mould, Rhi.zopus nigricans, has branching
filaments, or hyphae, which spread over the surface and grow into the
bread. If you look at the growth with a hand lens you can see lots of tiny
black pin heads. These are the structures carrying out asexual
reproduction for the fungal body, the mycelium, which you can see as a
mass of white filaments.
Fungi exhaust their food supplies, often very quickly, so they need to
produce spores to colonise new sources as follows:
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hyphae known as sporangiophores grow up into the air
the tip of each sporangiophore swells up to form a sporangium
haploid nuclei move from the hyphae into the sporangiun1 and divide
by mitosis
the columella - an extension of the sporangiophore - pushes up into
the sporangium separating it from the rest of the mycelium
cytoplasm forms around a group of several nuclei to form a spore
a wall forms around each spore
when the spores are ready, the sporangium splits open and the spores
dry and are blown away in air currents.
Fungal spores may be carried thousands of miles as they are so small and
light; most will not land on a food source, so there is great wastage.
Many organisms, both prokaryote and eukaryote, produce spores. They
vary considerably in their structure so the term spore is difficult to define.
They are best thought of as microscopic, reproductive bodies that contain
cytoplasm and one or more nuclei. They are often dispersed, but not
always as some are for surviving harsh conditions and not really for
dispersal in the sanie way as the spores of R. nigricans. The embryo sacs
of flowering plants are spores although they are not released nor are they
for surviving hash conditions (see page 151).
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bud
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Figure 1.1.2 These yeast cells are
budding. The nucleus divides and one of
the daughter nuclei enters the bud before
it breaks off.
Didyou know?
Rhizopus nigricans and R. tritici cause
soft rots in sweet potatoes. These
fungi damage the potatoes so they
cannot be sold.
Summary questions
1 Define the following terms:
asexual reproduction, binaryfission,
budding,fragmentation and spore.
2 According to the endosymbiosis
theory mitochondria are derived
from prokaryotes. The number
of mitochondria increases during
interphase of the cell cycle.
Describe how this happens.
3 Describe the functions of the
following in asexual reproduction
of R. nigricans: sporangiophore,
mitosis, sporangium and spore.
4 Suggest how you might
prevent the contamination of
stored sweet potato tubers by
R. nigricans.
5 Discuss the advantages and
disadvantages of asexual
reproduction for microorganisms,
such as bacteria, fungi and algae.
6 The sea anemone, Aiptasia is
occasionally introduced by
accident into marine aquaria.
It can very quickly spread
throughout aquaria causing harm
to other animals. Suggest why it
spreads so easily.
II
1.2
Asexual reproduction in flowering plants
Learning outcomes
On completion of this section, you
should be able to:
■
outline asexual reproduction in a
flowering plant
■
explain the role of meristems in
vegetative reproduction
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describe vegetative propagation
in crop plants
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describe how to take cuttings
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describe the process of tissue
culture
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describe the use of plant
hormones in vegetative
propagation and tissue culture.
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Meristematic tissue consists of
undifferentiated cells that divide by
mitosis. They are equivalent to
animal stem cells as referred to on
page 76.
Many flowering plants reproduce asexually. This is known as vegetative
reproduction oLvegetative propagation as it involves producing new
individual plants by modification.. of vegetative growth.rather.than growth
for..reproduction:involvingflowers. Propagation is a word meaning
multiplication or spreading. Plants can produce new individuals by
growth from:
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leaves, e.g. Kalanchoe daigremonti.anum and African violet,
Saintpaulia ionantha
stems, e.g. Irish potato, Solanum tuberosum, and ginger, Zingiber
officinale
roots, e.g. breadfruit, Artocarpus altilis, and dandelion, Taraxacum
officinale.
Ginger plants have a swollen horizontal stem known as a rhizome. The
stem has short adventitious roots that grow from it as it does not have a
main root. Leaves grow from leaf buds on the rhizome. There are axillary
buds at the point where the leaves grow. These buds have meristematic
tissue, which grows to provide branches on the rhizo1ne. The rhizo1ne
system can spread through the soil and cover much of the ground.
Eventually the rhizome will fragment into several separate plants.
Structures such.as rhizomes are swollen with stored energyJeserves and
nutrients for survival over unfavourable..times of the..year when it is too
hot, too dry or too cold for growth. Most stmctur_es that:rep.r.oduce
vcgct.1tivcl:y-ar_c.Jikc this. Structures, such as rhizomes, tubers..and bulbs,
store n1atcrials for perennation.
Plant hormones
The growth and development of plants are influenced by growth
substances that are often called plant hormones. There are three main
types of hormone.
Type of plant
hormone
Example
Source within
the plant
Effects on growth
auxin
indolyl acetic
acid (IAA)
shoot apex
stimulates elongation
of cells by •softening'
cell walls so they are
stretched when cell
absorbs water
cytokinin
kinetin
root apex
stimulates cell division
gibberellin
gibberellic
acid (GA3)
roots and
young leaves
stimulates cell
elongation and growth
in length of stem
Didyou know?
The Gros Michel variety of banana
was attacked by a fungal disease
known as Panama disease in the
1950s. Another variety, Cavendish,
is grown in plantations all over the
tropics. A new strain of Panama
disease (tropical race four) that
infects the Cavendish variety has
emerged in South-East Asia and may
well spread to the Caribbean and
Central America.
Synthetic plant hormones are used to control the growth of crop plants:
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the auxin naphthaleneacetic acid (NM) is used as a rooting
compound for cuttings
gibberellins are sprayed on seedless grapes in California to increase
the size of each grape and increase the distance between grapes so
reducing chance of disease spreading through the crop
kinetin is used in tissue culture to stimulate cell division.
Module 3 Reproductive biology ■
Artificial propagation
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Cuttings are taken from plants by removing stems, root or leaves from a
parent plant that has good features that are worth propagating. This can
be done by cutting:
Auxins stimulate root growth and
cytokinins stimulate shoot growth,
refer to Question 7 on page 157.
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a leaf at the point where it meets the stem, e.g. African violet
across a side shoot just below the point where a leaf joins the stem,
e.g. Joseph's coat, Coleus, and sugar cane, Saccharum officinarum
Didyou know?
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Plants that naturally produce structures for vegetative propagation can be
divided. Banana trees have suckers at their base which are removed for
propagation. This is the only way cultivated bananas, which are triploid
and therefore sterile, can be propagated.
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across the root, e.g. breadfruit, Artocarrpus altilis. "-• .. - •-.. -. • _. .. • --~-.. t· t
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Stem..cuttings:inay be dippedinto the NAAand then.placed into a
differentiation cells only kept the
suitable soil OLcompost:fm growing cuttings. After a while adventitious
genes they needed to carry out the
roots grow from the base of the cutting and the new plant can support
functions of specialised cells and
itself.
that they lost some or all of the
Not all cuttings are genetically identical to the parent plant from which
rest. Growing whole plants from
they have been taken. Some varieties of cultivated plants are chimeras,
differentiated cells shows this is not
which means that they are composed of cells of two different genotypes.
the case.
Root cuttings of thornless blackberry plants grow with thorns as the cells
. . . . . . . . 4' - - - ~ - ~4' - - "'"--- • ' " - • - - from which the shoots grow have the genotype for the thorned condition. .r.•
. , . . . . . . . . '-:, ........ :.-~ l.la
There are advantages of vegetative reproduction:
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new plants are easy to establish ■ plants have uniform appearance
(many seeds are not viable and
which gives a reliable supply
do not grow)
■ plants are uniform size which
most plants are clones and
helps harvesting and packing
genetically uniform
The main disadvantage is that genetically uniform crops are at risk of the
same pests and diseases for which they have no resistance. If there is an
epidemic of a pest or disease there is a chance that all plants with the
identical genotype will be wiped out.
ls~olution
Micropropagation
Many commercially important plants are propagated using n1ethods of
tissue culture. Small pieces of tissue are removed and cultured in a liquid
medium or on a solid medium. The media contain sucrose as a source of
energy, nutrients so the plant cells can make all the biological molecules
they need and plant hormones to regulate the type of growth. At the
beginning of culturing the number of cells increases, but at the end cells
are stimulated to differentiate into stems, roots and leaves.
Aseptic technique is used to ensure that no contaminants ruin the stock
of plants. This involves sterilising the media, using sterile containers and
implements; spores are removed by filtering the air and staff can take
precautions such as wearing appropriate clothing, masks and gloves.
/ cells in explant grow
_ / into callus tissueundifferentiated tissue
that can be divided to
grow into many plants
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Summary questions
1 Suggest the reasons why some plants are propagated vegetatively rather
than by seed.
2 Explain why plant hormones are used in tissue culture.
Figure 1.2.1 Tissue culture is used to
produce large numbers ofplants with
commercially important features
Advantages and Disadvantages of Asexual Reproduction
Advantages
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Only one parent is required. Where sexual reproduction involves two organisms, time and energy are used
in finding a mate, or, in the case of non-motile organisms such as plants, special mechanisms such as
pollination are required which may be wasteful of gametes. One solution to this problem is hermaphrodite
organisms which produce both male and female sex organs.
Genetically identical offspring. If the organism is well adapted to its environment, the fact that the offspring
are genetically identical may be an advantage. Successful combinations of genes are preserved.
Dispersal and spread. The methods of asexual reproduction often enable dispersal of a species. For example,
Penicillium and Mucor are common moulds which spread rapidly by means of asexually produced spores
which are light and easily dispersed by air currents. This enables the fungi to find fresh sources of food. Plants
that produce rhizomes, such as sea conch grass in sand dunes, bracken and Spartina (cord grass) in mud flats,
spread rapidly by this means.
Rapid multiplication. Bacteria can divide as often as once every 20 minutes allowing numbers to build up
very rapidly. Many parasites rely on one or more asexual stages where rapid multiplication compensates for
larger losses at other stages in the life cycle. The malaria parasite, tapeworm and liver fluke are good examples.
Disadvantages
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No genetic variation occurs among the offspring.
If spores are produced, many will fail to find a suitable place for germination and so energy and materials
used for their manufacture are wasted.
If an organism spreads in one area, it may result in overcrowding and exhaustion of nutrients.
Advantages and Disadvantages of Micropropagation
Advantages
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Rapid multiplication. When shoots are produced during cloning, they produce buds in the normal way. These
buds can be sued to generate more shoots using the same tissue culture techniques. By constantly recycling
buds in this way, the number of potential plants is multiplied at each stage. This can result in thousands or
even millions of plants being produced from one shoot over a period of a year. This is much faster than
traditional breeding methods, so new varieties can be introduced several years earlier than was possible by
the old techniques.
Genetic uniformity. Since plants produced are genetically identical, they all possess the desirable features of
the stock plant. It is difficult to produce plants that breed true (are homozygous for the required features) when
sexual reproduction is used.
Disease-free plants. As explained, virus diseases can be eliminated by using meristematic tissue for
propagation. Since the plants are prepared in sterile conditions, they are also free from surface bacteria and
fungi, some of which could cause disease.
Tissue culture can be carried out independently of seasonal changes in climate. One advantage of this is that
plants can be produced out of season, when they can be sold for higher prices.
Plant development is more closely controlled, guaranteeing product uniformity for customers.
Some plants, such as banana, are sterile and have to be propagated asexually.
The seeds of some plants, such as certain orchids, are difficult to germinate. The plants can be produced more
reliably by asexual means.
Micropropagation can be linked with genetic engineering to produce transgenic plants.
Airfreighting is economic because the cultures are not bulky. This increases the possibility of international
trade.
Disadvantages
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It is very labour intensive and not as convenient as sowing seed. The work is also skilled. Organisation and
training of staff presents problems when carrying out procedures on a large scale. It also adds significantly to
the cost of operation. The process normally only commercially viable for expensive products such as
ornamental plants and is not suitable for low-cost crops such as carrots.
Sterile conditions must be maintained. This adds to the cost and makes operations much more demanding.
Plants obtained from callus cultures sometimes undergo genetic changes. A small proportion of these changes
may be commercially useful, but most are undesirable.
Since clones are genetically identical, crops are very susceptible to new diseases or changes in environmental
conditions. Whole crops could be wiped out.
stimulate rooting
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shoot growth
~t
shoot growth
' ·itiidt~
c3
growth of embryos
stock plant with
desired features
liquid culture
i Methods for cloning from a stock plant
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plantJet
growth In
► greenhouse
and soil