Polyploidy

B.Sc.IIIyear
Paper-II,Unit-III
Dr.Pinky Dwivedi

CHROMOSOME NUMBER
 Chromosome number can be altered in two ways
 Variation in the number of sets of chromosomes
 Euploid variation
 Variation in the number of a particular chromosome
within a set
 Aneuploidy
2

CHROMOSOME NUMBER
 Phenotypes of all species are influenced by thousands
of genes
 ~35,000 genes in a single set of human chromosomes
 Most genes are expressed only in certain cell types or
during certain times in development
 Intricate coordination is required in the expression of
these genes
 Proper development often requires two copies of each
gene per cell
3

CHROMOSOME NUMBER
 Aneuploidy
 Alteration in the number of a particular
chromosome within a set
 Commonly causes an abnormal phenotype
 Due to an alteration in the amount of gene
product produced
 i.e., 150% if three copies, 50% if one copy, etc.
4

CHROMOSOME NUMBER
 Aneuploidy
 Harmful effects first discovered in Jimson weed
 Datura stramonium
 Various trisomies had morphologically different capsules
 Many other
morphologically
distinguishable traits
 Many detrimental
5

CHROMOSOME NUMBER
 Aneuploidy
 Causes abnormal
phenotypes in humans
 Tolerated best with sex
chromosomes
 Remember X
inactivation?
6

CHROMOSOME NUMBER
 Down syndrome
 Generally caused by trisomy 21
 Affected by maternal age
7

CHROMOSOME NUMBER
 Euploid organisms
 Chromosome number is an exact multiple of a
chromosome set
 e.g., Haploid, diploid (2n), triploid (3n), etc.
 Polyploid = three or more sets
 Euploid variation can occur
 Occasionally in animals
 Quite frequently in plants
8

CHROMOSOME NUMBER
 Variations in euploidy
 Occur naturally in a few animal species
 e.g., Honeybees
 Females are diploid
 Males (drones) are haploid (monoploid)
 Produced from unfertilized eggs
9

CHROMOSOME NUMBER
 A few vertebrate polyploid animals have been discovered
 Certain amphibians and reptiles
 Separate diploid and polyploid species
10

CHROMOSOME NUMBER
 Variations in euploidy can occur in certain tissues
within an animal
 Diploid animals sometimes produce polyploid tissues
 e.g., Human liver cells can vary greatly in ploidy
 3n, 4n, 8n, etc.
 “Endopolyploidy”
 Biological significance poorly understood
 Enhanced production of certain gene products?
11

CHROMOSOME NUMBER
 Variations in euploidy can occur in certain tissues
within an animal
 Diploid animals sometimes produce polyploid tissues
 e.g., Chromosomes in Drosophila salivary glands undergo
repeated rounds of mitosis without cell division
 ~9 Doublings  500 copies of each chromosome
 Polytene chromosomes are produced
 Provides a unique opportunity to study chromosome structure
and gene organization
12

CHROMOSOME NUMBER
 Drosophila polytene chromosomes
 Drosophila possess 8 chromosomes per diploid cell
 Homologous chromosomes synapse and replicate
 Polytene structure is formed
 Centromeres of all four types
of chromosomes are attached
to the chromocenter
13

CHROMOSOME NUMBER
 Lend themselves to microscopic examination
 Can be seen during interphase
 100-200 times larger than an average metaphase chromosome
 Normal chromosomes are
not visible in interphase
14

CHROMOSOME NUMBER
 Exhibit a characteristic banding pattern
 Each dark band is a chromomere
 ~5,000 bands total
 More compact than interband regions
 Over 95% of DNA is in these bands
15

CHROMOSOME NUMBER
 Allow the study of the organization and functioning of
interphase chromosomes in great detail
 Duplications, deletions, and other rearrangements readily
detectable
 Expression patterns of
particular genes can be
correlated with changes
in the compaction of
certain bands
16

CHROMOSOME NUMBER
 Common in plants
 30 – 35% of ferns and angiosperms are polyploid
 Important in agriculture
 Many food plants are polyploid
 e.g., Fruits and grains
 Often display outstanding agricultural characteristics
 Often larger and more robust
17

CHROMOSOME NUMBER
 Wheat (Triticum aestivum) is
hexaploid
 Arose from the union of three
closely related diploid species
 “Allohexaploid”
18

CHROMOSOME NUMBER
 Polyploid ornamental plants often produce larger
flowers than their diploid counterparts
19

CHROMOSOME NUMBER
 Some polyploids have an even number of chromosome
sets
 e.g., 4n, 6n, etc.
 Produce balanced gametes
 Equal segregation during meiosis I
 Fertile
20

CHROMOSOME NUMBER
 Some polyploids have an odd number of chromosome
sets
 e.g., 3n, 5n, etc.
 Produce highly
aneuploid gametes
 Unequal segregation
during meiosis I
 Generally sterile
21

CHROMOSOME NUMBER
 Sterility is generally a detrimental trait
 Can be desirable agriculturally
 e.g., Seedless bananas and watermelons are triploid
22

CHROMOSOME NUMBER
 Triploid domestic bananas are derived from ancestral
diploid species
 Small black spots in the center
are degenerate seeds
 Asexually propagated through
cuttings
23

CHROMOSOME NUMBER
 Triploid varieties of flowering plants have been
developed
 Unweakened by seed bearing
 Increased blooms
24

CHROMOSOME NUMBER
 Variations in chromosome number are fairly
widespread
 Generally have a significant phenotypic impact
 Various causes
 Nondisjunction
 Improper segregation of chromosomes during anaphase
 Interspecies crosses
25

CHROMOSOME NUMBER
 Nondisjunction
 May occur during meiosis
 “Meiotic nondisjunction
 May occur during mitosis
 “Mitotic nondisjunction”
26

CHROMOSOME NUMBER
 Meiotic nondisjunction
 Can occur during meiosis I
 All resulting gametes are aberrant (aneuploid)
 Can occur during meiosis II
 Half of the resulting gametes are aberrant (aneuploid)
27

CHROMOSOME NUMBER
 Meiotic nondisjunction
 Complete nondisjunction occurs in rare cases
 Diploid gamete is produced
 Chromosome number is not reduced
 Fertilization with a normal haploid gamete produce a triploid
individual
28

CHROMOSOME NUMBER
 Mitotic nondisjunction
 Improper segregation of chromosomes may happen
after fertilization
 Part of the organism will contain cells genetically
different from the rest of the organism
 “Mosaicism”
 Size and location or mosaic region depend on timing of
nondisjunction
29

CHROMOSOME NUMBER
 Mitotic nondisjunction
 Bilateral gynandromorph of Drosophila melanogaster
 Began as an XX individual
 X chromosome was lost in the first mitotic division
 Left half is XX and female
 Right half is XO and male
30

CHROMOSOME NUMBER
 Changes in euploidy can occur by various different
mechanisms
 Complete nondisjunction
 Results in autopolyploidy
 Increase in number of sets within a single species
 Result of interspecies crosses
 Generally between close evolutionary relatives
 More common
 Results in allopolyploidy
 Possess sets of chromosomes from different species
31

CHROMOSOME NUMBER
 Autopolyploid individuals possess additional sets of
chromosomes from the same species
32

CHROMOSOME NUMBER
 Allopolyploidy can produce various combinations
 Individuals possessing one set of chromosomes from
each of two species are termed allodiploid
 Allopolyploids contain a combination of
autopolyploidy and allodiploidy
33

CHROMOSOME NUMBER
 Hybrids between two species
 Such hybrids often possess desirable traits
 e.g., Traits of both parental species
 Often sterile
 Depends on the degree of similarity of the different species’
chromosomes
 May be fertile if the two genomes are very similar
34

CHROMOSOME NUMBER
 Roan antelope (Hippotragus equinus) and sable antelope
(Hippotragus niger) have similar chromosomes
 Evolutionarily related chromosomes
are homeologous
 Allodiploid hybrids are fertile
35

CHROMOSOME NUMBER
 Georgi Karpchenko (1928)
 First to recognize the relationship between chromosome
pairing and fertility
 Crossed a radish (Raphanus) and a cabbage (Brassica)
 Both are diploid with 18 chromosomes
 Allodiploids possessed 18 chromosomes
 Species are not closely related
 Chromosomes are distinctly different
 Cannot synapse in meiosis
 Hybrids are sterile
36

CHROMOSOME NUMBER
 Most hybrids are sterile
 Rare offspring are produced
 Karyotyping revealed that they were allotetraploid
 Contain 36 chromosomes
 Allotetraploids are fertile
37

CHROMOSOME NUMBER
 Allodiploids are sterile
 Allotetraploids are fertile
38

CHROMOSOME NUMBER
 Some fertile allotetraploids are fertile
 Sadly, they were useless
 Possessed the leaves of a radish and the roots of a cabbage
 Demonstrated that it is possible to artificially produce a
new self-perpetuating species
39

CHROMOSOME NUMBER
 The development of polyploids is of interest among
plant breeders
 Often exhibit desirable traits
 Various agents have been shown to promote
nondisjunction
 Leads to polyploidy
40

CHROMOSOME NUMBER
 The drug colchicine is commonly used to promote
polyploidy
 Binds to tubulin in the spindle apparatus
 Interferes with normal chromosome segregation
41

CHROMOSOME NUMBER
 Alfred Blakeslee and Amos Avery (1937)
 First to apply colchicine to plant tissue
 Able to cause complete nondisjunction
 Produced autotetraploids
 Can be propagated asexually from
cuttings
 Tetraploid flowers are capable of
sexual reproduction
42

CHROMOSOME NUMBER
 Several mechanisms can produce variations in
chromosome number
 Some of these processes can occur naturally
 Figure prominently in speciation and evolution
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CHROMOSOME NUMBER
 Individual cells can be mixed together and made to
fuse
 “Cell fusion”
 Can create new strains of plants
 Enables crossing of species
unable to interbreed naturally
44

CHROMOSOME NUMBER
 The development of diploid crop strains homozygous
for all of their genes is a goal of some plant breeders
 Two such true-breeding strains can be crossed to
produce hybrids heterozygous for many genes
 Hybrid vigor (heterosis) can result
45

CHROMOSOME NUMBER
 True-breeding strains can be produced after several
rounds of self-fertilization
 Alternately, monoploids can be used to produce such
strains
 Have been used to improve wheat, rice, corn, barley, and
potato
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CHROMOSOME NUMBER
 Sipra Guha and Satish Maheshwari (1964)
 Developed a method to produce monoploid
plants directly from pollen grains
 “Anther culture”
 Used extensively to produce entirely homozygous
diploid strains
47

Polyploidy

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Polyploidy