Download Introduction The phenomenon of change in chromosome number is

Introduction
The phenomenon of change in chromosome number
is called heteroploidy, which includes aneuploidy and
euploidy. The change in chromosome number may
involve one or a few chromosomes of a genome, and the
phenomenon is called aneuploidy, or the change may
involve one or more complete sets of chromosomes or
genomes and the phenomenon is called euploidy. The
chromosome number of euploids is an exact multiple of
the basic chromosome number of the species. The basic
chromosome number of a species is represented by ‘X”
whereas ‘n’ represents the gametic chromosome number
and ‘2n’ represents the somatic chromosome number of a
species. Euploidy is designated by ‘X’, ‘2X’, ‘3X’, ‘4X’,
‘6X’, etc. depending upon the number of genomes
involved. Diploidy is considered as the normal condition in
plants and animals. A diploid individual contains two sets
of chromosomes (i.e. each chromosome is in a pair form),
one set contributed by each parent during sexual
reproduction. Thus diploids are 2X with two sets of
chromosomes. Any deviation from this 2X condition
involving whole set of chromosomes results in euploidy.
The level of euploidy in an organism is, however,
determined by the tolerance power of the nucleus, mitotic
and meiotic behaviour of chromosomes, and gene
expression and interaction. Most of the euploids are
tetraploids (4X), some are hexaploids (6X) and rarely
octaploids (8X) are found.
Classification of Euploidy
When one or more complete sets of genomes are
involved, the condition is called euploidy.Euploidy is
classified as:
1. Monoploidy / Haploidy
2. Diploidy
3. Polyploidy
• Autopolyploidy
• Allopolyploidy
Heteroploidy
Euploidy
Aneuploidy
Monoploidy Diploidy (2X)
Hyperploidy
Haploidy (X)
(2n + 1)
Tetrasomy
(2n + 2)
Hypoploidy
Polyploidy
(3X, 4X, 6X)
Monosomy
Trisomy
Nullisomy
(2n – 2)
(2n – 1)
Autopolyploidy Allopolyploidy
(AAA, AAAA)
(AABB, AABBCC)
1. Monoploidy / Haploidy
Monoploids have a single basic set of chromosomes
represented by ‘X’, whereas haploids have half the
somatic
chromosome
number
of
an
individual,
represented by ‘n’, e.g. in diploid barley with 2n = 14;
both monoploid and haploid chromosome number = 7.
On the other hand in tetraploid barley with 2n = 28;
Monoploid number = 7 but haploid number = 14.
Haploids of polyploids are called polyhaploids, whereas
those of aneuploids are called aneuhaploids. In general,
the term haploidy is more commonly used than
monoploidy.
Origin and production of haploids
Haploids are produced by many
spontaneously or induced artificially, e.g.
ways,
either
• Parthenogenetic development: Haploids can
originate from any gametophytic cell without
fertilization, e.g. egg, synergids, antipodal cells,
pollen tube nucleus, etc. All these cells are haploid
produced after meiosis of megaspore mother cell.
These cells occasionally give rise to embryo directly.
The first case of haploidy was reported in Datura by
Blakeslee et al (1922), which was due to
parthenogenetic
development
of
egg.
In
Hymenoptera insects queen and drones are diploid
females, while males are haploid. The male insects
are produced parthenogenetically. Similarly in
flowering
plants,
haploids
originate
due
to
parthenogenetic development of egg (e.g. tomato,
cotton). Haploids originating from pollen tube
nucleus are called androgenic haploids.
• Delayed pollination: A delay in pollination induces
parthenogenetic development of egg into haploid
embryo, e.g. in maize.
• Distant hybridization: Commonly called as
Chromosome elimination technique or Bulbosum
technique. This technique was developed by Kasha
and Kao 1970 for the production of haploids in
barley. When Hordeum vulgare and H. bulbosum are
crossed, the chromosomes of H. bulbosum are
eliminated in early zygotic divisions, and the embryos
produced are thus haploids. These embryos can be
cultured to get haploid barley plants. Bulbosum
technique has been widely used to produce haploids
in wheat, barley and maize etc.
According to Subrahmanyam and Kasha (1973),
normal fertilization between H. vulgare and H.
bulbosum is follwed by gradual elimination of
bulbosum chromosomes from embryo to endosperm
development. Bennet et al (1976) concluded that
sudden shortage of proteins in the developing
embryo and endosperm and the better ability of
vulgare chromosomes to form spindle attachments
relative to bulbosum chromosomes may be
responsible for chromosome elimination. In short,
chromosome elimination is under genetic control.
• Anther or pollen culture: It is the most important
technique of haploid production and was developed
by Guha and Maheshwari (1970). Pollen grains or
anthers are cultured on suitable medium to produce
pure haploid plants. This technique has been
successfully employed in several crop plants for
haploid production.
• Haploids can also be produced by X – rays, colchicine
treatment or through transfer of haploid inducing
genes.
Morphology and Cytology of Haploids
Haploids are generally weak and show reduction in
the size of all vegetative and floral parts. The size of
seed, pollen, stomata, etc is also smaller in haploids as
compared to diploids. Due to the expression of recessive
lethal genes and other harmful genes at various stages of
growth and irregular meiotic behaviour, the haploids
generally die, or do not produce seeds for future
generations. Haploids are maintained by vegetative
propagation only.
Since haploids have no homologous chromosome
pairs, therefore all chromosomes are seen as univalents
at metaphase –I of meiosis. The anaphase-I segregation
is irregular resulting in the formation of defective
gametes. Consequently, a considerable amount of sterility
and almost zero percent seed set is found in haploids.
Seeds can, however, be produced in haploids through
apomixis and parthenogenesis. In polyploids like
autotetraploids, haploids behave like diploids during
meiosis.
Uses of Haploids
 Haploids are used for the production of homozygous
diploid lines (purelines) which may be directly used




as cultivars or may be used in breeding programmes.
Generally, it takes 8-10 years to develop a
homozygous diploid line (pureline) in conventional
breeding programmes. However, pollen of a superior
plant can be cultured, followed by chromosome
doubling of the haploid plant, to produce
homozygous diploid line also called as pureline.
Haploids are used for hybrid sorting in hybrid
breeding: One of the essential steps in haploid
breeding involves selection of superior plants among
haploids derived from F1 hybrids through anther
culture. It is popularly described as hybrid sorting,
and virtually means selection of recombinant superior
gametes.
Haploids are very useful in basic and applied
research on induced mutations. Due to the presence
of only one set of chromosomes, even a recessive
mutation is expressed. After diploidization, the useful
recessive mutations can be made stable in
homozygous condition.
Haploids are used in cytogenetic research for
production of aneuploids, alien addition and
substitution lines, and determination of basic
chromosome number.
Haploids are currently used in transgenic technology.
This helps in stability and better expression of the
gene transferred during pollen or anther culture
followed by diploidizing the transgenic haploid.
2. Polyploidy
Polyploids have more than two sets of genomes.
There are two main types of polyploids.
 Autopolyploids
 Allopolyploids
A. Autopolyploidy
Autopolyploids are those where the same basic set of
chromosomes is multiplied. i.e. they contain more than
two sets of genomes from the same source. For example
if a diploid species contains two similar sets of
chromosomes or genomes designated as AA, different
series of autopolyploids obtained from this diploid set
would be:
14)
AA = 2X (diploid) – normal, e.g. X = 7, 2n =
AAA = 3X (autotriploid, e.g. X = 7, 2n = 21)
28)
42) etc.
AAAA = 4X (autotetraploid, e.g. X = 7, 2n =
AAAAAA = (autohexaploid, e.g. X = 7, 2n =
Autotriploids and Autotetraploids are found more
commonly in nature than the higher polyploids. This is
due to the fact that the cell of a species can
accommodate a certain limit of chromosome number in
its nucleus, beyond which it may affect the cell
functioning or even lead to cell death. Autotriploids have
been reported in grapes, watermelons, sugar beet,
tomato, banana, lemon, apple, etc., where as
autotetraploids have been reported in rye, corn, berseem,
marigold, flox, snapdragon, grapes, apples and several
vegetables.
Origin and production of Autopolyploids
Autopolyploids
are
generally
produced
by
chromosome doubling. This chromosome doubling may
occur spontaneously in nature, or may be induced
artificially by physical and chemical agents.
• Spontaneous production
Autopolyploids are spontaneously produced by
the formation of unreduced gametes followed by
their fertilization. They can also be produced by
chromosome doubling followed by failure of cell
division or failure of 2nd meiotic division. Similarly,
autopolyploids are also produced during tissue
culture. Plants with various ploidy levels are
regenerated from the callus in tissue culture.
Naturally produced autopolyploids can be inferred by
their mitotic and meiotic behaviour, e.g. karyotype
analysis during mitosis, formation of multivalent (tri,
tetra and hexavalents, etc.).
Fig. Chromosome duplication due to fertilization
between unreduced gametes (Downloaded from
internet)
• Induced Autopolyploids (Colchicine Treatment)
The best method of inducing polyploidy is by
treatment with colchicine. Colchicine is an alkaloid
obtained from seeds and corms of Colchicum autumnale.
It has been used with great success in a large number of
crop plants belonging to both dicots and monocots. The
successful doubling of chromosomes by colchicine was for
the first time described by Levan, 1938 and Eigsti, 1940.
Colchicine has the property of arresting and breaking the
spindle or preventing spindle formation during cell
division, so that chromosome duplication, followed by
failure of movement of chromosomes to anaphase and
failure of cell division leading to doubling of chromosome
number, in the same cell occurs. Colchicine treatment is
given to seeds, meristematic tips or axillary buds, etc.
The concentration and duration of treatment may vary
from plant to plant. Autotriploids are generally produced
by crossing diploid and tetraploid genotypes of the
concerned species.
Figs. Chromosome doubling by colchicine treatment
(Downloaded from internet).
Different types of autopolyploids can be produced as
follows.
Diploid genome AA (2n = 20, X = 10)
AA
2n = 2X = 20
Colchicine treatment
AAAA
2n = 4x = 40
(Autotetraploid)
X
AA
2n = 20
AAA
2n = 3X = 30
(Autotriploid)
treatment
Colchicine
AAAAAA
2n = 6X = 60
(Autohexaploid)
Fig. showing various ploidy levels (Downloaded
from internet)
Cytology and breeding behaviour
Autopolyploids generally show multivalent associations at
metaphase. In case of autotriploids, trivalents of various
shapes, or a bivalent plus one univalent, are observed at
metaphase. The anaphase segregation of each trivalent is
irregular. For example the trivalent can segregate in 2:1
or 1:1 (with one chromosome lagging). The overall
chromosome segregation in triploids is highly irregular,
leading to the formation of defective or sterile gametes,
which fail to take part in fertilization. Thus triploids do not
produce seeds due to their irregular meiotic behaviour.
Triploids can, however, be maintained by vegetative
propagation, or by crossing diploids and tetraploids. On
the other hand, autotetraploids show tetravalents of
various shapes at metaphase. Occasionally, there can be
two bivalents or a trivalent plus a univalent at
metaphase. At anaphase, generally there is 2:2
segregation of a tetravalent producing balanced 2X
gametes which on fertilization result in the production of
tetraploid progeny. Sometimes, there can be 1:3 or 2:1
(one chromosome lagging) segregation at anaphase.
Thus, autotetraploids mostly produce fertile gametes;
however, some sterility is also found.
(Fig. downloaded from internet)
Morphological
features
and
applications
of
Autopolyploids:
• One of the important features of autopolyploids is
their gigantism. The fruits, flowers, seeds, leaves etc.
are generally larger in size than in the normal
diploids.
• Polyploids have larger cell size than diploids. Guard
cells of stomata are larger and number of stomata
per unit area is lower in polyploids than in diploids.
• Pollen grains of poyploids are generally larger than
those of the corresponding diploids.
• Polyploids are generally slower in growth and show
delayed flowering and maturity.
• Polyploids generally show reduced fertility due to
irregularities during meiosis and due to genotypic
imbalance leading to physiological disturbances.
• Different species have different tolerance levels in
ploidy. Some species survive up to 3X level while
others can tolerate up to tetraploid (4X) and
hexaploid (6X) levels.
• Autotetraploids are produced in those plants where
large size of seeds and fruits, increased flower and
leaf size is the economic requirement, e.g. several
fruit crops, vegetables, ornamentals and forage
crops.
• Autotriploids are produced in those plants where
seed is not the economic product, e.g. banana,
oranges, lemon, watermelon, grapes, apple, tomato,
etc.
• Autotetraploidy
is
used
to
overcome
selfincompatibility in some plants, viz. Nicotiana, clover,
and petunia.
• Some distant crosses fail at diploid level, but are
successful at tetraploid level because the hybrid
produced is stable here. e.g. 4x Brassica. oleracea X
4x B. chinensis
• Some examples of autopolyploids are
 Potato
(Solanum
tuberosum)
4x = 48
 Coffee (Coffea arabica)
4x
= 44
 Alfalfa (Medicago sativa) 4x = 32
 Pea nut (Arachis hypogea) 4x = 40
 Sweet potato (Ipomoea batatus) 6x
= 90
 Banana (Musa sapientum)
3x
= 33
 Apple
(Malus domestica)
3x
= 51
Fig. Seedless fruits (Autotriploids)
2n and 4n grapes
Fig. Comparison of stomata size between diploids
and polyploids (Downloaded from internet)
B. Allopolyploidy
Allopolyploids are formed by hybridization between
two or more than two different species (interspecific or
intergeneric) followed by doubling of chromosomes. The
two or more species can hybridize naturally, or can also
be crossed artificially. Similarly, chromosome doubling of
the hybrid can occur spontaneously, or can be induced by
colchicine treatment. Genomes of two different species
cannot survive in diploid condition in a hybrid due to lack
of chromosome pairing and genetic disharmony. That is
why evolution has favoured chromosome doubling so that
they survive at tetraploid or hexaploid level.
Schematic
representation
of
Allopolyploidy
AA x BB
production
of
AB
F1 hybrid sterile
colchicine
Chromosome doubling /
AABB
X
Allotetraploid
fertile
CC
ABC
Allotriploid (sterile)
doubling /
Chromosome
colchicine
AABBCC
Allohexaploid (fertile)
Since allopolyploids (AABB, AABBCC) have two or more
different genomes, they behave like diploids during
meiosis and are thus also called amphidiploids, as the
genomes of two different species behave like diploids.
The
production
of
allopolyploids
has
attracted
considerable attention as they result into the evolution of
new species. Sometimes, the genomes of two or more
than two different species have some homology so that
multivalents are occasionally formed during meiosis. Such
polyploids are called segmental allopolyploids.
It is believed that natural allopolyploids might have been
produced due to following reasons.
• Failure of chromosome segregation at anaphase
after zygote formation.
• Irregular mitotic cell division in apical meristem,
leading to the formation of allopolyploid branch.
• Fertilization between unreduced gametes
Fig. One possible mechanism of origin
allopolyploid (Downloaded from internet)
of
Schematic representation of origin of allopolyploidy
(unreduced gametes)
AA x
2n egg (AA)
BB
2n pollen (BB)
4n zygote (AABB)
Allotetraploid
Synthetic alloplyploids can be produced artificially by
hybridization between two diploid species followed by
colchicine treatment or between two autotetraploid
species.
AAAA
x
BBBB
Gametes
AA x BB
AA
BB
F1
sterile
AABB
treatment
Allotetraploid
(fertile)
Important features of allopolyploids
AB
hybrid
Colchicine
AABB
Allotetraploid
 Allopolyploids generally show the characteristics of
both parents. However, one cannot predict whether
all desirable characters of the two species would
come into the hybrid or not, e.g. Raphanobrassica
was produced to combine the root of radish
(Raphanus sativus) and leaves of cabbage (Brassica
oleracea). But the hybrid showed undesirable
characters, like root of cabbage and leaves of
raddish. On the other hand the first man made
cereal, Triticale developed from a cross between rye
(Secale cereal) and wheat (Triticum aestivum),
showed all desirable characters i.e. the hardiness of
rye and yielding ability of wheat.
 In general allopolyploids are more vigorous than
diploids.
 Allopolyploids have better adaptability than diploids.
 Many allopolyploids are apomictic in nature, i.e.
produce seeds without fertilization, e.g. Poa,
Taraxacum, Parthenium, Rubus, Fritillaria, Tulipa etc.
 Allopolyploids show meiotic behaviour like diploids,
unlike auto-polyploids which show multivalent
associations during meiosis.
 Allopolyploidy helps in creation of new species with
characteristics from two or more parents.
Role of allopolyploidy in evolution
Allopolyploids have been found more successful as crop
species than autopolyploids due to their better
adaptability. Many of our present day crop species are
allopolyploids, and have thus contributed to a great
extent in the evolution of plants. It is estimated that
about 1/3rd of the present angiosperms are polyploids,
and that majority of them are allopolyploids. Some of our
major crops, such as wheat, brassica, cotton, tobacco,
etc. are all allopolyploids. Natural allopolyploids are
identified by karyotype analysis and meiotic behaviour. It
has been possible to trace back the evolutionary history
of many allopolyploid crop species and heir diploid
progenitors have been identified with some degree of
certaintity. The identification of diploid parents is
primarily based on pairing between the chromosomes of
diploid progenitor and the allopolyploid species. The
homology between some of the chromosomes suggests
that diploid species may be one of the parental species of
the allopolyploid. Additional evidence on the parental
diploid species is obtained by synthesising the naturally
occuring allopolyploid from parental diploid species. The
resemblance between synthetic and natural allopolyploid
species gives an evidence of possible progenitors of the
allopolyploid crop. Protein and enzyme analysis and
chromosome banding provide the further confirmatory
evidence. The evolutionary history of some important
crop species has been worked out, some of which are
discussed below.
1. Evolution of bread wheat (Triticum aestivum)
The common bread wheat is an important example of
alloplyploidy. It is hexaploid with 2n=42 and has evolved
from three diploid species during the course of evolution.
These are:
I. Triticum monococcum: AA (2n=14)
II. Aegilops speltoides: BB (2n=14)
III. Aegilops squarrosa: DD (2n=14)
The hexaploid wheat is designated as AABBDD. A cross
between Triticum monococcum and Aegilops speltoides
produced an unstable hybrid in which spontaneous
chromosome doubling has occurred resulting in an
allotetraploid Triticum diccocoides with 2n=28 (AABB).
This was followed by hybridization between tetraploid
Triticum diccocoides and Aegilops squarrosa. The
chromosome doubling of the hybrid has finally resulted
into Triticum aestivum with 2n=42(AABBDD). The present
evidence suggests that A, B and D genomes from three
diploid species as shown above are not much different
from one another. For this reason common bread wheat
is now considered as a segmental allopolyploid.
Triticum monococcum X
2n=14(AA)
Aegilops speltoides
2n=14(BB)
F1 – 2n= 7+7 (AB)
Chromosome doubling
Aegilops squarrosa
2n=14 (DD)
X
Triticum diccocoides
2n=28 (AABB)
F1 – 2n= 7+7+7 (ABD)
Chromosome doubling
Triticum aestivum
2n=42 (AABBDD) (Allohexaploid)
Evolution of bread wheat
2. Evolution of Triticale:
Triticale is the first man made cereal. Tritical was
derived from a cross between wheat (Triticum sp.) and
Rye (Secale cereale). Depending upon whether the
tetraploid Triticum durum (2n=28) or hexaploid Triticum
aestivum (2n=42) is used in the cross, Triticale is
hexaploid or octaploid, respectively.
a.
Triticum durum
2n=28 (AABB)
X
Secale cereale
2n=14 (RR)
F1 – 2n=3x=21(ABR)
Chromosome doubling
2n=6x=42 (Hexaploid Triticale)
AABBRR
b.
cereale
Triticum aestivum
X
Secale
2n=42 (AABBDD)
2n=14 (RR)
F1 – 2n=4x=28(ABDR)
Chromosome doubling
2n=8x=56 (Octaploid Triticale)
AABBDDRR
3. Evolution of Raphanobrassica (Raphanus sativus
x Brassica oleracea)
G.D. Karpechenko (1927) made a cross between
Raphanus sativus (2n=18) and Brassica oleracea
(2n=18). The F1 hybrid with 2n=18 was completely
sterile due to lack of paring of chromosomes. However,
among these sterile plants, he found some fertile plants.
On cytological examination, these fertile plants were
found to have 2n=36 chromosomes, indicating that
chromosome doubling has occurred leading to normal
pairing of chromosomes.
(Table:
Chromosome
numbers
of
different
autopolyploids - downloaded from internet)
(Table:
Chromosome
numbers
of
different
allopolyploids - downloaded from internet)