The following points highlight the four main types of permanently specialized chromosomes. The types are: 1. Nucleolar Chromosomes 2. Sex Chromosomes 3. B Chromosomes 4. Chromosomes with Diffuse or Multiple Centromeric Regions.
Permanently Specialized Chromosomes: Type # 1.
Nucleolar Chromosomes:
The chromosome forming the nucleolus (plural = nucleoli) is called the nucleolar chromosome. Normally one or more chromosomes in the haploid set (genome) are nucleolar chromosomes.
For example, in maize (2n = 20), chromosomes 6 forms the nucleolus, whereas in barley (2n = 14), two chromosomes, namely, 6 and 7 are nucleolar chromosomes. In human, (2n = 46), chromosomes of the D group (13, 14, 15) and G group (21, 22) are nucleolar chromosomes. The region of the chromosome which is active in nucleolus formation is called nucleolar organizer region or nucleolar organizing region (Fig. 7.2.); it contains ribosomal DNA (rDNA) or rRNA cistrons.
In interphase nucleus, the nucleoli can fuse and therefore, the observable number of nucleoli does not correspond to the number of nucleolar organizer chromosomes. The number of nucleoli in human interphase cell never exceeds 6 although the number of nucleolar chromosomes is 10.
The nucleolar organizer region negatively heteropycnotic and is called the secondary constriction. The portion of the chromosomal segment separated by the secondary constriction from the main body of the chromosome is called satellite.
The chromosome bearing a satellite is termed as SAT-chromosome or satellite chromosome (Fig. 7.2). The size of satellite varies in different species and in different chromosomes in the same species. In barley, chromosome 6 possesses a larger satellite as compared to that of chromosome 7.
The chromosomes of D and G groups in human have satellites so small that they are often not manifested in every cell. Satellites may be of different types such as micro-satellite and macro-satellite; linear satellite, intercalary satellite and terminal satellite.
The nucleolar organizer region can be trans-located from one chromosome to the other. When the nucleolar organizer is lost, a large number of small nucleoli are formed in all the chromosomes. This indicates that the main nucleolar organizer functions to form nucleolus and suppresses other loci.
Nucleolar suppression may be observed in allopolyploids where genomes of two different species are brought into a single nucleus. In such cases, nucleoli of one species may not be formed due to suppression. In man-mouse cell hybrids, only mouse ribosomes are produced.
Nucleolus disappears at the end of prophase and reappears at telophase. When metabolic activity is shut off, the nucleolar organizer becomes more condensed and the nucleolus disappears. During telophase, the synthetic activity resumes and the nucleolar organizer begins to de-condense and nucleolus reappears.
Composition of isolated nucleolar fractions:
Isolated nucleolar fractions contain three main components:
DNA, RNA and protein. DNA is not detectable in some cases, such as in some amphibian oocyte nucleoli, but in some cases, relatively high amount of DNA is found. If the DNA and its associated histones are excluded, the RNA content ranges from 3 to 13% of the nucleolar dry weight with an average of 8%. Protein content ranges from 87 to 97%, the average being 92%. Thus the “protein/RNA ratio” varies between 30: 1 and 5: 1 with an average of 11.5 : 1.
Structure and function of nucleolus:
Nucleolus is roughly a spherical structure and consists of two regions when observed under an electron microscope: the inner fibrous region, and the outer granular region.
(a) Fibrous region or Core:
Fibrils of about 5 nm diameter are observed in this region. It contains the ribosomal RNA genes (rDNA) on which the precursor molecules of ribosomal RNA are synthesized. In eukaryotes, the gene coding for rRNA is present in about 1000 copies, and each gene coding for a 45S RNA is separated from the others by spacer DNA (Fig. 7.3).
In Xenopus laevis, the nucleolar organizer contains 800 rRNA cistrons (rDNA) and each is transcribed by 100 RNA polymerases. If the entire nucleolar organizer locus is involved in RNA synthesis, there are at least 80,000 RNA polymerases simultaneously transcribing 45S RNA molecules within the nucleolus of this organism.
In HeLa cells the estimated time for transcription of a 45S RNA molecule (4.4 x 106 Daltons M.W.) is about 2.3 minutes.
(b) Granular region or Cortex:
The rRNA is coated with protein to form ribonucleoprotein (RNP) particles of about 15-20 nm size. The granular region consists of a well defined fibrillar element to which granules are irregularly attached. These granules are RNA and protein (RNP) complexes. A newly formed nucleolus is composed only of the fibrillar component; when RNA synthesis begins, the granular component gradually accumulates.
Synthesis and processing of ribosomal RNA:
In eukaryotic cells, RNA polymerase I transcribes the 18S and 28S rRNAs; this enzyme is localized within the nucleolus. The rRNA is transcribed as a large 45S RNA precursor molecule (Fig. 7.4). Then methylation of specific regions of this precursor occurs; the main site of methylation is the 2′-hydroxyl group of the ribose sugar.
The methyl groups (-CH3) are derived from methionine through trans-methylation reaction which occurs as the 45S RNA is being transcribed. Studies using 3H-Uridine have indicated that the radioactive methyl groups incorporated into 45S RNA are associated with 28S and 18S RNAs.
It seems that the methylation protects specific regions of the 45S RNA from degradation during the processing. The cleavage of the 45S RNA occurs through the action of ribonucleases; it is first cleaved into a 41S RNA which is then cleaved into the molecules of 32S and 20S RNA (Fig. 7.4). The ribonuclease cleaves 32S RNA into one 28S RNA and one 5.8S RNA molecules. The 20S RNA is cleaved into one 18S RNA molecule.
Another type of ribosomal RNA is the 5S RNA which is synthesized outside the nucleolar organizing region. An eukaryotic ribosome contains one molecule each of 28S, 18S, 5.8S and 5S RNAs. The 5S RNA is synthesized in excess of the 45S RNA; the excess of 5S RNA is degraded.
Formation of eukaryotic ribosomes:
In eukaryotes, the ribosomal proteins are synthesized in the cytoplasm but the assembly of ribosomal subunits occurs within the nucleolus. The 45S RNA and 5S RNA are combined with proteins (synthesized in the cytoplasm) to form the RNP complex of about 80S.
This 80S particle is the precursor of both the large and the small ribosomal subunits. In human, the large ribosomal subunit is a 55S particle and contains molecule each of 28S, 5.8S and 5S RNAs, and 49 specific proteins (Table 2.3). But the small subunit contains one molecule of 18S RNA and 33 specific proteins.
The small subunit is rapidly transported while the large submit is slowly transported to cytoplasm where they form 80S polyribosome. Therefore, the large ribosomal subunits are readily detected in nucleolar extracts. The schematic model giving the main steps of ribosome synthesis is presented in the Figure 7.4.
Prokaryotic ribosome formation:
Prokaryotic cells do not contain nucleoli. Their rRNA cistrons are present in a few copies, about 6 copies which are transcribed like other genes. In contrast to eukaryotes, the prokaryotic 5S RNA gene is closely linked with the 16S and 23S RNA genes in the order 16S-23S-5S. The large subunit (50S) of ribosomes contains one molecule each of 23S and 5S RNA, while the small subunit contains one molecule of 16S RNA (Table 2.3).
In addition to the three rRNA species (23S, 16S and 5S RNAs), some transfer RNA (tRNA) molecules are also produced from the primary transcript of rRNA genes (Fig. 7.5). In E. coli, one tRNA gene is located between the 16S and 23S genes, while another tRNA gene is located just beyond the 5S RNA gene. There are 31 specific proteins in the large subunit and 21 specific proteins in the small subunit (Table 2.3).
Permanently Specialized Chromosomes: Type # 2.
Sex Chromosomes:
Generally a genetic mechanism regulates the determination of sex in various organisms. There may be a single gene or gene complex that governs sex determination, e.g., in papaya, Asparagus and several fishes. One of the two sexes is homogametic in that it produces a single type of gametes, while the other sex is heterogametic and produces two types of gametes, while the other sex is heterogametic and produces two types of gametes. Thus the progeny in any generation consist of both the sexes in equal ratio.
In maize, male and female flowers (unisexual flowers) are produced on the same plant (monoecious condition). A simple system involving two pairs of genes (Ba ba and Ts ts) converts this monoecious plant into a dioecious one, i.e., the male and female flowers are produced on different plants.
The dominant gene Ba produces normal female flowers in the cob, but its recessive allele ba in homozygous state interferes with the cob development and produces rudimentary female flowers. Thus ba ba plants are functionally male.
The dominant allele Ts of the other gene produces normal male flowers in the tassel, but its recessive allele ts (tassel seed) in homozygous condition causes seed setting in the tassel making the plant functionally female.
When both the dominant genes Ba and Ts are present, the plant is monoecious. The double recessive plant (ba ba ts ts) is functionally female. Similarly, the plant of genotype ba ba Ts/Ts develops into male, while Ba Ba ts ts plants develop into females. The plants of ba ba Ts ts genotype are heterogametic male. On crossing a ba ba ts ts (female) and ba ba Ts ts (male) plants, male and female plants are obtained in 1: 1 ratio in the progeny. Thus maize plant can be made dioecious by two genes ba and ts.
In many cases, there operates a chromosomal mechanism of sex determination. There are specific chromosomes which carry the genes responsible for sex determination. Such chromosomes are called allosomes or sex chromosomes, while the remaining chromosomes are called autosomes (symbolized by “A”). One of the sexes is homogametic while the other is heterogametic.
The sex chromosome of the homogametic sex is designated as the X-chromosome. One sex chromosome of the heterogametic sex is the same as the X-chromosome, while the other (if present) is different from the X; it is designated as Y-chromosome.
Several different chromosomal mechanisms of sex determination exist in the nature (Table 7.1). In certain plant species, such as, Humulus lupulus and Rumex acetosa, compound sex chromosomes are known to occur.
The X chromosome also carries genes that have no role in sex determination; they are called sex-linked genes. In some organisms, e.g., human, the Y- chromosome is smaller, while in others, such as, Drosophila, Melandrium, it is larger than the X-chromosome.
Further, the Y chromosome is often more heterochromatic than the X. The Y chromosome carries genes for maleness in several organisms, e.g., human, Melandrium and Coccinia indica. Genes located on the Y chromosome are inherited from father to son; such genes are called holandric genes.
In the system where female is heterogametic (XY) and male is homogametic (XX), the Y chromosome is inherited from mother to daughter, and the genes located on this chromosome are called hologynic genes. A few genes have been located on the human Y chromosome, e.g. histocompatibility gene (H-Y) and the testis determining factor (TDF).
Meiotic behaviour:
A very small homologous segment is present at one end of the X and Y chromosomes in which pairing occurs; as a result, the X and Y chromosomes form a bivalent during meiosis and move to opposite poles during Al.
In case of compound sex chromosomes, say, XY1 Y2 system, both the arms of the X chromosome possess pairing segments. The Y1 and Y2 chromosomes pair in such a way that the X chromosome moves to one pole, while both the Y chromosomes move to the opposite pole (Fig. 7.6). In the X, Y,X2Y2 system, both the X chromosomes move to one pole, while the Y chromosomes move to the opposite pole.
In the XX-XO mechanism, the XO individuals have a single X chromosome. Their X chromosome orients itself at the metaphase plate and moves to one pole at Al, leaving the other pole devoid of this chromosome. At All, the X chromosome divides and its chromatids move to the opposite poles. Thus only 50% of the gametes possess an X chromosome, while the remaining 50% of the gametes have no X chromosome.
Genie Balance Theory:
This theory was given by Bridges in 1921 based on his study of the progeny of crosses between triploid females and diploid males of D. melanogaster. According to this theory, the X chromosome of Drosophila carries the genes for femaleness, while the autosomes carry the genes for maleness. A balance between the number of X chromosomes and the number of sets of the autosomes determines the sex.
If the ratio “X/autosomal set (A)” is equal to 1.0, the fly develops into a normal female, while if the ratio is 0.5, it develops into a normal male. The ratios falling between 1.0 and 0.5 lead to development of intersexes (Table 7.2).
Later, it was shown that the male determining factors are carried on the 2nd and 3rd chromosomes of Drosophila. The Y chromosome has no role in determination of sex but it is essential for male fertility; as a result, XO flies are phenotypically males but they are sterile.
X-Y balance:
In several organisms male determining genes are present on the Y chromosome while the female determining genes are located on the X chromosome; sex is determined by the balance between the X and Y chromosomes.
In human, Y chromosome is strongly male determining (Table 7.3). In the absence of Y chromosome, the phenotype of the individual is female. Thus a female phenotype develops in individuals of XO, XX, XXX, XXXX, XXXXX constitution. However, in presence of a single Y chromosome, they all (XY, XXY, XXXY, XXXXY, XXXXXY) develop into males.
In plant Silene (Melandrium), XX individuals bear female flowers while XY individuals bear male flowers. The X chromosome carries genes for femaleness, while Y chromosome carries genes for maleness. A balance between X and Y chromosomes determines the sex. The X/Y ratio above 1.5 causes occasional formation of hermaphrodite flowers in an otherwise male plant (Table 7.2).
In the absence of a Y chromosome, female flowers are produced. When the X/Y ratio is 4.0, the plant is hermaphrodite (♀) but occasional male flowers are also produced. Thus in Silene one Y chromosome balances 4X chromosomes.
In plant Coccinia indica (family Cucurbitaceae), the XX-XY mechanism of sex determination operates. In this plant species, individuals having 2A + XX, 3A + XXX, and 4A + XXXX are females, whereas those having 2A + XY, 3A + XXY, and 4A + XXXY are males. Thus the presence of a single Y chromosome causes the development of a male phenotype.
Y chromosome:
The Y chromosome is the sex chromosome confined to the heterogametic sex in the XX-XY system of sex determination (in contrast, X chromosome is found in both the sexes although the heterogametic sex has only one copy).
The size and function of the Y chromosome vary in different organisms. In Drosophila, the Y chromosome is slightly larger than the X chromosome; it is heterochromatic and does not carry the genes for maleness.
However, it is necessary for the fertility of the males since XO male Drosophila are sterile. In contrast, in the case of humans and several plants like Melandrium, the Y chromosome possesses male determining genes. In human, the Y chromosome is very small, but it is strongly male determining so that individuals with XXXXY and mosaics with XXXY/XXXXY/XXXXXY constitution develop the male phenotype.
Genes for maleness are located on the short arm of human Y so that XY individuals with deleted short arm of the Y chromosomes develop the female phenotype. The segments bearing male determining genes in the short arm of Y may be trans-located to the X chromosome; in this case the XX individuals develop the male phenotype.
In Silene (Melandrium), Y chromosome is much larger than the X chromosome. It possesses the following four distinct regions (Fig. 7.7).
(I) Female suppressor region:
It is located at the end of the Y chromosome and carries the genes for suppression of the female reproductive organs. In the absence of this region, hermaphrodite (bisexual) flowers are produced on the XY plant.
(II) Male promoting region:
This region lies next to the first (female suppressor) region and carries genes for initiation of anther development. Absence of this region causes the production of female flowers on the XY plant.
(III) Male fertility region:
It lies next to the second (male promoting) region and carries the genes for male fertility. When this region is absent, anthers abort and the plant is male sterile.
(IV) Pairing region:
It is located at one end of the Y chromosome and is homologous to a region in the X chromosome. During meiosis, the X and Y chromosomes pair in this region which ensures their proper separation during AI. The X chromosome of Silene possesses a differential region (V) which carries genes for the development of female reproductive organs (Fig. 7.7).
Dosage compensation:
In the sex determining system where XX individual is female and XY individual is male, females contain two X chromosomes, whereas males contain only one X chromosome. Apart from the genes governing the sex, other genes are also present in the X chromosome; they are called sex-linked genes.
These genes are in homozygous or heterozygous condition in the females (XX), but they are in hemizygous condition in the males (XO, XY) since they do not have corresponding alleles in the Y chromosome. But males and females are morphologically and physiologically similar in expression of these genes.
The mechanism by which the effects of sex- linked genes in males (XO, XY) are equalized to their effects in females (XX) is known as dosage compensation.
In Drosophila, this mechanism operates by enhancing the activity of the X-linked genes in males, while the activity of these genes in the two X chromosomes of females is restrained. The mechanism of dosage compensation in man and other mammals differ from that in Drosophila.
One of the two X chromosomes of the females is inactivated through heterochromatinization so that only one X chromosome remains active. Thus there is a balance of X-linked gene activity in the females and males.
Single active X-hypothesis or Lyon hypothesis of dosage compensation: The dosage compensation in human and other mammals is regulated by the inactivation of one X chromosome in the females. This is known as single active X hypothesis or Lyon hypothesis proposed by Lyon in 1961 and elaborated subsequently.
The main genetic evidence for this hypothesis comes from the mosaic phenotype of female mice heterozygous for sex-linked recessive genes that affect coat colour.
According to this hypothesis:
(i) One of the two X chromosomes in the cells of normal female mammals is genetically inactive.
(ii) Inactivation occurs early in embryonic development.
(iii) The inactive X chromosome may be maternal paternal one in the different cells of the same animal.
(iv) The decision as to which X chromosome becomes inactive is taken at random. Once an X chromosome is inactivated in a cell, the same X chromosome will always be inactivated in all its progeny cells.
(v) The inactivation occurs due to heterochromatinization. The hetero-chromatinized X chromosome forms the sex chromatin observed during interphase, and is late replicating.
There occurs a preferential heterochromatinization of abnormal X chromosome (Table 7.4). If one X is normal and the other is an iso-X-chromosome, the iso-chromosome is always heterochromatic. In man, inactivation of sex-linked genes has been demonstrated at cellular level, for example, glucose-6-phosphate-dehydrogenase (G6PD), Hunter-hurler syndrome, Juvenile hyperuricaemia.
Sex chromatin (Barr body) Drum sticks:
Sex chromatin is the hetero-chromatinized X-chromosome observed as a condensed body in interphase nuclei of mammalian females. It was discovered by Barr and Bartram in 1949 in the neurons of cat and was called Barr body after M.L. Barr.
Generally the sex chromatin is observed as a planoconvex body lying adjacent to the inner surface of the nuclear membrane. A detailed study shows that it has V or U-shaped structure and its apex points towards the centre of the nucleus.
The size of sex chromatin ranges from 0.7 x 1.0 µm to 1.0 x 1.4 µm with an average of 0.8 x 1.1 µm in the different tissues and species. Sex chromatin is not visible in all the interphase nuclei of females; the frequency of cells showing sex chromatin varies in the different tissues of the same species.
The frequency of “sex chromatin positive” nuclei is 85% in the nervous tissues, 96% in the whole mounts of amnion epithelium and from 20-25% to 60-70% in oral smears. It has been found that the number of haploid autosome complements influences the number of late replicating X chromosomes.
The relationship between numbers of sex chromatin, autosomal set and the X chromosome has been expressed by the following formula:
B = X – (p/2) (7.1)
where, B = number of sex chromatin bodies
X = number of X chromosomes
P = number of autosomal sets
The above relationship holds for all even degrees of ploidy, viz., 2n, 4n, 6n, 8n, etc. Thus in human, a tetraploid cell (2n = Ax = 92, XXXX) has two sex chromatin bodies. But a tetraploid cell with XXYY constitution (92, XXYY) will not show any sex chromatin. In triploids (69, XXX), some cells have one and others have two sex chromatin bodies (average 1.5 per cell).
The abnormal males, such as, XXY (Klinefelter’s syndrome) also show sex chromatin in their cells (Table 7.4). Abnormal X chromosome is, as a rule, always hetero-chromatinized. The Y chromosome has no role in sex chromatin formation; XYY males do not show any sex chromatin. Thus sex chromatin can be used as a direct clinical test to determine the number of X chromosomes in an individual.
Davidson and Smith in 1954 found some bodies similar to sex chromatin in the circulating polymorphonuclear neutrophil leucocytes of human blood. This body is like a drum stick and is attached to one lobe of the polymorph nucleus.
However, the frequency of drum sticks is very low, viz., 1 in 40 leucocytes in normal females and < 1 in 500 leucocytes in normal males. The drum sticks probably represent the hetero-chromatinized X chromosome.
Permanently Specialized Chromosomes: Type # 3.
B Chromosomes:
A special class of chromosomes may be found in some animal and plant species in addition to their basic set of chromosomes constituting the haploid genome; these chromosomes are called supernumerary chromosomes, accessory chromosomes or accessory fragments.
They were first discovered by Wilson in 1905 in the insect Metapodius (Hemiptera). In 1928, Randolph coined the term B-chromosomes for the additional chromosomes and A-chromosomes for normal chromosomes of the genome.
Origin and occurrence:
B chromosomes are found in the natural populations of many plant and animal species as a numerical chromosome polymorphism. In animal kingdom, they have been observed in most of the insect orders, e.g. Heteroptera, Lepidoptera, Diptera, Coleoptera, mollusks, flatworms and vertebrates, except fish and primates.
In plant kingdom, B chromosomes are known to occur in about 1000 species including angiosperms, they have been reported in 51 dicotyledonous and 14 monocotyledonous families.
‘The occurrence of B chromosomes seems to be related to the breeding system of the species: they occur is cross pollinated species. Among crop plants, they are known to occur more frequently in Gramineae. Most of the species containing B chromosomes are forage grasses, such as, Bromus, Agrostis, Festuca, Deschampsia, Loliunt, Dactylis, Poa and Anthoxanthum.
Further, B chromosomes are also found in graminaceous crop plants like maize, Avena, Pennisetum, Sorghum and rye.
In plants, B chromosomes have unknown origin and function. But in animals, they appear to have arisen from sex chromosomes. There are several views regarding the origin of accessory chromosomes.
According to one view they have been derived from some heterochromatic elements of the normal chromosomes. The other view proposes that B chromosomes originated in interspecific hybrids and involved a non-disjunction system which promotes chromosome accumulation rather than elimination.
Alternatively, repeated duplication of small chromosome fragments caused by structural changes might have resulted in the origin of B chromosomes.
B chromosomes differ from the normal chromosomes (A chromosomes) in various aspects like morphology and structure, genetic constitution, variation in number, effects on different characters, meiotic behaviour and mitotic behaviour.
Structure and genetic constitution of B chromosomes and their effect on meiotic behaviour of A chromosomes:
B chromosomes are normal in structure but they are shorter than the A chromosomes. Maize B chromosomes are about 2/3 in size of the smallest A chromosome. In several plant species, such as, maize and in many insects, they predominantly euchromatic (Fig. 7.8).
These chromosomes exhibit their own unique pattern of heterochromatin distribution. Generally, B chromosomes are telocentric or acrocentric; consequently, they are easily distinguishable from the normal (A) chromosomes.
Polymorphism in B chromosomes has been observed in several plants but the morphological changes do not show any genetic consequences. Generally, B chromosomes do not show pairing affinity with the A chromosomes.
The base composition of DNA of B chromosomes of a grasshopper species showed a higher A = T content than that in the A chromosome set. The nuclear DNA content increases due to the presence of B chromosomes (Table 7.5), e.g., the DNA content of maize increases 154.5% over the normal in the presence of 34 B chromosomes.
Presence of 8 B chromosomes in rye causes 55% increase of DNA content over normal. The increase in DNA content in turn, increases the duration of cell cycle. RNA and protein contents of nucleus also increase due to presence of B chromosomes. As a result, cell size and plant development are affected.
Chiasma frequency of A chromosomes is influenced by the presence of B chromosomes; this effect varies in different plant species. In Secale cereale, Briza elatior and others there is no change in the chiasma frequency, but in some other plant species like Aegilops speltoides and Lolium perenne (perennial rye grass) the chiasma frequency in reduced due to the B chromosomes.
In contrast, chiasma frequency is increased due to B chromosomes in certain plant species like Zea mays, Pennisetum typhoid’s and Festuca arundinaceous, and in grasshopper. Experiments using A-B translocations have shown that the genetic elements of maize B chromosomes affecting A chromosome recombination are distributed throughout the B chromosome.
In interspecific crosses involving B chromosomes, chromosome pairing is affected in some hybrids. In wheat, chromosome 5B contains a major suppressor locus (Ph) that suppresses homologous chromosome pairing so that only bivalents are formed.
Mochizuki in 1964 reported that the B chromosomes of Aegilops mutica could suppress homologous pairing in the F, hybrid “T. aestivum x Ae. mutica” which was mullisomic for 5B chromosome of wheat. Thus B chromosomes of Aegilops could compensate for the 5B effect of wheat.
On the basis of hybridization experiments involving B chromosomes. Dover and Riley in 1972 advanced the theory that the 5B pairing control locus of wheat could have been derived through the translocation of a critical segment of an Aegilops B chromosome into the genome of hexaploid wheat.
The effects of B chromosomes on chromosomes pairing have been observed in other cases also. When Lolium temulentum (2n = 2x = 14) is crossed with L. perenne (2n = 2x = 14) at the tetraploid level, and there are no B chromosomes, both the homologous and homologous pairings occur giving rise to bivalents and multivalent.
But when B chromosomes are present in the cross, homologous pairing is suppressed so that only bivalents are formed. This phenomenon is of importance in plant breeding because it provides genetic means for diploidization of polyploid iter-specific hybrids between grass species.
Variation in number of B chromosomes and their effects:
The number of B chromosomes varies among different cells/tissues of an individual, among individuals of a population and among generations. A maize plant with one B chromosome, on selfing, can produce offspring with 0, 1, 2, 3, and 4 B chromosomes. Maize plants having up to 34 B chromosomes have been observed. In rye, a maximum of 10 B chromosomes have been reported.
There are many species in which B chromosomes are constant within an individual, but are variable among individuals. B chromosomes may be eliminated from one organ of the individual but may be present in the other organ.
For example, in the grass Poa alpina, B chromosomes are present in primary roots and central part of germ cells, but are eliminated from leaves, and adventitious roots. In case of animals, occasionally they are lost from the somatic tissues and are present only in the cell lines leading to the development of reproductive organs.
Their number increases mainly due to nondisjunction following which they are accumulated. In animals, their accumulation is usually restricted to females. Up to certain number, B chromosomes have no effect, but above this limit, they exert deleterious effects on the individual. Their most serious effects are upon seed and pollen fertility.
In maize, the presence of 10 to 15 B chromosomes does not have an effect on vigour of plant, but the presence of 16 to 25 B chromosomes leads to increased fertility and vigour. However, plants with 30 to 34 B chromosomes are sterile and low in vigour. The B chromosomes of rye have relatively greater euchromatin content and they are not tolerated well by an individual.
As the number of B chromosomes increases, fertility declines; there is 99.9% sterility when B chromosomes become 7 (Table 7.6). No seed set occurs when the B chromosomes become 8 or more in number in this plant species. On the other hand, the presence of a single B chromosome can cause male sterility in plantain (Plantago coronapus).
B chromosomes produces physiological effects also. Seed germination is variably affected by B chromosomes depending on plant species. In Festuca pratensis, there is no effect on seed germination, but in Allium porrum (leek) and in several other species a low number of B chromosomes enhances the germination of seeds. On the other hand, germination is delayed by B chromosomes in certain species like Secale cereale.
The presence of B chromosomes delays flowering. Quantitative characters, such as, plant height, number of tillers etc. are adversely affected by B chromosomes. In rye, straw weight increases when 2 B chromosomes are present.
Odd and even number of chromosomes have differential effect on straw weight and tiller number. The odd numbers of B chromosomes have more deleterious effect as compared to the even numbers, however, the reasons for this differential effect are not known.
Meiotic behaviour and non-disjunction:
B chromosomes may show normal meiotic behaviour. If there is a single B chromosome, the univalent can divide at Al (Precocious centromere division) or at AIL If it divides at Al, it is subjected to elimination during the second meiotic division; the elimination rate varies in different species.
In Ae. speltoides, about 20% of the univalents reach one pole at Al and they divide at AIL The 80% univalents divide at Al and are eliminated at a high rate. If two B chromosomes are present, they may form a bivalent, however, univalents may also occur.
The frequency of bivalent formation differs in different species and in different varieties of a single species. In rye, the frequency of B chromosome bivalents varies from 19.0 to 92.4% in the different varieties. The presence of more than 2 B chromosomes shows pairing irregularities.
B chromosomes show non-disjunction at AI or All of meiosis. Thus two chromosomes at AI or two chromatids at All remain together and are included preferentially in a single cell that will develop into egg.
Non-disjunction of B chromosomes is classified into two main groups according to the stage of cell division:
(i) Non-disjunction during meiosis and
(ii) Non-disjunction during post meiotic mitosis.
Nondisjunction during meiosis:
Nondisjunction of B chromosomes occurs during first meiotic division of the megaspore mother cell; both the chromosomes pass to the same pole selectively so that 75 to 85% of the eggs contain B chromosomes. This type of nondisjunction occurs in Lilium callosum, trillium grandiflorum, Plantago serrasia and several other species.
Nondisjunction during post-meiotic cell division:
Different types of nondisjunction during post-meiotic mitosis were distinguished by Battaglia in 1964:
(1) Meiotic behaviour of B chromosomes in normal. Nondisjunction occurs during the first post-meiotic mitosis. The centromere of B chromosome divides but both the chromatids remain attached together near the centromere (Fig. 7.9).
The dividing nucleus occupies a position at the periphery of the cell and the B chromosome remains at the position of the original nucleus and it is directed towards the pole responsible for the formation of sperm.
Thus the vegetative nucleus becomes devoid of B chromosome and the generative nucleus receives both the chromatids. In the second mitotic division, each of the sperm nuclei receives 2 B chromosomes. On the female side also, B chromosomes are preferentially directed towards the pole that is responsible for egg formation. Such type of preferential nondisjunction occurs in rye (Secale cereale)
(2) Meiotic Behaviour of B chromosomes is normal and the first pollen mitosis is also normal. Nondisjunction occurs during the second pollen mitosis and both the chromatids of B chromosome pass in the same sperm nucleus.
The sperm possessing the two B chromosomes preferentially fertilizes the egg. The frequency of such selective or preferential fertilization is about 60 to 70%. This type of non-disjunction occurs in maize (Zea mays).
(3) The first pollen mitosis is normal. Then one or more fast divisions of vegetative nucleus occurs; some of the division products give rise to supernumerary generative nuclei. This division is so rapid that the B chromosomes pass undivided to the generative pole. This type of nondisjunction occurs in Sorghum.
Causes of non-disjunction:
The factor responsible for nondisjunction has been found to be located in the B chromosome. It causes a delay in the replication of DNA in the heterochromatin. In rye, the factor for nondisjunction is located in the large terminal heterochromatic knob in the long arm of the B chromosomes. In the absence of this knob, nondisjunction of B chromosomes does not occur.
In maize, this factor is located in the euchromatin proximal to the large block of heterochromatin. When translocation occurs and the distal heterochromatic part of B chromosome is transferred to an A chromosome, the A chromosome also shows nondisjunction. The B chromosome deficient for the heterochromatic part also shows non disjunction as long as the heterochromatic portion is present in the same cell.
Permanently Specialized Chromosomes: Type # 4.
Chromosomes with Diffuse or Multiple Centromeric Regions:
In general, centromere is located on plant and animal chromosomes and one centromere per chromosome is the most common situation. Sometimes two centromeres may be located in the same chromatid (dicentric) due to chromosome aberrations.
Occasionally, 3 centromeres may be present in a chromosome giving rise to the tricentric condition. The dicentric, tricentic etc. situations are given the general notation of polycentric chromosomes; such chromosomes arise due to chromosome aberrations.
In certain animals and plant species, the centromeric activity is spread over a considerable part or even the entire chromosome; such chromosomes are called holocentric or holo-kinetic chromosomes. They occur in Hemiptera and in some other insects, and in monocotyledonous plants of the order Juncales and Cyperales. Some primitive dicotyledonous plants, such as, Myristica (order Ranales) are also known to possess chromosomes with diffuse centromeric regions.
Holocentric chromosomes have been studied most widely in the plant genus Luzula (family Juncaceae). The species L. purpurea has the lowest number of chromosomes which are the largest in size among those having diffuse centromeres. Endo-nuclear polyploidy is also found in these species.
Luzula spicata has three different races; one race has 12 large chromosomes (2n = 12), the other has 10 large and 4 small chromosomes (2n = 14), while the third has 24 small chromosomes (2n = 24). The total chromosomes volume is nearly comparable in the three races. The small chromosomes are believed to be formed through fragmentation of the large chromosomes of the ancestral form having 2n = 12.
In Cyperaceae, there is a large variation in chromosome number characterized by aneuploid series. The species of Carex forms an aneuploid series where gametic chromosome number varies form n = 6 to n = 66. Some of the species possessing diffuse centromeres are unique in the sense that chromosome number of germinal tissue differs from that of somatic tissue.
In Ascaris megalocephala univalens, germ line cells contain two chromosomes, while somatic cells possess 42 chromosomes that are produced through fragmentation of the germ line chromosomes.
The criteria for identification of holo-kinetic chromosomes are:
(i) Specific behaviour of chromosomes at metaphase and anaphase,
(ii) Peculiar structure of bivalents at MI, and
(iii) Chromosome fragmental have centromere activity and behave like complete chromosomes so that they are not lost.
Cell Division:
The entire body of the holo-kinetic chromosome attaches to chromosome fibres which form a sheet like appearance on the pole-ward surface. The sister chromatids are not associated at their centromeric region, and spatial separation among them is possible.
The orientation of chromatids at the mitotic metaphase is parallel to the equator; one chromatid orients towards one pole, while the other chromatid orients towards the opposite pole. At anaphase, the separated chromatids remain parallel to the equatorial plane and to each other; they do not assume a V or J shape (Fig. 7.10).
Meiosis in such species differs from that in species having mono-centric chromosomes; in addition different species with holo-kinetic chromosomes also show remarkable differences. Luzula (family Juncaceae) is representative of one meiotic type.
Meiosis is normal up to prometaphase I. At anaphase I, two chromatids move to one pole and two to other pole. The chromosomes remain associated by their ends during metaphase I while the central portions separate. Due to kinetic activity, the terminal regions are stretched out, while the central parts remain condensed.