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Acknowledgement
I would like to thank all the people who helped and supported me with writing this
dissertation project.
Firstly, I would like to express my deepest gratitude to my Supervisor, Dr. R.N. Singh
for his unwavering support, collegiality and mentorship throughout this project. Without
his guidance and persistent help this dissertation would have not been possible.
I would also like to thank our college librarian who provided me the reuired books
which helped me a lot to prepare my dissertation project.
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Contents
1. Introduction2. Aneuploids or Heteroploids;
!onosomic
"ullisomic
#risomic
#etrasomic
$. Euploidy
%aploidy&!onoploidy'iploidy
(olyploidy
$. Types of polyploids )
a. *utopolyploids
b. *llopolypliods
5. Appliction of !olyploids.
". Role of polyploidy in plnts
#. $onclusion
% References.
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INTRODUCTION
*n individual carrying chromosome numbers other than true monoploid or diploid
numbers is called heteroploids +Sharp, -$/. %eteroploidy is divided into euploidy and
aneuploidy.
In polyploids, x is the basic +monoploid/ chromosome number, n is the gametic
chromosome number of chromosomes, and 0n is the 1ygotic or somatic chromosome
number for 2x) the genomic formula of #riticum aestivum is 0n30x3. In both cases,
the basic chromosome number x3is seven.
#he basic set of chromosomes in a diploid is called a genome.
2very cell in the body of living organism is provided with a packet of genetic material
+the nuscles/ which contains chromosomes. #he number of chromosomes in the nulei of
a given species is fixed.
#he following chart gives the idea about the chromosome number in different organisms.
species of plants and animals have identical chromosome numbers. In such cases one
should not think that these organism will show similar characters, because it is not the
number of chromosomes which differentiates various species from one another, but
rather them nature of hereditary material in the chromosomes determine the character of
species. In most of plant and animals the chromosome number ranges between 4 and
5o.#he number above and below this range are comparatively rare. #he highest number
of chromosomes reported so far is about 44 found in 6adiaolaria , an unicellular
marine proto7oa. In plants, chromosome number ranges from two pairs +in a member of
compositeae, %aplopappus gracilis/ to several hundreds found in ferns. In ophioglossum
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89: chromosomes have been counted in each nucleus. In chromosomes of pea there
are 8 chromosomes of different shapes, si7es and structures and each kind of
chromosome has its own homologue. So there are 8 pairs of chromosomes in pea. #his
basic number is denoted by the letter ;x< or ;n
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ii/ #he growth rate decrease due to slower rate of cell division. #his leads to a
decrease in auxin supply and decrease in respiration.
iii/ #he time of blooming is delayed and prolonged due to slow growth rate.
iv/ *t higher ploidy level) autoocotaploids or higher the adverse effects are highly
pronounced and lead to the death of the plants.
Induction of !olyploidy-)
In recent years, a number of methods have been worked out to induce polyploidy in plants. Some of them are described below.
1. *y Rdition-)
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(olyploidy can be induced in plants by exposing their certain parts, such as vegetative
buds and flower buds, to ultraviolet rays, @)rays or other rays of shorter wavelengths.
Irradiation increases the rate of cell division and also cause the multiplication of
chromosome number.
2. *y Inury-)
When the merismatic 7ones of a plant are injured the cells at the points of injury grow
rapidly and form a callus. Aallus growth is enhanced by a chemical substance named
coumerine which also brings about somatic doubling of chromosomes. Begetative buds
generally developing from callus tissus are ployploid in nature, from injured part of
tomato plants it is possible to produce tetra ploids plants.
/. *y $he+icl Tret+ent-)
* number of chemicals are now known which induce polyploidy in plants. Important
among them are colchicine, granosan. Ahloroform, chloral hydrate, some narcotics and
alkaloids, *cenaphthene. Indolacetic acid, 2thyl)mercury chloride etc. colchicine is the
best chemical for this purpose. It is a chemical compound which was first discovered by
(ernice in ::-.
Aolchicine was first demonstrated by levan to be a specific and efficient chemical in
creating polyploidy restitution nuclei. Aolchicine is obtained from the extract of roots
and corms of the colchicine outumnale. In india. It is obtained from colchicium luteum
and glorios a superba.
0ethods tested for colchicine tret+ent
. #wigs could be immersed in solutions of various concentration for difference periods
0. Seeds were immersed in colchicine solution for different periods
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$. Seeds were germinated on blotting paper soaked in different concentrations of
colchicine solution.
. Coung flower buds could be immersed in test tubes filled with different
concentrations of colchicine for different periods.
5. Begetative buds could be immersed in D.5 per cent and D.05 per cent aueous
solutions of colchicine for different periods
9. Aolchicine could be applied as a paste in anhydrous lanolin with D.)D.5 per cent
solution.
8. Aolchicine solution could be sprayed on the growing buds.
:. Erowing buds could be wetted with aueous solution.
-. Solution of colchicine in agar may be applied to growing buds on interval.
Adpt,ility of plnts to colchicine tret+ent
#he different crop plants show different range of adaptability to the induction of
polyploidy by colchicine treatment. "ot the least, even different genotypes within a
species too show different response to double the chromosomes. *ccording to loue
+-5$/ and evan +-:/.
. (lants low in chromosome numbers are more likely to respond well in doubling thechromosome than those higher in chromosome number.
0. Aross)fertili7ing plants are more likely to successful for doubling the chromosome
numbers than self)fertili7ering species.
$. Begetatively propagated plants may be more successful as autopolyploid than seed
grown plants.
. Fertility of autotetraploids may be reduced to a great extent.
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Effects of tret+ent
#he changes brought by chromosome doubling are of great significance. >oth
individually as well as doubling of chromosomes bring morphological, physiological
and genetic changes.
Morphological : eaves become large, darker in colour, leathery in texture ? leaf lobes
get overlapped and twisted. Stomata, bracts, (ollen grains, bolls and seeds become
bigger than control. !aturity is delayed.
Immediate effect of seed treatment causes swelling of radicles and retardations of their
growth. Seedling show slow growth with thicker stem, darker and broader leaves.
Si7e of pollen grain and guard cells are bigger than control plants.
#he stems of polyploids are stouter and thicker, leaves are larger and broader, hair on the
vegetative parts are coarse and thicker. #he floral, fruits and seeds are larger than
diploid.
"ormally, polyploids form gigas character because individual cell si7e becomes larger.
Eigas character occurs in both naturally as well as artificially)induced polyploids.
Physiological : >ecker in -$ reported that osmotic concentration in autopolyploids is
proportional top the chromosome number.
*utotetraploid mai7e takes longer time to mature than diploid. In fruits and vegetables,
vitamin content is increased. In tobacco, nicotine content is increased and also nitrogen,calcium potas and magnesium but carbohydrate, sulphur and phosphorus decreases. In
triploid beet, sugar content is increased.
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#he decrease in growth rate slowly leads to the formation of perennial from annual types
in cotton. Sharp -$ reported that autotetraploid 1ea race is perennial, while diploid is
annual.
!olyploidy in !lnts-)
Ahromosome doubling in Somatic and Eerm cells. #wo basic irregular processes
have been discovered by which polyploids may evolve from diploid plants and become
established in nature.
. With somatic doubling cells sometimes undergo irregularities at mitosis and give rise
to meristematic cells.0. 6eproductive cells may have an irregular reductional division in which the sets of
chromosomes fail to separate completely to the poles in anaphase. >oth sets thus
become incorporated in the same GrestitutionH nucleus, which doubles the
chromosome number in the gamete.
$. #wo main kinds of polyploids, autopolyploids and allopolyploids, may be
distinguished on the basis of their source of chromosomes. It occurs freuently in
single cells of many plants, but they not survive, *llopolyploids result when different
genomes come together through hybridi7ation. *mong the surviving plants.
The chnge in the chro+oso+e-)
"umber involve addition or elimination of individual chromosomes or of complete
chromosomal set +genome/.on the basis of two main classes of ploids can be recogni7ed)
. *neuploids or %eteroploids0. 2uploids
"umerical change in chromosomes or variation in the chromosome number
+heteroploidy/ can be mainly of two types. "amely)*neuploidy, 2uploidy.
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*neuploidy means the presence of chromosome number, which is different than a
multiple of the basic chromosome number. 2uploidy, on the otherhand, means that the
organism should possess one or more full sets of chromosomes, let us imagine that 8 is
the basic chromosome number +x/ in a particular class of individuals where the diploid
number +0n/, is . In this case, the chromosomes number 0n35 and 0n3$ would be
aneuploids, while those having 0n 38,0,0:,$5 or 0 would be euploids. * classification
of different kinds of numerical changes in chromosomes is presented in fig)
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Numerical changes in chromsomes
!"#lo$d% Ane"#lo$d%
&ono#lo$d% d$#lo$d% 'ol%#lo$d% (%#o#lo$d% (%#e)#lo$d%
*+, *2+, *3+- 4+- 5+-6+ etc,
monosom% n"ll$som%
*2n.1, *2n.2,
T)$som% tet)/som%
*2n1, *2n2,
Fig: D$e)ent k$nds o n"me)$c/l c(/nges $n c()omosome *+/s$c
c()omosome n"me) 2nsom/t$c c()omosome n"me),
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Aneuploidy or Heteroploidy-)
It is a condition in which nuclei contain chromosomes whose number is not true
multiple of basic chromosome number or genome. #he organism in which such
abnormal conditions exist are called *neuploids.
Aneuploidy-)
#he aneuploidy may be arranged in several groups, some of which are as follows?
/ 0onoso+ic &2n)1'-)
*n organism lacking one chromosome of a diploid complement is called monosomic
+0n)/. Suppose in an organism diploid set is formed of pairs of chromosomes. In this
by chance one chromosome is missing. So in this case one may fine $ normal pairs of
chromosomes and one singlet chromosome. #his should, there for, be called as
monosomic.
#he monosomics are very rare. %owever, viable monosomics have been studied in
'rosophila, tobacco, 'atura, !ai7e, tomatoes and many other organisms.
!onosomics show abnormal type of meiosis. #hey have chromosome constitution 0n),
so they produce 0 kinds of gametes some with n and others with n) chromosome. #he
odd chromosome, which has no homologue to pair tends to pass at random to one pole in
meiosis. Freuently, however, it acts as a laggard at anaphase and is not induced in any
of the daughter nuclei. For this reason the gametes having ;n< chromosomes occur less
freuently than the gametes of +n)/ type. In plants, the nuclei with a missing
chromosome seldom survive. #he n) gametophytic generation fails, presumably
because the complete absence of certain genes that occur on the missing chromosome
means the failure of certain biochemical reactions which are accomplished in presence
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of en7yme formed by these genes. #hus monosomic analysis makes it clear what genes
are found on the particular chromosome or linkage group.
In some plants, e.g. 'atura, monosomics are not viable. In "icotiana tobaccum with
basic chromosome number 0, clausen and his co)workers have found all 0 possible
monosomics as expected and have noted that they were very much different from one
another. In wheat +triticum vulgare/ haploid number ;n< is 0. So one may expect that
0 monosomics of different types should be in bread wheat, but these do not differ
much from the normal diploids.
Nulliso+ic &2n)2'-)
In these both the homologues of a particular chromosome set are somehow lost so
that a chromosomal pair is completely missing from the normal 0n set. #he phenotypic
expression of such a plant, when compared with that of normal plant, will indicate the
number of genes contained in the missing chromosomes and the respective character
they governed. "ullisomics are generally nonviable like the monosomics. In mai7e and
wheat, several nullisomics have been reported. "ullisomics are generally obtained by
selfing monosomcis. When the gametes containing n) chromosomes fuse they form
nullisomics. "ullisomics are those individual, which lack a single pair of homologus
chromosomes, so that the chromosome formula would be 0n)0, and not 0n)), which
would mean a double monosomic. Sears had isolated all the 0 nullisomics in wheat.
Triso+ic &2n1'-)
*n organism containing two complete genomes plus one extra chromosome is termed
trisomic +0nJ/. In the normal process of meiosis, sometimes chromosomal pairs
separate in such a way that one member of each pair goes to one daughter nucleus and
other member goes to other daughter nucleus. >ut very rarely one pair of chromosomes
fails to divide and finally it moves as such to one spindle pole. So half of daughter cells
receive an extra chromosome and half of them lose one. In this way gametes with +n)/
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and +nJ/ chromosomes will develop. If by chance the gamete with +nJ/ chromosomes
fuses with normal gamete carrying ;n< chromosomes, the resulting 7ygote will have
nJJn or 0nJ chromosomes and it will produce trisomic organism. Eametes with +n)/
chromosomes on fusion with gamete carrying ;n< chromosomes will give rise to
monosomic +0n)/ 7ygote. !any of the mutations in oenothera noted by 'evries are
trisomics. #risomis are widespread in nature and have been extensively studied in
'atura, mai7e, tomatoes, wheat, tobacco and 'rosophilla.
>lakeslee and >elling +-0/ announced a mutant type in 'atura having 05
chromosomes rather than normal 0 chromsomes +0n30/. In that, at the meiotic
metaphase one of the 0 pairs was found to have an extra member, i.g. there were normal pairs and one trisomic. #he trisomic individulas exhibit several specific features
which are uite different from those seen in the wild types.
#he additional traits were associated with the extra chromosome which gave to the
plants extra dose of all the genes that are already contained in the normal duplicated
chromosomes.
It is thus possible to identify some genes while are associated to extra chromosomes.
#his supports the statement that genes are contained in particular chromosomes. Since in
'atura there are 0 chromosome pair with different genic composition and since the
triplication of every chromosome pair is possible,0distinguishable trisomics can be
expected in it. >lakeslee and his associates have succeeded in producing all 0 different
trisomics as expected. #hese trisomics were grown in >lakeslee
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will express themselves less freuently than in normal diploid plants because
comparatively more wild type alleles are present to check their expression. It has been
observed by geneticists in 'atura that extra chromosome enters the.
!roduction of Triso+ics-)
#risomics may originate spontaneously due to production of nJ type of gametes
rarely as a result of non disjunction of a bivalent .
owee)
Fig: ')od"ct$on o t)$som$cs d"e to o)m/t$on o n1 t%#e o g/metes
$n d$#lo$d *2+, /nd t)$#lo$d *3+, $nd$$d"/ls mo)e /ten t)$som$cs /)e
#)od"ced /)t$c$/ll% e$t(e) % selng t(e t)$#ol$ds *#)od"ced %
c)oss$ng d$#lo$d /nd /"totet)/#lo$ds, o) % c)oss$ng t(ese t)$#lo$ds /s
em/les w$t( d$#lo$ds /s m/le *3+× 2+,
$ytology of triso+ics-)
15
0x $x
"ondis)
junction
n) nJ n nJ
0nJ 0nJ
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#he trisomics have an extra chromosome which is homologous to one chromosome
of the complement. #herefore, it forms a trivalent this trivalent may take a variety of
shapes in primary and secondary trisomics as shown in fig. in a tertiary trisomic a
characteristic pentavalent is observed.
Type of triso+ic So+tic chro+oso+es 0etphse I configurtion
&'(rimary trisomics 1 1 1
2 2 2
1 1 1 1 1 1 1 11 1
2 2 2
2 2 2 2 2 2 2 2
1 1
&i' &ii' &iii' &i'
&,'Secondary trisomics 1 1 1
2 2 2
1 1 1
2 2 2
1 1 1 1 1 1 2 2 1 1 2
2 2 2 2 2 2 2 2
2 2 2 2
&i' &ii' &iii'
&c' #ertiary trisomics 1 1 1 /
/
2 2 2 3
3
2 1 1 3 3
2 1 3
& pentlent'
4ig- 'ifferent types of trisomics and their meiotic configurations at metaphase I .
Triso+ic nlysis-)
#risomics are also used for locating genes on specific chromosomes. If a particular
gene is located on the chromosome involved in trisomy.
Dou,le Triso+ics &2n11'-)
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Sometimes an organism receives two complete genomes plus two different extra
chromosomes. #his is called double trisomic. #he extra chromosomes are homologues of
two different chromosomal pairs in normal diploid set. #hus two different chromosome
pairs in the normal diploid set will have one additional homologue with them and thus
they form two trisomics. #he phenotypic expression of genes on extra chromosomes in
trisomics will be deeper than in normal diploids.
Tetrso+ic &2n2'-
*neuploid nuclei which have normal diploid genome and one extra pair of
chromosomes are called tetrasomic +0nJ0/. In this one pair of chromosomes in the
diploid set is further reduplicated due to additional chromosome pair. (henotypic
expression of the genes in tetrasome may be deeper than the affects found for the genes
in corresponding trisomics.
In meiosis the four homolouges of tetrasomic set after tend to form a uadrivalent. If
disjunction by twos is regular, a fair genetic system will operate. Eenerally uadrivalents
are not formed and disjunction +Separation/ is not always regular. "evertheless,
tetrasomics are more regular than trisomics in meiosis.
Aneuploid segregtion in plnts-)
#he first critical study of *neuploid plants was initiated in -0 by *.F. >lakeslee
and K. >elling when they discovered a Gmutant typeH with 05 chromosomes in the
common jimsonweed, 'atura stramonium, which normally has 0 chromosomes in the
somatic cells. *t the meiotic metaphase, one of the 0 pairs was found to have an extra
member, that is, one trisome +0nJ/ was present along with disomes. #his trisomic
plant different from wild type plants in several specific ways, particularly in shape and
spine characteristics of seed capsules. >ecause the complement was composed of 0
chromosome pairs differing in the genes they carried, these were grown in >lakeslee
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garden, each was found to have a distinguishable phenotype that was attributed to the
extra set of genes contained in one of the 0 chromosomes.
It is also possible to identify some traits that were determined by genes on particular
chromosomes by trisomic ratios that is, 5?, 8? and $5?, in contrast to $? and ?
expected from monohybrid crosses in regular 0n plants. #he extra chromosome in
poinsettia, for e.g. was found to carry the locus for alleles (J and (, for purple or white
flowers, respectively. *ny one of these chromosome arrangements with the dominant
gene (J +(J (J (J , (J (J( or (J ((/ produced poinsettia plants with purple flowers,
whereas only one, the fully recessive.
ses of Aneuploids
/ #hey have been used to determine the phenotypic effects of loss or gain of different
chromosomes.
0/ #hey are used to produce chromosomes substitution lines. Such lines yield
information on the effects of different chromosomes of a variety in the same genetic
background.
$/ #hey are used to produce alien addition and alien substation lines. #hese are useful in
gene transfers from one species into another.
/ *neuploid analysis permits the location of a gene as well as of the linkage group onto
a specific chromosome. #his is one of the most important applications of aneuploidy.
5/ Studies on nullisomic)tetrasomic combinations made it possible to establish
homoeology among the chromosomes of *, > and ' genomes of wheat.
9/ !ultivalent formation in nullisomic 5> of wheat led to the discovery of the (h gene.
#his gene suppresses pairing between homoeologous chromosomes.
8/ *neuploids are useful in identifying the chromosomes involved in translocations.
:/ *neuploids are also useful in preparation of molecular maps.
-/ #hey may be used for obtaining chromosome L specific probes. * probe is a '"*
seuence that is used in nucleic acid hybridi7ation for detecting the presence of the
same '"* seuence in test '"* samples.
Euploidy-)
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+EM. 2u3true and ploid3unit/. #his is a condition in which the change in the
chromosome number involves additions or elimination of complete genome.
#he basic chromosome number of euploids are represented by the haploid +n/. the
euploids above the haploid or monoploid level are called polyploids. #hey may be
diploid +0n/, triploid +$n/, tetraploid +n/, pentaploid +5n/, hexaploid +9n/ and so on. In
all these cases of euploids we see that the chromosome number are exact multiples of
their original haploid number +n/.
Whereas aneuploids differ from standard 0n chromosome complements in single
chromosomes, euploids differ in multiples of n or x if n3x.
!onoploids +n/ carry one genome. #he n or x chromosome number is usual for gametes
of diploid animals,>ut unusual for somatic cell.!onoploidy is seldom observed in
animals, but is found in the male honeybee and other insects in which male haploids
occur.
>y contrast, plants have gametophyte stage in their cycle that is characteri7ed by the
reduced +n/ chromosome number. In higher plants , this stage is brief and in
conspicuous, but in some lower plant groups it is the major part of the some lower plant
groups it is the major part of the cycle. 4ccasionally plants in natural populations or
plots can be recogni7ed as monoploids by abservation and verified by cytological
producer. #hese plants are usually frail in structure with small leaves, low viability, and a
high degree of sterility. Sterility is attributed to irregularities at meiosis. 4bviously, no
pairing is possible because only one set of chromosomes is present. #herefore, if the
meiotic process succeeds at all, the dispersal of chromosomes to the poles is irregular,and the resulting gametes are highly in viable, because monoploids undergo no
segregation and carry a single set of gene.
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It is seerl types. So+e of the+ re s follo6s-)
. %apolyploidy or !onoploidy +a condition in which nucleus of an organism has only
single genome ;n
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the cell. #hey may constitute restitution nucleus including all chromosomes and may
thus give rise to gametes having a complete haploid set of chromosomes. Sometime the
distribution of chromosomes to the two poles is irregular and the gametes so formed are
highly non)viable +i.e. they die soon/.
"ormal gamete is possible if by chance all the chromosomes pass to one pole during the
irregular meiosis. !onoploids do not undergo segregation and they carry a single set of
genes. So gemetically they are pure.
se of Hploids-)
When the chromosome number of a haploid is doubled by colchicine treatment. #he
derived diploid will be completely homo7ygous for all genes so that the most important
use of haploids is in the production of homo7ygous diploids. Seven varieties in rice,
three varieties in tobacco and two varieties in wheat have also been released using
haploids in china consult Gcytogenetic of crop plantsH.
#hus they can be used experimentally to a good advantage
Diploids-)
!ost common type of euploid is the diploid which has two complete genomes. *s
mentioned earlier this chromosome arrangement is nearly universal among animals and
common in plants. #hese organisms are normal in their behavior, so they are not
discussed here in detail.
!olyploids-)
(olyploidy is commonly met within plant world. In animals, it is indeed rare. *bout
one half of total species of flowering plants are polyploids +stabbins, -5D/. *mong
certain families the proportion of polyploids to diploids is higher than 5D percent. *bout
two)third species of all grasses are polyploids. #he cases of polyploidy may be found in
many other common plants, e.g. Ahryasanthemum, Solanum, >rassica, Wheat,
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"icotiana. In the genes chrysanthemum, the basic number of chromosomes is -. In this
genes species are known with $9,5,9$,80 and -D chromosomes. In wheat the basic
chromosome number is 8. In this , several varieties are known with , 0: and 0
chromosomes which are diploid, tetraploid and hexaploid respectively. In the nicotiana
+0/, varieties with 0, :, 80 chromosomes are known.
Similarly the genus Solanam, in which basic chromosomes number is 0, consists of
several polyploids varieties with 0,$9,:,9D,80,-9,D:,0D and chromosomes. In
6ose, the basic chromosome number, is 5. In this, diploid, triploid, tetraploid, hexaploid
and octaploids have D,5,0D,$D and D chromosomes respectively.
Types of !olyploids-)
#wo features could account for the origin of polyploids from diploids.
/ 'oubling of the chromosome number in somatic tissue capable of giving rise to
gametes.
0/ Failure of reduction in the chromosome number during the formation of gametes. 4n
the basis of their origin, the polyploids are of the following two types)
i/ Autopolyploids-)
#hese individuals contain more than two identical genomic sets which are derived by
self)duplication of parental genomic sets.
In more scientific language, in these organisms, multiple genomes are identical or
very nearly so. Eenome reduplication within a normal species give rise to
autopolyploids. It arises due to failure of anaphase during meiosis.
>ecause of these multiple sets, segregation problem may occur, such that the gametes
may not receive full sets of chromosomes and, as a conseuence, the organism will
suffer reduced fertility.
For example, autotriploids +0n3$x have three complete chromosome sets. *t any one
point only two chromosomes can pair effectively +although small sections of triple L
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paired synaptonemal complex have been seen in some triploids and trisomics. #his
means that combinations can occur of three univalents, one univalent and a bivalent
or a trivalent. =nivalents, will generally be lost from the cell via micronucleation.
>ivalents will segregate normally. #he orientation of the trivalent on the metaphase
plate determines how many chromosomes go to each pole.
#o ensure full fertility, all chromosomes must form trivalent, all trivalent must
orientate in a convergent manner and all convergent trivalents must segregate two
chromosomes to one pole and one to the other. #his would produce two diploid
gametes and two haploid gametes. "eedless to say, this rarely happens and triploids
form all types of configuration at metaphase I. this produces unbalanced gametes thatare generally non L viable, and conseuently fertility is reduced.
*s a general rule, the higher the basis number, the lower the fertility. 4ther
polyploids with odd numbers of chromosomes sets L 5x, 8x, etc. L experience similar
problems to triploids because the odd set, which will either form univalents, trivalents
or complex multivalent. For example in a pentaploid when 0n35x the chromosomes
can pair in any combinations of between one and five chromosomes.
When a triploid has a total chromosome number of -, fertility is reduced to 05N of
the diploid level as a result of the probability of all chromosomes forming trivalents
and all trivalents orientating the same way. When 0n3$x30, fertility is further
reduced to 0.5N compared with diploid. %igher basic numbers reduce fertility
further and are almost completely sterile. In triploid apples such as >lenheim 4range
and >ramley
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#etraploids have four complete sets of chromosomes. 2ach set of four homologues
has the following pairing options? four univalents two bivalents a bivalent and two
univalents a univalents and a trivalent or a uadrivalent. #he freuency with which
these are produced depends on the pairing arrangements. #he same basic rule as
described for triploids also applies here? segregation of complete sets of
chromosomes produces viable gametes other combinations either produce non viable
gametes, or result in viable or deleteriously affected offspring.
!any tetraploids have mostly bivalent and uadrivalent pairing as this allows for
maximal pairing in the synaptonemal complex, producing balanced segregation at
anaphase I.
$ytology of utopolyploids-)
In an autopolyploids, there will be more than two sets of homologous chromosomes.
#his leads to the formation of multivalent instead of bivalents as found in the diploids. In
an autotriploids there three sets of homologous chromosomes.If these three sets are
normally paired, trivalent as described in primary trisomics will be observed. #he
trivalents can not disjoin normally and will either disjoin 0? chromosomes to two pales
or will disjoin ? leaving one chromosomes as a laggard. #he number of chromosomes
in the gametes of a triploid organism therefore, will vary from n to 0n. !ost of these
gametes are unbalanced leading to high degree of sterility.
In autotetraploids, since there are four sets of chromosomes uadrivaents are formed
which disjoin in a normal 0?0 manner giving diploid gametes. 6arely, however, a
uadrivalent may disjoin $? or may leave one or more chromosomes, as laggards atanaphase I. #herefore, autotetraploids also have certain degree of sterility, although it
will not be as high as in autotriploids.
7enetics of utopolyploids &triso+ic nd tetrs+oic inheritnce'-)
#he segregation pattern in polyploids is uite different than what we find in diploids.
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In a polyploidy there are more than one kind of hetero7ygotes, because there are more
one kind of hetero7ygotes, because there are more than two homologues. #he different
possible genotypes and expected the ratios are based on the assumption that the gene is
close to centromere leading to what we cell chromosome segregation. If gene is away
from centromere, crossing occur between gene and the centromere will take place and
the ratios will be modified. #his will then be called chromatid segregation, because
segregation is taking place not at the chromosome level but at the chromatid level due to
crossing over.
Autotriploids-)
Such plants have three genomes per nucleus and they are commonly designated as $n
plants. *utotriplids occur in a number of plants, e.g. 4enothera, 'atura, rose, rice and
many others. #hey are usually formed as a result of fertili7ation between a diploid +0n/
and a haploid gamete +n/, +0n gameteJ n gamete3$n 7ygote/.
'iploid gametes are produced by normal tetraploids in meiosis or in sectors of otherwise
diploid organisms where automatic doubling of somatic chromosome number has taken
place. Somatic doubling may be spontaneous or may be induced.
$hrcters-)
i/*utotriploids are more vigorous than normal diploids, they are more leafy and show
tendency towards perenniality.
ii/ Sometimes floral abnormalities may be observed.
iii/ (ollengrains, stomatal guard cells and wood cells of xylem in triploids are larger
than those of diploids.
iv/ (lants are highly sterile and seeds are formed rarely. In nature, seed propogated
triploid plants are ordinarilly by vegetative means this is advantageous in
horticulture, specially for ornamental plants, e.g. chrysanthenum roses. 'ahlias etc.
and also in the production of seedless fruits, some of the commercially important
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fruit plants, e.g. apples, pears, bananas, grapes, orange, guavas, pineapples are
triploids.
$ytologicl ,ehior of Autotriploids -)
In autotriploid it is irregular. #he centromeres of three homologous chromosomes
have no way to orient themselves so as to give euivalents at the two poles. In this
condition, the only possibility is that the components of trivalent separate in such a way
that two chromosomes go to one side and third goes to other. If it is so then the meiotic
products may contain either ;n< or ;0n< chromosomes or any number in between. #his is
true irrespective of whether three homologous chromosomes align either as trivalent or
as one bivalent and one univalent. #herefore, the gametes arising from triploids, have
unbalanced genomes. In fact, probability of formation of haploid and diploid gametes
are vary rare.
Autotetrploids)3n-)
*utotetraploids have four similar genomes per nucleus. #hey may originate in one of
the following ways.
i/>y fusion of two diploid gametes.
ii/ #hey may result from the duplication of somatic chromosomes +0n/ and following
failure of mitotic division. #he resulting nucleus contains copies of each
chromosomes, instead of the usual two. It this tetraploid perpetuates itself through
normal mitosis, the increased chromosome number may become established in a
group of cells or tissues within the body of organism. (lants capable of vegetative
propagation may be manipulated to produce pure tetraploids.
$hrcters-)
i/*utotetraploids usually show marked phenotypic variations, sometimes marked disease
resistance, they are commercially more valuable than corresponding diploids.
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ii/ Some are larger and more deeply coloured than diploids and sometimes they bear
large seeds and fruits.
iii/ #hey have larger cells and epidermal cells are larger and are arranged in different
patterns in tetraploids than those in diploids.
iv/ #he stomatal guard cells and epidermal cells are larger and are arranged in different
patterns in tetraploids than those in diploids.
v/ Flowers are larger and more showy in tetraploids than those in diploids.
vi/ %igh content of vitamin A +ascorbic acid/ have been reported in tetraploids.
vii/ #he tetraploids show tendency towards perenniality and may show low fertility.
$ytologicl ,ehior in Autotetrploids-)
In autotetraploids, each chromosome is present four times. #hus, chromosome
associations such as uadrivalents, trivalents, bivalents and univalients are expected
based on random association of four homologus chromosomes. Ahromosomes pairing is
usually studied at diakinesis or metaphase I of meiosis, because chromosomes at these
stages are in condensed forms, chromosome migration at anaphase)I in autotetraploids is
determined by the co)orientation of their kinetochores at metaphase I.
It may be normal or abnormal. When four homologues align in tetravalents, normal
diploid gametes are formed. Sometimes, two bivalent or one trivalent and one univalent
may also occur. In these cases, products of meiosis are irregularly formed. Incomplete
genomes in gametes may result in total sterility.
Allopolyploids-)
*lloployploids are obtained when crosses are made between two member of distinct
taxonomic groups. #he hybrid so obtained are highly sterile because the genomes
coming from two different individuals are so different that their chromosomes do not
synapse during 7ygote stage of meiosis I. #hese hybrid allopolyploids owing to complete
sterility may face extinction unless they are able to reproduce vegetatively. *s far as
meiosis is concerned, the situation in allopolyploids is exactly like that in haploids.
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6egular meiosis and formation of gametes containing one complete genome from each
of the two parents are possible only if the number of chromosomes in somatic cells is
doubled.
When such gametes fuse they give rise to a 7ygote containing the complete diploid
complement of original parents. 'ouble diploids of this sort are called amphidiploids.
#hey are fertile, since each chromosome has now got its pairing partner. #hese
amphidiploids behave normally like a diploid with relatively large number of
chromosomes. #he origin of amphidiploids is explained in the figure.
* very important case of allopolyploid is 6aphanobrassica which was experimentally
evolved by the 6ussian geneticist E.'. Marpechenko who made intergeneric cross
between radish +6aphanus sativus/ and cabbage +>rassica oleracea/. *lthough these
plants were distantly related they were enough alike to be crossed successfully with each
other. >oth had - pairs of chromosomes. #he diploids hybrid 6aphanobrassica had :
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chromosomes. - chromosomes from radish and - chromosomes from >rassica parent.
>ut it could not perpetuate itself largely because of failure of pairing between unlike
chromosomes during meiosis. When chromosomes of sterile 6aphanobrassica were
doubled by artificial means a fertile polyploid 6aphanabrassica was produced with $9
chromosomes : of radish and : of cabbage. #hus the homologous chromosomes
could pair themselves and the regular meiosis resulted normal gametes, each with :
chromosomes. #his pairing of chromosome is called autosynapsis, as actually
chromosomes of two genomes are pairing with their respective replicas. #his experiment
had theoretical significance since it provided a method by which fertile inter specific
hybrids could be produced. #his also suggests the possibility of incorporating the
desirable genotypes of two different species into a new polyploid species.
6adish +6aphanus Sativus/ × Aabbage +>rassica 4leracea/
+0n3:/ ↓ +0n3:/
F hybrid sterile
+n 3 J n 3 /
#he allopolyploidy thus represents the method by which new species originate in nature.
Fruits of amphidiploids are characteristically someuehat larger and plants as whole
possess a good deal of the robustness.
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2volution of new species due to allopolyploidy is seen among the cultivated verities of
wheat. Aytogenetically analysis of cultivated wheat plants has yielded important
information in this regard. Wheat verities fall under three groups? diploid, allotetraploid
and allohexapeoid. #hese are given in the following table.
#he genome A present in bread wheat is supposed to have been derived form a grass
aegilops surarrosa +0n3/ which grows in the region extending from *rmenia to
*fghanistan. #he different varities of wheat with higher chromosome number have
apparently arisen from this type and other related grasses through hybridli7ation
followed by chromosome doubling.
#he individuals in which the genomes making a multiple set are not alike are called
allopolyploids. #he different genomes are derived from two or more distinct species by
hybridi7ation. #hus, it involves addition of new genomic sets from outside.
!any even) numbered polyploids may be allopolyploids with little similarity between
the different contributing chromosome sets. #hus, in order to maintain fertility, bivalents
are only formed between homologous chromosomes, for example the hybrid formed
between >rassica oleracea +0n30x3: and 6aphanus sativus +0n30x3:. #he F is
sterile owing to lack of pairing between the two chromosomes sets, but if these are
doubled the allotetraploid +0n3x3$9 is fertile with the formation of : bivalents.
>ivalents may also be formed in even) numbered polyploids owing to the effects of
various pairing control mechanism +such as (b in wheat which suppress homoeologous
pairing.
#he addition of > chromosomes to polyploid cell can also increase the rate of bivalent
formation of approximately 5N when compared with the same variety without >s.
Some specific 2xamples of *utopolyploids?
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A' Eolution of 6het-)
Aommon cultivated wheat is another example of allopolyploidy, although its
allopolyploid nature has now been uestioned. #here are three different chromosome
numbers in the genus #riticum, namely 0n 3 , 0n 3 0: and 0n 3 0. #he common
wheat is hexaploid with 0n 3 0and is derived from three diploid species? ** 3 #riticum
aegilopoides +0n 3 /, +ii/ >> 3*egilops speltoides, +0n 3 / and +iii/ '' 3 *egilops
suarrosa +0n 3 /.#he hexaploid wheat, therefore, is designated as **>>'', the
tetraploid +0n 3 0:/ as **>>. and diploid +0n 3 / as **. #here is, however, evidence
available now which suggests that *, > O' genomes from three diploid species.
>/ Synthesi8ed llopolyploids-)
Aertain allopolyploids were artificially produced in order to find out the origin of
naturally occurring allopolyploids. Aommon hexploid wheat and tetraploid cotton
furnish two such examples.
i/#ritium spelta, a hexaploid was artificially synthesi7ed in -9 by 2.S. !cFadden and
2.6. Sears and also by %. Mihara. #hey crossed an emmer wheat +tetraploid 0n30:/
with *egilops suarrosa +diploid 0n3/ and double the chromosome number in
the F hybrid. #he hexaploid or amphidiploid synthesi7ed in this manner was found
to be similar to the primitive wheat #. spelta when this synthesi7ed hexaploid was
crossed with naturally occurring #. spelta, the F hybrid was completely fertile and
showed normal pairing of chromosomes into bivalents. #his suggested that
hexaploid wheat must have originated in the past due to natural hybridi7ation
between tetraploid wheat and goat grass +*egilops suarrosa/ followed by
subseuent chromosome doubling.
T)$t$c"m d$cocco$des × Aeg$lo#s s:"/))os/
*tet)/#lo$d w(e/t, *go/tg)/ss,
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AA;; DD
2n28< 14 $/lents 2n14< 7 $/lents
A;D
2n21< 21 "n$/lents *t)$#lo$d (%)$d,colc($c$ne
AA;;DD
2n42< 21 $/lents
s%nt(es$=ed (e+/#lo$d w(e/t
*T)$t$c"m s#elt/,
Fig: A)t$c$/l s%nt(es$s o (e+/#lo$d w(e/t
ii/ Eossypium hirsutum, popularly known as upland cotton, is another interesting example
of amphidiploidy. 4ld world cotton has $ pairs of larg chromosomes +genome *h/,
while *merican or Gupland cottonH has $ pairs of smaller chromosomes +genome
'h/. #he new world cotton, the cultivated long)staple type has 09 pairs of
chromosomes +*h *h 'h 'h/, $ large and $ small, K.4. >easley crossed the
*merican and old world cottons and double the chromosome number in the F
hybrids. #he amphidiploid thus produced resembled the cultivated new world cotton
and when crossed with it gave fertile F hybrids. #hus, showed that "ew world
cotton, cultivated gossypium hirsutum +tetraploid/ originated from two diploid
species namely E. herbaceum +0n309/ and E. raimondii +0n309/.
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>oss%#$"m (e)/ce"m × >oss%#$"m )/$mond$$Old wo)ld Ame)$c/n *"#l/nd,
Cotton cotton
2n26< 13 $/lents 2n26< 13 $/lents*?m/ll c()omosomes, *l/)ge c()omosomes,
@1 (%)$d2n26< 26 "n$/lents*13 ?m/ll 13 l/)ge,
Colc($c$ne
2n52< 26 $/lents*13 sm/ll 13 l/)ge,
New wo)ld cotton
*>oss%#$"m ($)s"t"m,
Fig: A)t$c$/l s%nt(es$s o New wo)ld cotton
iii/Spartina townsendii, commonly called as #ownsend
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@1 (%)$d
2n63< 63 "n$/lents
c()omosome
do"l$ng
2n126< 63 $/lents
*? townsend$$,
Fig: ')o/le o)$g$n o ?#/)t$n/ townsend$$
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A/ Triticle9 ne6 +n +de cerel-)
In recent years, considerable emphasis has been laid on the possibility of utili7ing a
new man made cereal known as triticale +@ #riticose cale wittmack/. on a commercial
scale. It is already grown on about one million hectares +05 lakh acres/ of landcommercially and research work is in progress all over the world to improve this man
made crop.
#riticale is the first man made crop, in so far as it resulted as an artificial allopolyploid
derived by crossing wheat +tritium/ and rye +secale/. 'epending upon whether, tetraploid
+0n3x30:/ or hexaploid tritium +0n39x30/ is utili7ed for the synthesis, one would get
hexaploid triticale +0n39x30/ or octoploid triticale +0n3:x359/ respectively.
In each case, only diploid rye +0n30x3/ was used. #he derivations of triticales are
shown in fig.
In a similar manner, tetraploid triticales +0n30:/ at some places were obtained, by using
diploid wheats. 'ecaploid triticale, using hexaploid wheat +0n39x30/ and tetraploid rye
+0n3x30:/ were also obtained by *.!unt7ing in Sweden. %owever, at present only the
hexaploid triticales +0n30/ are considered to hare the potentiality of becoming a new
crop. #hese hexaploid triticale are being crossed to wheat for making improvement.
*s a result of these crosses a large no. of GSecondary triticalesH have now been derived
from the original G(rimary triticalesH some of these secondary triticales have been,
released as commercial cultivars. Some of the most significant work on triticales was
done by *. !unt7ing of Sweden, who passed away on 8 th Kanuary -:. For a more
detailed account on triticales, readers are advised to consult either the book G#riticale)
6esults and (roblemsH written by * !unt7ing in -8-.
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*/, T)$t$c"m d")"m × ?ec/le ce)e/l2n28 2n14
@1 (%)$d *?te)$le,2n21
C()omosome do"l$ng2n42
e+/#lo$d t)$t$c/le
*, T)$t$c"m /est$m × ?ec/le ce)e/l2n42 2n14
@1 (%)$d *ste)$le,
2n28C()omosome do"l$ng
2n56
Octo#lo$d t)$tc/le
Fig: A)t$c$/l s%nt(es$s o */, (e+/#lo$d t)$t$c/le /nd *, oct/#lo$d
t)$t$c/le
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:rieties Diploid chro+oso+es
nu+,er
7eno+e constitution
1. Small grained
2inkorn types or
wheat of 2urope
and *sia.
0n3 AA
2. 2mmer wheat of
"orthern 2urope
which has thick
kernels and is used
mainly as stock
feed.
0n30: AA **
*llotetraploid
/. >read wheat or
vulgar wheat
0n30 ** >> AA
*llohexaploid
>y colchicine treatment in order to get diploid gametes from these two plant species. >y
crossing these two colchicine treated plants they found a fertile hybrid with 0
chromosomes. #he hybrid resembled bread wheat and was given name #.spelta. #hese
researches showed how a moderately useful wheat and useless grass were combined in
nature to produce most valuable variety of wheat.
Appliction of !olyploidy-)
*mong the cultivated varities of wheat, three different chromosome number are
represented? ,0: O 0 +x38/ for example?
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#he primitive small grained einkorn type of 2urope and *sia. #riticum
monococcum, has chromosomes in its vegetative cells. Its yield is low and it is of
comparatively little value.
#he bread wheats, #. aestivum, with 0 chromosomes, were postulated by
K.(ercival in 2ngland to have come from a cross between emmer wheat and goat grass,
both of which are native to the >abylonian region where bread wheat originated.
Wheat techniues for artificial chromosome doubling became established,
investigations of the origin of bread wheat confirmed (ercival
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2vidently, hybridi7ation and chromosome duplication occurred somewhere in the
ancestry of the new world cotton.
!ri+rose Hy,ridi8tion-)
#he primo some primula kewensis, is an allotetraploid with $9 +0n/ chromosomes. It
was derived from a cross between two diploids. (. floribunda +x3-/ and (. verticillata
+x3-/. (lants from these two species crossed readily. (roducing hybrids with :
chromosomes in their vegetative cells, but the hybrids were sterile, the fertile
allotetraploid are shown.
Desese Resistnce in T,cco-)
Induced polyploidy has been exploited to a great extent. (ractical applications may
become more common as additional data are accumulated.
>y artificially induced polyploidy, disease resistance.
!olyploid 4ruits9 4lo6ers nd 6het-)
Some varities of plants that serve human needs more effectively than other have now
been identified as polyploids.
!any polyploids were selected and cultivated because of their large si7e, vigor,
and ornamental values, before their chromosome numbers were know. Eiant GsportsH
from twigs of !clntosb apple trees that were found to be tetraploid +n/ were propagated
into whole trees, which produce extra large fruit. >artlitt pears, several varieties of
grapes, and cranberries have also produced sports with giant fruits. Some of these show
promise of practical usefulness, with colchicine treatment, a number of polyploids have
been developed artificially. #his techniue has provided a way to explore the mechanism
involved in polyploidy formation, and to make use of the good ualities of opolyploids.
#etraploid +n/ mai7e is more vigorous than the ordinary diploid and produces some 0D
percent more vitamin *. its fertility is somewhat reduced. (olyploid watermelons have
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been developed from colchicine treatment by kihara and others. #he tetraploid with
chromosomes is large and has practical value.
* constant threat to the wheat crop is rust a fungus that attacks the stems and leves
of the growing plants and destroys the ripening grain. Spores are borne by wind and,
when conditions are right, the disease can be combated by developing rust)resistance
strains.
In plnt ,reeding-)
Barious cereal crops and vegetables with several desirable characters have been
evolved by inducing polyploidy and crossing the polyploids. #he polyploidy plants are
better in uality, taste, yield, nutritional value and sugar contents. !any polyploids are
disease resistance. #hus the knowledge of polyploidy is of great help in plant breeding.
!any species of wheat are polyploids. #here are three categories in wheat, namely
2inkorm group +0n3/. Bulgare wheat +0n30/. #he third verities is polyploidy and is
better than any of the other two groups, in Solanum and "icotiana many good varities
could be evolved artificially by the knowledge of polyploidly. %ybrids which are proved
useless in the beginning, the application of polyploidy inducing agents made it possible
to double the chromosome number in the sterile hybrids.
#he genome analysis shows that first species is amphidiploid hybrid of the last two
species. "icotiana tabaccum is susceptible to downy mildew disease but ". glauca, an
*ustralian species, is disease resistance polyploidy which contains more nicotine content
and shows more vigour.
*mong ornamental varieties, certain triploids are of outstanding attraction in comparison
to diploids because of their large flowers, longer flowering period. Some of the best
varieties of apples are triploids.
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Role of !olyploidy in !lnt-)
#he polyploidy has played an important role in evolution of new varieties in nature.
*ngiosperms and pteridophytes have very high number of polyploid species in nature.
!ore than $5N angiosperms are polyploids. *ccording to !anton +-5D/ among all the
plant groups, ferns show highest degree of polyploidy. It is generally noted that with the
increase in chromosome number the adaptability and variabilities of species increase
progressively. !any mutation in diploids are deleterious or fatal, but in polyploids,
where a gene is represented more than twice the chances of appearance of deleterious
effects due to mutation are lesser.
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$(N$
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polyploidy causes nuclear enlargement and increases the complexity of the processes
that are involved in managing and partitioning chromosomes during cell division.
(erhaps the most striking evidence of change comes from the discovery of epigenetic
remodelling, which leads to both the activation and suppression of gene expression.
*lthough some of these changes are potentially advantageous, many cause instability of
the neopolyploids and might be disruptive. #his review of how polyploids form and
adapt will not only be useful to readers who are interested in the evolutionary role of
polyploids. It will also be useful to those who are interested in how parental gene
interactions can lead to %2#264SIS or 'CSE2"2SIS, in how epigenetic patterns are
maintained or altered, and in how structural features of genomic organi7ation affect
cellular functions.
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