surabhi project-1.docx

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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


35/44

@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


40/44

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.

surabhi project-1.docx

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