Download TEXT Definition Chromosomal alterations are variations from the

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Definition
Chromosomal alterations are variations from the wild-type
condition in either chromosome structure or chromosome
number.
Classification
Chromosomal alterations are easily classified, but the origin,
the effects, and the genetic consequences are quite diverse.
Chromosomal alterations fall under two major classes-changes in
chromosome structure, and changes in chromosome number
(Table-1).
Table 1 Various types of chromosomal alterations:
TYPE OF
NAME
DEFINITION & ORIGIN
ALTERATION
Loss
of
chromosome
segment. May occur through
Deletion
two breaks and loss of
(Deficiency)
intermediate
segment
or
through
loss
of
a
chromosome tip.
Duplication of chromosome
Chromosome
segment
in
tandem
or
structure
elsewhere in the genome.
May occur through unequal
Duplication
recombination (tandem) or by
(Repeat)
distribution of normal and
translocated
chromosome
segments to the same pole
during
meiosis
of
heterozygoty
for
a
Chromosome
number
A.
translocation.
Inversion of a segment of
chromosome.
Arises
by
Inversion
inverting
intermediate
segment
between
two
chromosome breaks.
Translocation of chromosome
segment to elsewhere in the
genome. May be reciprocal,
which
chromosomes
Translocation in
exchange parts by improper
rejoining after breaks in
different chromosomes.
Abnormal number of whole
sets of chromosomes; may
follow failure of nucleus to
divide
after
chromosome
Euploidy
duplication, or fertilization of
an abnormal diploid gamete
by a normal haploid gamete
Ex. tetraploidy, triploidy.
Unusual number of individual
chromosome(s). Failure of
disjunction of a chromosome
Aneuploidy
in mitosis or meiosis. Ex.
trisomy-21
(Down’s
syndrome).
Changes in Chromosome Structure
In this class are included those chromosomal alterations
which change the chromosome structure, i.e., the number and
the sequence or the kind of genes present in chromosome(s), and
do not involve a change in chromosome number. There are four
common types of structural chromosomal alterations (Fig.1):
a)
Defeciencies.
b)
Duplications.
Inversions.
Translocations.
Duplications, inversions and translocations can revert back
to the wild-type state by a reversal of the process by which they
were formed. However, deficiencies cannot revert because a
whole segment of chromosome is missing. Deficiencies and
duplications alter the number of genes present, inversions change
the gene sequence, while translocations change the location of
genes from one to other non-homologous chromosome.
The first cytological demonstration of plant chromosomal
rearrangements was made in maize by Barbara McClintock.
Working with pachytene and other meiotic prophase stages that
present large chromosomes for microscopic observations, she
eventually demonstrated that irregular configurations made by
chromosomal rearrangements in the pairing process lead to all
the above kinds of structural changes i.e., deficiency, duplication,
inversion and translocation.
c)
d)
Deficiency
Deficiencies are macrolesions in which genetic material (part
of chromosome, terminal or interstitial) has been removed from a
chromosome (Fig.2). These are often lethal, even in heterozygous
form, owing to loss of vital genes or to gene imbalances. This
may be true to very small deletions, particularly in haploid
organisms, suggesting that genes are close together, and many
of them have indispensable functions. However, in some
genetically well-known species, notably Drosophila, use has been
made of small deletions to map very small areas of
chromosomes. Such deletions are often viable, if not wholly
normal, in heterozygous form. Consider a heterozygote in which
one homolog is structurally a normal chromosome bearing a
recessive mutation, and the other homolog has a small deletion
that removes the gene in question. Such a heterozygote will be
hemizygous for the recessive mutation and will express it
phenotypically or at biochemical level. This “uncovering” of the
recessive mutation is called pseudodominance. If a number of
overlapping deletions are available in a chromosome region,
together with recessive mutations in the same region, it is
a)
possible to map the end points of the deleted DNA and the order
of the mutations by their expression in mutant/deletion
heterozygotes.
Duplications
Duplication is a chromosomal alteration that results in the
doubling of a segment of a chromosome (Fig. 3). Duplications of
chromosome regions may be separated from one another, or
may be adjacent. Large duplications of chromosomal material
lead to gene imbalances that may be lethal to a zygote or even,
in the case of plants, to the pollen or ovules that carry hem.
Duplications in which the duplicated copies lie at different
positions on the same chromosome or on different chromosomes
may arise through matings: normal gametes may fuse with
abnormal gametes in which the same chromosomal segment has
been translocated to another location in the genome.
Small tandem duplications, in which duplicate segments
lie adjacent to one another, occur frequently in complex
organisms. Tandem duplications are able to produce even more
copies of the duplicated region by means of a process called
unequal crossing over, which is actually a type of ectopic
recombination. Fig.4A illustrates the chromosomes in meiosis of
an organism that is homozygous for a tandem duplication (brown
region). When they undergo synapsis, these chromosomes can
mispair with each other, as illustrated in Fig.4B. A cross over
within the mispaired part of the duplication Fig.4C will thereby
produce a chromatid carrying a triplication and a reciprocal
product (labeled “ single copy” in Fig.4D) that has lost the
duplication.
Duplications have other significance. An extra copy of a gene is
free to evolve through the acquisition of mutations into a gene
having a more specialized or different function. This process does
not compromise the original function, since an intact copy of the
original gene remains in the genome. Duplications are, therefore,
raw material for evolution, and many examples of duplication and
specialization are known. One example is the evolution of several
forms of the β-chain of hemoglobin (Fig.4). The beta-globin gene
cluster in humans contains 6 genes, called Epsilon (ε) [an
b)
embryonic form], Gamma G (γG), Gamma A (γA) [the gammas
are fetal forms], Pseudo Beta (ψβ) [an inactive pseudogene],
Delta (δ) [1% of adult delta-type globin], and Beta (β) [99% of
adult beta-type globin]. γG and γA are very similar, differing by
only 1 amino acid.
If mispairing in meiosis occurs, followed by a crossover
between delta and beta, the hemoglobin variant Hb-Lepore is
formed. This is a gene that starts at delta and ends as beta.
Since the gene is controlled by the DNA sequences upstream, HbLepore is expressed as if it were delta. That is, it is expressed at
about 1% of the level that beta is expressed. Since normal beta
globin is absent in Hb-Lepore, the person has severe anemia.
A chromosome carrying a duplication or a deficiency yields a
characteristic loop when it pairs at meiosis with a normal
homolog (Fig.5). The loop represents the material that has no
counterpart in the homolog.
Inversions
An inversion is a chromosomal alteration that results when a
segment of a chromosome is excised and then reintegrated in an
orientation 1800 from the original orientation (Fig 6). Obliviously,
the gene sequence in an inverted segment is exactly the opposite
of that in its normal homologous (noninverted) segment.
Therefore, inversions can be readily detected by a comparison of
linkage maps of the normal and the inverted chromosomes. The
existence of inversions was first detected by Sturtevant and
Plunkett in 1926 in this manner. Inversions have no physiological
effects if the break-points (inversion points) do not fall within a
gene. However, there can be phenotypic consequences when the
break-points occur within a gene or within regions that control
gene expression. The meiotic consequences of a chromosome
inversion depend on whether the inversion occurs in a
homozygote or a heterozygote. If the inversion is homozygous
(e.g., ADCBEFGH/ADCBEFGH), where the BCD segment is the
inverted segment in both the chromosomes, then meiosis will
take place normally and there are no problems related to the
gene duplications or deletions. However, crossing-over within
c)
heterozygotes (e.g., ABCDEFGH/ADCBEFGH, where only one
homologue has an inverted BCD segment) has serious problems
in chromosome pairing at meiosis (Figs.7,8) and recombination
within the characteristic loop leads to chromosomes with
duplications, deficiencies and in some cases two centromeres
(dicentric chromosomes), after recombination in meiosis. These
abnormalities are usually not recovered in the next generation,
because the gametes or the zygotes receiving them are inviable.
Therefore, heterozygotes for inversion are partially sterile.
d)
Translocations
Translocation is a chromosomal alteration in which there is a
change in position of chromosome segments and the gene
sequence they contain (Fig.9). There is no gain or loss of genetic
material involved in a translocation. Two simple kinds involve a
change in position of a chromosome segment within the same
chromosome; this is called an intrachromosomal (within a
chromosome) translocation. The other kind involves the
transfer of a chromosome segment from one chromosome into a
non-homologous
chromosome;
this
is
called
an
interchromosomal (between chromosomes) translocation. If
this latter translocation involves the transfer of a segment in one
direction from one chromosome to another, it is non-reciprocal
translocation; if it involves the exchange of segments between
the two chromosomes it is reciprocal translocation.
No serious meiotic disturbances accompany translocations, if
they are in homozygous form. In organisms homozygous for the
translocations, the genetic consequence is an alteration in the
linkage relationships of genes. For example, in the nonreciprocal
intrachromosomal translocation shown in Fig. 9a, the BC segment
has moved to the other chromosome arm and has become
inserted between the F and G segments. As a result, genes in the
F and G segments are now farther apart than they are in the
normal strain, and genes in the A and D segments are now more
closely linked. Similarly in reciprocal translocations new linkage
relationships are produced. A heterozygote carrying normal and
translocated sequences, however, also encounters a problem of
chromosome pairing that creates duplications and deficiencies in
meiotic products. The most problematic process is the distribution
of centromeres to the poles in Anaphase I of meiosis. Consider a
heterozygote in which two normal chromosomes N1 and N2 are
paired with the translocated chromosomes T1 and T2 (Fig.10).
while homologous centromeres go to opposite poles, we may
nevertheless get a distribution that yields meiotic products N1 +
T2 and N2 + T1. Such combinations are duplicated for some
regions of the genome and deficient for others.
In addition to the above four classes of structural
chromosomal alterations, some relatively less common altered
chromosomes are also known (Fig.11). These are:
a)
Ring chromosomes: Which are produced when both the
ends of a chromosome are damaged and the two damaged ends
of the centric segment reunite with each other.
b)
Dicentric chromosomes: Produced when the chromatids
of a centric segment of a broken chromosome unite with each
other.
c)
Isochromosomes:
Rarely,
the
centromere
of
a
chromosome may misdivide, i.e., divide vertically instead of
longitudinally, and the two chromosome arms may undergo
replication and produce two isochromosomes. The two arms of an
isochromosome are identical with each other in morphology as
well as gene content.
d)
Breakage-fusion-bridge cycle: Sometimes, the broken
ends of two sister chromatids of a chromosome may reunite with
each other; it would produce a chromatid bridge at the following
anaphase. The chromatid bridge would break at a random point
along the length between the two functional centromeres; when
such chromosomes with broken ends replicate during the
following interphase their sister chromatids are likely to fuse with
each other at the broken end. This will again generate a
chromatid bridge at the following anaphase. Thus, breakage
would be followed by fusion of sister chromatids leading to bridge
formation at anaphase. This would again be followed by
breakage, fusion and bridge formation giving this phenomenon
the name breakage-fusion-bridge cycle. It was first described by
Barbara McClintock in duplication heterozygote of maize.
B.
Changes in Chromosome Number
Somatic cells of higher plants and animals are usually diploid
i.e., two copies of the same genome are present i.e., 2n=2x (“n”
represents gametic chromosome number and “x” represents the
basic chromosome number or genomic number), while their
gametes contain a single genome i.e., n=x (note=it is not true for
polyploid species, wheat is a hexaploid with 42 chromosomes; in
this case x=7 and n=21). A deviation from the diploid (2n=2x)
state represents a numerical chromosome alteration which is
often referred to as heteroploidy; individuals possessing the
variant chromosome numbers are known as heteroploids.
The various heteroploid states may be grouped into two
classes:
a)
aneuplody, and
b)
euploidy.
The various terms describing the different states of
heteroploidy are listed in Table 2.
Table 2: Types of changes in chromosome number.
TYPE OF
TERM
DEFINITION
SYMBOL
HETEROPLOIDY
Nullisomic
One
chromosome
2n-2
pair missing.
Monosomic
One
chromosome
2n-1
Aneuploid ( One
missing.
or a few
Double
Two
chromosomes
monosomic nonhomologous
extra or missing
(each
from
a
from 2n i.e.,
2n-1-1
different
pair)
2n±few)
chromosomes
missing.
Polysomy (the general condition in which
organism have varying numbers of extra
chromosomes from one to many)
Trisomic
One
chromosome
2n+1
extra.
Double
Two
trisomic
nonhomologous
(each
from
a
2n+1+1
different
pair)
chromosomes
extra.
Tetrasomic
One
chromosome
2n+2
pair extra.
Monoploid
Only one genome
x
present.
Haploid
Gametic
chromosome
number
of
the
n
concerned species
present.
Autopolyploid (More than two copies of the
same genome present).
Autotriploio Three copies of the
3x
d
same genome.
Euploid (
Autotetraplo Four copies of the
4x
Number of
id
same genome.
genomes different
Autopentapl Five copies of the
from 2)
5x
oid
same genome.
Autohexaplo Six copies of the
6x
id
same genome.
Autooctaploi Eight copies of the
8x
d
same genome.
Allopolyploid (Two or more distinct
genomes: generally each genome has two
copies).
Allotetraploi Two
distinct
(2x1+2x2)
d
genomes; each has
*
two copies.
Allohexaploi
d
Three
distinct
genomes; each has
two copies.
Allooctaploi Four
distinct
d
genomes; each has
two copies.
* In general, this situation occurs;
situations may also occur.
(2x1+2x2
+2x3)*
(2x1+2x2
+2x3+2x4
)*
other
Aneuploidy
The word aneuploidy is a Greek word meaning “uneven
units”. In aneuploidy, one or several chromosomes are lost from
or added to the normal set of chromosomes. In most cases,
aneuploidy is lethal in animals, so in mammals it is detected
mainly in aborted fetuses. It is estimated that about 4% of
human zygotes are chromosomally abnormal, but only 10% of
them (i.e., 0.4% of the total zygote) survive to be borne. The
remaining 90% of abnormal embryos either fail to implant in the
uterus or abort in the early stages of embryonic development
after successful implantation. An estimated 20% of all
spontaneous abortions bear chromosomal defects. Triploid and
tetraploid fetuses invariably abort, but one triploid baby is
reported to have survived for one hour after birth. However,
aneuploid zygotes survive in relatively larger frequencies, and
several types of aneuploid variations are known in man (Table-3).
Plants are more often aneuploid. In most plants, a single extra
chromosome (or a missing chromosome) has a more severe
effect on phenotype than the presence of a complete extra set of
chromosomes. Each chromosome that is extra or missing, results
in a characteristic phenotype. The first critical study of aneuploidy
in plants has been made by Blakeslee and Belling in Jimson weed,
Datura stramonium. It shows a considerable amount of
morphological variation in many traits, particularly in fruit
characters. The normal chromosome number for this plant is
2n=24, but several particular morphological variants had 25
chromosomes. One of the 12 kinds of chromosomes was found to
be present in triplicate; that is, the somatic cells were 2n+1.
Such trisomic plant has three of each of the genes of the extra
a)
chromosome. Because the Jimson weed has 12 pairs of
chromosomes, 12 recognizable trisomics should be possible, and
Blakeslee and his colleagues succeeded in producing all of them
(Fig.12). Trisomics usually arise through nondisjunction, so that
some gametes contain two of a given chromosome.
Table 3: Human Aneuploid Conditions:
Formula
Chromosome
Condition
Constitution
Down’s
Syndrome
2n+1
47, +21
Edward’s
Syndrome
2n+1
47,+18
Patau
Syndrome
2n+1
47,+13
Turner’s
Syndrome
2n-1
45,X
Klinefelter’s
Syndrome
2n+1
2n+2
2n+2
2n+3
2n+4
47,XXY
48,XXXY
48,XXYY
49,XXXXY
50,XXXXXY
Phenotype
Round,
broad
head,
simian
palm, narrow,
high
palatte,
low IQ.
Mental
retardation,
multiple
congenital
defects of all
organs; death
within
06
months.
Similar
to
Edward’s
Syndrome;
death within 03
months.
Retarded
development of
female
sex
organs;
sterility.
Poor male sex
organ
development,
breast
development,
subfertility.
Euploidy
The term is drawn from the Greek word meaning “even
events”. Euploids have one or more complete genomes which
may be identical with or distinct from each other. The most
common condition of euploidy is the diploid state, in which two
copies of the same genome are present in a cell; it is represented
as 2x. Euploid variations are designated with reference to the
diploid(2x) state and not to the somatic complement (2n). These
variations may be grouped into two broad categories:
I)
Monoploids, including haploids, and
II)
Polyploids
b)
I) Monoploidy and Haploidy: Monoploidy denotes the
presence of a single copy of a single genome, and is represented
by x. On the other hand, haploidy represents the gametic
chromosome number of a species irrespective of whether it is
diploid or a polyploid species. Thus, monoploids are in essence
haploids of diploid species while haploids from polyploid species
are not. A classification of haploids is presented in Table 4.
Table 4: Classification of Haploids:
Euhaploids
Monohaploids
Allopolyhaploids
Autopolyhaploids
Disomic haploids (n+1)
HAPLOIDS
Addition haploids (9n+1. etc.)
Aneuhaploids Nullisomic haploids (n-1)
Substitution haploids (n-1+1)
Misdivision haploids
Monoploids are very rare in nature, because recessive lethal
mutations become unmasked and, thus, they die before they are
detected. These alleles normally are not a problem in diploids
because their effects are masked by dominant alleles in the
genome. Certain hymenopteran male insects (e.g. wasps, ants,
bees, etc.) are normally monoploid, because they develop from
Polyhaploids
unfertilized eggs. Consequently, these individuals will be sterile. A
stage in the life cycle of some fungal species can also be
monoploid.
II) Polyploidy: Presence of more than two genomes in an
individual is known as polyploidy. As a general rule, polyploids
can be tolerated in plants, but are rarely found in animals. One
reason is that the sex balance is important in animals and
variation from the diploid number results in sterility. However,
there are some polyploid animal species, such as North Americansucker (a freshwater fish), salmon, and some salamanders.
Recently, researchers in Chile have identified a new rodent
species, which may be the product of polyploidy (Fig.13).
Before we discuss polyploidy in plants(Fig. 14) in detail, first
a distinction must be made between the two major classes of
polyploids:
i)
autopolyploids and
ii)
allopolyploids.
The following definitions will rely on these chromosomal
descriptions. Two species will be considered, A and B. The
chromosomal composition of one species is:
A = a 1 + a2 + a3 . . . an
where a1, a2, etc. represent individual chromosomes and n is the
haploid chromosome number. The chromosomal composition of
the second species will be:
B = b1 + b2 + b3 . . . bn
i)
Autopolyploid - an individual that has an additional set of
chromosomes that are identical to the parental species; an
autotriploid would have the chromosomal composition of AAA
and an autotetraploid would be AAAA; both of these are in
comparison to the diploid with the chromosomal composition of
AA.
ii) Allopolyploid - an individual that has an additional set of
chromosomes derived from another species; these typically occur
after chromosomal doubling and their chromosomal composition
would be AABB; if both species have the same number of
chromosomes then the derived species would be an
allotetraploid.
An autotriploid could occur if a normal gamete (n) unites
with a gamete that has not undergone a reduction and is thus 2n.
The zygote would be 3n. Triploids could also be produced by
mating a diploid (gametes = n) with a tetraploid (gametes = 2n)
to produce an individual that is 3n. The difficulty arises when
autotriploids try to mate. They because of pairing problems,
produce
unbalanced gametes having additional chromosome
sets. Thus, these are invariably sterile.
Autotetraploids occur due to doubling of the chromosome
sets. This can occur naturally by doubling, sometime during the
life cycle, or artificially through the application of heat, cold or a
plant derived chemical called colchicine. Because an additional set
of chromosomes exists, autotetraploids can (but not necessarily
in all cases) undergo normal meiosis.
One generalization that has been made is that
autopolyploids are larger (but not
) than their diploid
counterpart. For example, their flowers and fruits are larger in
size which appears to be the result of larger cell size than cell
number. This increased size does offer some commercial
advantages. Important triploid plants include some
potatoes, bananas, watermelons and Winesap apples. All of
these crops must be propagated asexually. Examples of
tetraploids are alfalfa, coffee, peanuts and McIntosh
apples. These also are larger and grow more vigorously.
The chromosomal composition of allopolyploids is derived
from two different species. The classic experiment that initiated
research in allopolyploids was performed by G. Karpechenko in
1928. He knew that cabbage and radish, both had a diploid
number of 18 chromosomes, and he surmised that if he crossed
these two species, he should be able to derive offspring with 18
chromosomes. His applied goal was to develop a new plant that
contained radish roots and cabbage heads. To his disappointment
all of the progeny from the cross appeared to be sterile. It is
suggested that this occurred because correct pairing was not
possible between the two sets of chromosomes and synapsis and
normal disjunction were not possible. Thus, all of the gametes
were non-functional.
Surprisingly, though, one day he noticed that some seeds
did appear. These were grown, and chromosomal analysis
revealed that their diploid number was 36. Apparently,
chromosomal doubling had occurred. Therefore, balanced
gametes were generated because each chromosome had a
partner with which to pair. This type of situation where a
polyploid is formed from the union of complete sets of
chromosomes from two species and their subsequent doubling is
called amphidiplpoidy and the species is called an amphidiploid.
ANEUPLOIDY vs EUPLOIDY
Aneuploid and euploid variations again reveal the
importance of the quantitative balance of gene activities. In
plants, euploid variants, such as monoploids, triploids
and
tetraploids are very similar in appearance and function to the
diploid from which they were derived. However, aneuploids, with
gain or loss in individual chromosomes, can seriously disturb the
normal phenotype., often to the point of lethality. This is true
even in monosomics and trisomics of diploid organisms, neither of
which actually lacks any given gene entirely. The usual
explanation is that the greater harmful phenotypic effects in
trisomics are related to the imbalance in the number of copies of
different genes. A polyploid organism has a “balanced” genome in
the sense that the ratio of the numbers of copies of any pair of
genes is the same as in the diploid. For example, in a tetraploid,
each gene is present in twice as many copies as in diploid, so no
gene or group of genes is out of balance with the others. The
physiological effects of these imbalances are much more severe
in animals than in plants. In addition, the phenotype of
aneuploids is characteristic of the chromosome(s) by which they
differ from the diploid (Fig.15).
ORIGIN
A) Origin of structural alterations
All four classes of chromosomal alterations result from
breakage and improper rejoining of chromosome fragments or
from illegitimate recombination events. Chromosomal breakage
occurs spontaneously in a low frequency (Ca. 1% of the cells
studied) in almost all the tissues studied. The cause of
spontaneous chromosome breakage is not definitely known, but
several possible factors have been suggested, e.g., cosmic
radiations, nutritional deficiencies and environmental conditions,
such as temperature. The frequency of spontaneous chromosome
breakage is modified by several factors, viz., age, oxygen
availability, temperature and the metabolic stage of the cell. Age
is one of the most potent natural factors affecting the frequency
of chromosome breakage; the older the organism or tissue, the
higher the rate of spontaneous breakage. Similarly, root-tips from
older seeds show a higher frequency of breakage than those from
fresh seeds. Chromosome breakage is induced in a relatively high
frequency by several radiations (e.g., X-rays, γ-rays α-rays, βrays, neutrons), chemical agents (alkylating agents, e.g.
ethylmethane sulphonate; base analogues; many insecticides,
herbicides and fungicides, etc.) and by several viruses (e.g.,
measles virus). In addition, some genes are also known to induce
chromosome breakage, e.g., Ac-Ds of maize described by
McClintock and other transposable elements of eukaryotes, and
some other gene mutations, e.g. in soybean, etc.
Since broken chromosome ends lack telomere, they are
highly unstable and are prone to unite with damaged ends. When
a break occurs in a chromosome, the two broken ends thus
produced often join with each other, producing the same original
chromosome; this is known as restitution (Fig.16). Sometimes,
the two broken ends may heal and the acentric fragment thus
produced is generally lost, producing a terminal deficiency. An
acentric fragment:
i)
may move to one of the two poles during the division,
following its production, or
ii)
it may ordinarily lag behind, and be lost.
If it moves to a pole , it is included in the nucleus; at the next
division it is discernible as a micronucleus. Micronuclei generally
lag behind in the cytoplasm and are digested by exonucleases.
The presence of micronuclei is a clear indication of chromosome
breakage and deletion.
Most structural alterations, however, involve two breaks. A
chromosome may be folded on itself, and the two breaks may
occur at or close to the point where the chromosome passes over
itself; thus the following chromosome segments will be produced:
i)
segment AB containing the telomere
ii)
segment CDE (acentric segment), and
iii) centric fragment (containing centromere) FGHIJ (Fig
16B).
Often the broken ends of the three segments will join in their
original sequence (i.e., end B of AB with end C of CDE and end E
of CDE with end F of FGHIJ) producing the normal chromosome
ABCDEFGHIJ (restitution). But there may be non restitution, that
is, the broken ends of a chromosome may not unite in a pattern
other than their original sequence, thereby producing structural
alterations. For example, the segment CDE may be lost as an
acentric fragment, while end B of segment AB may unite with end
F of the centric fragment; this will produce interstitial
deficiency for CDE. Alternatively, the sequence of CDE may be
reversed, i.e. inverted, so that end E of CDE reunites with end B
of AB, while end C of CDE reunites with end F of the centric
segment; this produces inversion of CDE (Fig 16B).
When two nonhomologous chromosomes pass over each
other, breaks may occur in them at or close to the point of
contact. A reunion may occur between the centric segment of one
chromosome with the acentric fragment of the other chromosome
and vice-versa. Such a reunion will generate reciprocal
translocation between the two chromosomes (Fig 16C).
B)
Origin of numerical alterations
The origin of numerical alterations can be studied on the
following lines.
i)
Origin of Aneuploidy
Aneuploid individuals may be obtained in several ways:
1.
2.
3.
4.
5.
Meiotic irregularities, like non-disjunction or lagging of
one chromosome, occur spontaneously in low
frequencies, and produce n+1 and n-1gametes. When
such gametes unite with normal (n) gametes, 2n+1
and 2n-1 individuals are obtained (Fig 17).
Triploid plants are the best source of aneuploids as
they produce a high frequency of aneuploid gametes.
Many univalents are regularly present at MI of
asynaptic and desynaptic plants. Consequently, most of
their gametes are aneuploids and they produce
aneuploid progeny.
The ring of four in translocation heterozygotes may
disjoin 3:1; aneuploid gametes thus produced would
generate tertiary trisomics and monosomics.
Progeny from a cross between Tetrasomic (2n+2) and
disomic plants show a high frequency of trisomics.
ii)
I)
Origin of Euploidy
Origin of Monoploids (Haploids): haploids in some cases
as in male insects (Hymenoptera) are found as a routine and
are produced due to parthenogenesis. In these insects,
queen and drones are diploid females. Haploids may also
originate
spontaneously
due
to
parthenogenetic
development of egg in flowering plants. Such rare haploids
have actually been obtained in tomatoes and cotton under
cultivation. Rarely, haploids may originate from the pollen
tubes rather than from the eggs, synergids or antipodals of
the embryo sac. These haploids will be called androgenic
haploids. Haploids can be artificially produced by any one of
the following methods:
1. X-rays treatment.
2. Delayed pollination.
3. Temperature shocks.
4. Colchicine treatment.
5. Distant (interspecific and intergenic) hybridization.
II)
Origin of Polyploids: Polyploids may arise naturally or be
artificially induced. In plants it appears that diploidy is more
primitive and that polyploids have evolved from diploid
ancestors (Fig. 18). In natural populations this may arise as
a result of interference with cytokinesis, once chromosome
replication has occurred and may occur either :
i)
in somatic tissues, giving tetraploid branches, or
ii)
during meiosis, producing unreduced gametes.
It has been found that chilling may accomplish this in
natural populations.
Applications of the alkaloid colchicine, derived from the
autumn crocus (Colchicum autumnale), either as a liquid or
in a lanolin paste, induces polyploidy. Although chromosome
replication is not interfered with, normal spindle formation is
prevented and the double number of chromosomes becomes
incorporated within a nuclear membrane. Subsequent
nuclear divisions are normal, so that the polyploid cell line,
once initiated, is maintained. Polyploidy may also be induced
by other chemicals (acenaphthene and veratrine) or by
exposure to heat or cold.
Consequences
A)
Position effect of gene expression
When a chromosome rearrangement involves no change in
the amount of genetic material, but only in the order of genes,
the term position effect is used to describe any associated
phenotypic alteration. These effects have been studied
extensively in Drosophila, and also in the Yeast. The first
example, from the studies of Sturtevant and Bridges was noticed
on the bar eye duplication in Drosophila (Fig.19). These
investigators found a relation between the number of
chromosome sections (16A) present and the number of facets in
the eye. Further, critical experiments showed, however, that it is
not a strictly proportional relation. The arrangement of the
chromosome segments with respect to each other, as well as
their presence or absence, influences the size of the eye. The
effect of different arrangements was demonstrated by
manipulating the chromosomes through appropriate matings and
counting the facets in the eyes of the female offspring.
When section 16A was duplicated and the extra segment
occurred in homozygous condition with a total of four segments
(B/B genotype), the number of facets in the eyes averaged 68.
But when three16A sections were side by side in one homolog
and one section in the other homolog (BD/B+ genotype), the eyes
averaged 45 facets. Since the same number of 16A units is
presented in the eyes (B/B genotype and BD/B+ genotype), the
difference depends on the arrangement or position of the genes
with respect to each other. This phenomenon was interpreted as
a position effect. This phenomenon provided one of the earliest
indications that rearrangements of chromosome segments can
affect gene expression.
B)
Chromosomal alterations and evolution
A consequence of chromosomal structural alterations in a
population is related to evolutionary change, including speciation.
Chromosomal alterations are associated with position effects, that
may be significant in natural selection. More important for
evolution is the genetic isolation, that is mostly caused by
inversions and translocations. Speciation in Drosophila group of
dipterous insects, for example, has been related to chromosome
inversions. These structural changes occur in chromosomes of
individual flies, and are carried homozygous in populations.
Populations have developed over periods of time with different
chromosome inversions. Each may be isolated, because matings
of flies from a particular population with those of another
population carrying a different inversion, result in sterile or
inviable hybrids. This strengthens the boundaries around a
particular population and prevents the exchange of genes
between related populations. Speciation in Drosophila has been
associated with a series of different inversions that occurred by
chance in breeding populations and were eventually recognized in
different taxonomic groups. Translocations have been shown to
occur in certain plant groups, and to cause genetic isolation, thus
promoting evolutionary stability in populations.
A good example of polyploid evolution is provided by wheat
(Triticum species), exhibiting variable chromosome number within
the species. The domestication of wheat was a major event in
world civilization because it allowed humans to change from
nomadic hunter gathers to permanent residents of specific
locations. The following is the current suggested development of
modern bread wheat.
Triticum urartu (AA) X Aegilops speltoides (BB)
Triticum turgidum (AABB) X Triticum tauschii (DD)
Triticum aestivum (AABBDD)
Archaeological evidence has shown that Triticum turgidum
(AABB) was being grown in both Mesopotamia (Tigris and
Euphrates River Valley) and in the Nile River Valley, 10,000 years
ago. Because wild T. tauschii is found only in the mountain region
of southern Russia, western Iran and northern Iraq, it is thought
that the hybridization that produced T. aestivum occurred in
these regions. It has been suggested that this occurred as
recently as 8,000 years ago, which coincides with the
development of collective settlements by man. The wheats that
were developed by the above hybridization scheme are all
cultivated today.
Cultivated T. turgidum is called durum wheat. North Dakota
is essentially the only state in the US that grows durum wheat.
This wheat is processed and used for pasta. Bread, cookie and
pastry wheats are cultivated varieties of T. aestivum. North
Dakota is also a leading producer of these wheats, and North
Dakota is often the #1 producer for all types of wheat.
Another example is the recent development of a new
saltmarsh grass species. In the early nineteenth century, seed of
American saltmarsh grass (Spartina alterniflora) was accidentally
transported to the southern coast of England and the northern
coast of France. The grass began growing in the same location
where European saltmarsh grass (S. maritima) was grown. Soon
a new species of saltmarsh grass appeared, called Townsend's
grass (S. townsendii). The growth pattern of this species was
more vigorous and soon it had crowded out the other two native
species. These characteristics were recognized and soon it was
introduced into Holland to stabilize the dikes, and subsequently
into other locations for the same reason.
Chromosomal analysis suggested that Townsend's grass was
an amphidiploid because its chromosome number, 2n=122, could
be derived from the American (2n=62) and European (2n=60)
chromosome numbers. Apparently a hybridization occurred on
the beaches, followed by a chromosomal doubling to produce the
current species. An important point to consider is how quickly
speciation can occur due to allopolyploidy. Clearly, the
Townsend's grass species appeared and became established
within 100 years because of its vigorous growth.
It has been estimated that about 50% of all angiosperm
taxa (flowering plants) are polyploid. The following are some
examples of common cultivated plants that are autopolyploids.
Wild Species
Cultivated Species
Wild potato (2n=24) Cultivated
Potato
(2n=48)
Wild Cotton (2n=26) Cultivated
Cotton
(2n=52)
Dahlia (2n=32)
Garden
Dahlia
(2n=64)
Wild
Tobacco Cultivated
Tobacco
(2n=24)
(2n=48)
For some plant species, a series of successive ploidy levels are
seen. To describe these species it is necessary to introduce the
final symbol x. x is the base number of chromosomes for a
specific series of species. For roses, diploid roses having 2n=14,
the base number of chromosomes (x) is 7. The tetraploid rose
species have 2n=4x=28, the pentaploids have 2n=5x=35 and
the hexaploid rose have 2n=6X=42. Fern species exhibit some of
the largest chromosome numbers, and these are a result of
polyploidy. Adder's tongue fern (Ophiglossum sp.) has a base
number of 120 chromosomes. The diploid species has
2n=2x=240. One related decaploid species has 2n=10x=1200.