The below mentioned article provides an overview on Chromosomes in the Nucleoplasm:- 1. Subject-Matter of Chromosomes 2. Historical Note of Chromosomes 3. Number 4. Morphology 5. Size 6. The Structure of Eukaryotic Chromosomes 7. Principal Landmarks 8. Chromosome with Diffuse kinetochore and Polycentric Chromosome.
Contents:
- Subject-Matter of Chromosomes
- Historical Note of Chromosomes
- Number of Chromosomes
- Morphology of Chromosomes
- Size of the Chromosomes
- The Structure of Eukaryotic Chromosomes
- Principal Landmarks in the Chromosomes
- Chromosome with Diffuse kinetochore and Polycentric Chromosome
1. Subject-Matter of Chromosomes:
In the nucleoplasm, a dark staining network is seen which is termed nuclear reticulum or chromatic reticulum. The threads of reticulum are formed of nucleic acid-protein complex called chromatin which is now-a-days referred to as nucleohistone.
During the interphase or metabolic phase, the condensed portions of chromatin threads (heterochromatin) can be distinguished from the less dense regions (euchromatin) in electronmicrograph of a cell.
The chromatin of interphase nucleus represents an aggregation of chromosomes. During cell division the chromatin fibres become greatly condensed and appear as chromosomes. They are invisible in metabolic nucleus due to high water content in nucleoplasm.
Chemical analyses consistently show that chromatin is composed of DNA, RNA, histones and non-histones (acidic proteins) (in sperm nuclei of some fishes and birds histones are replaced by more basic proteins, the protamines).
The term chromosome refers only to the deeply staining DNA containing filamentous bodies observed in the dividing cells of nucleate organisms. The chromosomes are nuclear components possessing special organic individuality and functions. They bear genes or hereditary units and are capable of reproducing themselves without involving any change in morphology and physiology at successive generations.
The chromosomes can be readily stained with basic dyes such as basic fuchsin (feulgen’s stain), carmine, haematoxylin, crystal violet etc.
2. Historical Note of Chromosomes:
It was W. Fleming (1879) who found that the nuclei of cells contain some material that became intensely coloured at certain time. He coined the word chromatin for the stainable fibrous material of the nucleus. The present name chromosome (Gr. Chroma = colour, soma = body) was coined by W. Waldeyer (1888) for the dark staining thread-like bodies of nucleus.
W.S. Sutton and T. Boveri (1902) suggested that chromosomes were the bearers of hereditary potentialities.
The fine coiled filamentous component of chromosome was named chromonema by Vejdovsky (1912). Morgan (1933) suggested that chromosomes were responsible for the transmission of hereditary traits from one generation to another. By 1953, it was established that the genes of all higher organisms were made of DNA.
The chromosome number simply represented the number of packages into which the DNA of a species is divided. De Robertis (1964), Setelo and Wettstein (1965), Good Enough and Levine (1974), Du Praw (1965, 1968), Solari (1965), Bradbury et al. (1981) have made good contributions on the structure and molecular organization of chromosomes.
3. Number of Chromosomes:
Every cell of the body is provided with a packet of genetic material, the nucleus which contains chromosomes. Normally, all the individuals of a species have fixed number of chromosomes in their nuclei. The characteristic group of chromosomes in a cell is spoken of as its complement.
The number of chromosomes is not necessarily sufficient for identification of species, since several species may have the same chromosome number, as for example, white squirrel, human and evening bat all have forty-six chromosomes and rice, tomatoes, pines, white oaks and grasshoppers all have same chromosome number i.e., twenty four yet they are separate species.
The chromosomes of different species do not have the same quality and quantity of DNA. Here quality means the composition and arrangement of nucleotide sequences and quantity refers to their number.
Generally, the somatic cells of animals and higher plants are diploid (i.e., they have two sets of chromosomes-2n) while their gametes, as a rule, are haploid as they contain only one set of chromosomes. The number of chromosomes present in the gamete is referred to as haploid or gametic chromosome number or ‘n’.
In diploid organisms half of the chromosomes of a nucleus are contributed by male gamete (paternal chromosomes) and the remaining chromosomes are contributed by the female gamete (maternal chromosomes) in the sexual reproduction.
The haploid set of chromosomes is also known as genome. In eukaryotes, the minimum visible complement of indispensible chromosomes, each represented only once is referred to as basic chromosome set and the number of chromosomes in a basic set is called basic chromosome number or base number. This is symbolized by ‘x’.
In diploid cells, each chromosome of one haploid set has a partner or homologue in the other set. Homologous chromosomes are those which are identical in size and carry identical and complementary genes.
The chromosome number may change in some organisms and this change may be of two kinds :
(i) Euploidy; and
(ii) Aneuploidy.
Euploidy refers to a condition where the set of chromosomes is kept balanced. If the change in chromosome number is brought about by the increase in the number of set of chromosomes beyond 2n so that the nucleus contains three (3n), four (4n) or more sets, this condition is referred to as polyploidy.
When the change in chromosome number involves loss or gain of one or more chromosomes causing the set to become unbalanced, the situation is called aneuploidy.
The different types of aneuploids are monosomies (2n – 1), trisomies (2n + 1), nullisomics (2n – 2), tetrasomics (2n + 2), double monosomics (2n – 1 – 1), double trisomies (2n + 1 + 1), etc.
In polyploid individuals which possess more than two haploid sets of chromosomes in their somatic cells, it becomes necessary to establish an ancestral primitive chromosome number or basic chromosome number (x). In common wheat (triticum aestivum), for example, with 2n = 42, n = 21 and x = 7, the somatic chromosome number is, therefore, hexaploid (6x).
In some organisms, chromosome number may vary in the cells of different tissues of the body, as for example, in mammals liver cells show chromosome numbers which may be multiple of the normal diploid number.
The number of chromosomes may vary from species to species. The lowest chromosome number in animals has been recorded in round worm (ascaris megalocephala) with 2n = 2. In mesotoma (flat worm) and ophryotrocha puerilis (polychaete), haploid chromosome number (n) is 2.
In ascaris megalocephala which has a single chromosome in the gamete, the chromosome is compound which divides into as many as 42 chromosomes in the somatic cells. In plants, the smallest chromosome number has been reported in a member of asteraceae (compositae), Haplopappus gracilis with 2n = 4. In some organisms the chromosome number may be as high as several hundred in diploid cells.
The highest chromosome number in animals is reported in aulacantha, a radiolarian, in which 2n is approximately 1,600 chromosomes. In animals chromosome numbers up to 50 are not uncommon. The higher plants, such as the angiosperms and animals such as primates, the members of the taxonomic groups often have chromosome numbers within a limited range.
In angiosperms the most frequent haploid number is 12 and the great majority of the members of this group have at an average haploid numbers 8 to 16.
Similarly, fungi also have a definite range of chromosome number, the haploid number ranging from 3 to 8. In primates, the haploid number ranges from 16 to 30.The man has 23 pairs in somatic cells. Some species of plants and animals are listed with their respective chromosome numbers in Table 9.1.
4. Morphology of Chromosomes:
The chromosomes are filamentous bodies found in the nucleus and are visible during cell division. They are linkage structures each consisting of a linear sequence of genetic information or genes.
The chromosomes of prokaryotes and eukaryotes are essentially similar in two main activities:
(i) They are concerned with the transmission of genetic information from cell to cell and generation to generation.
(ii) They are concerned with the ordered release of genetic information to control cellular functions and development. They are auto-duplicating structures of differential complexity in eukaryotes and prokaryotes and their morphology and organization are organism specific. The chromosomes of prokaryotes are generally double stranded circular DNA molecules but in some viruses they may be single stranded.
In eukaryotes they present a complex structure. Most of the chromosomes in a cell are called autosomes which carry genes for most of the body characters. Besides, there may be one, two or more chromosomes which carry genes for determination of sex as well as for sex-linked characters.
These are called sex chromosomes (as for example, X-chromosomes and Y-chromosomes). The standard chromosomes forming the chromosome complement of a cell are called A-chromosomes. In some species one or more extra chromosomes are present in addition to A chromosomes. They are called B chromosomes or supernumerary chromosomes or accessory chromosomes.
5. Size of the Chromosomes:
The chromosome size may vary in different closely related genera as well as in different species and varieties of the same genus. The chromosome size is measured normally at mitotic metaphase. They may be as short as 0.2µ in fungi and birds and as long as 30µ in some plants like Trillium.
Generally, most of the chromosomes at mitotic metaphase fall within a size range of 3.5µ in fruit-fly (drosophila) to 6µ in man and 8-10µ in maize.
The plants in general have larger chromosomes than the animals. Further a great variation may be seen in the sizes of chromosomes in different tissues within the same organism. The giant chromosomes of salivary gland in dipteran insects may attain a length of nearly 300µ and a breadth 10µ.
In amphibian oocytes, lampbrush chromosomes may be 800µ in length whereas their somatic chromosomes are only a few microns in length.
In plant mediola the root tip chromosomes are 50% longer than the shoot tip chromosomes. The organisms with less number of chromosomes have, in general, comparatively larger sized chromosomes than those with many chromosomes. Monoctyledons, in general, have larger chromosomes than dicotyledons which often contain greater number of chromosomes.
The plants, in general, have large-sized chromosomes in comparison of animals. Further, all the chromosomes in a cell may not be alike. In single set of chromosomes the largest chromosome may be as much as 50 times larger than the shortest one. On the other hand, in some organisms the size difference among the chromosomes of the set may not be well marked and all the chromosomes are of about the same length.
6. The Structure of Eukaryotic Chromosomes:
The detail structure of chromosome varies from cell to cell and major changes are associated with cell division. Regardless of differences in detailed morphology, both direct and indirect evidences make it clear that each chromosome is an individual entity which, barring accidents, is retained throughout the life of the cell.
The detailed morphology of the chromosome is best studied during cell division, particularly at metaphase and anaphase stages when the chromosomes are in maximum contraction state. The morphology of chromosomes differs from phase to phase in the continuous process of cell growth and division.
During the interphase, when cells are not dividing, the nucleus has a granular appearance because the chromosome material or chromatin remains dispersed throughout the nucleus.
At the time of cell division the chromosome material becomes highly condensed and appears in the form of delicate, contractile, poorly stainable interwoven thread. In the initial stage of cell division they gradually become thick and filamentous and show distinct individuality.
When completely differentiated, each chromosome consists of two chromatids held together at a point along their length. The point at which the two chromatids of a chromosome are held together appears as a constriction this is primary constriction or kinetochore or centromere.
In the metaphase chromosome the centromere stains lighter than other segments. The centromere divides the chromosome into two parts, each called a chromosome arm. In some of the chromosomes there may be additional constriction, the secondary constriction which indicates the position of nucleolar organiser region (Fig. 9.1).
Thus the structures present in the chromosome include:
(i) The chromonema,
(ii) Primary constriction,
(iii)Sedcondary constriction and nucleolar organiser region, and
(iv) Telomeres.
The Chromonema:
Each chromosome is formed of two identical spirally twisted delicate threads which lie so close to each other that it is difficult to see them separately by ordinary microscope (Figs. 1.43 A and B). These two sub-units are called chromatids. If the chromosomes are treated with trypsin to remove proteins, each chromatid is seen to consist of two or more sub-units.
These are termed chromonemata (singular- chromonema) by Vejdovsky (1912). The question as to how many such chromonemata are there in a chromatid has long been a controversial matter. Some workers have suggested that a chromatid may be composed of two chromonemata while others have suggested a strong possibility for more than two chromonemata.
The number of threads may vary in different phases of cell division because at one stage the chromatid may contain one thread and at the other phase it may contain two, four or more chromonemata. The chromonemata may remain coiled with each other.
The coils may be of the following types:
(i) Paranemic coils:
When the chromonemal fibrils are coiled in such a way that they are easily separable from each other, such coils are called paranemic coils (Fig. 9.2).
(ii) Plectonemic coils:
When the chromonemal threads are intertwined in such a way that they cannot be separated easily. This type of coiling is referred to as plectonemic coil (Fig. 9.2).
The chromonemata are embedded in the achromatic substance called the ‘matrix” which, in turn, is enclosed in a sheath or interface. Both matrix and sheath are non-genetic substances which appear when nucleolus disorganizes during the prophase. Electron microscopic studies have denied the occurrence of membranous pellicle surrounding chromosome.
The chromonema of a chromosome appears like a fine string or thread bearing a number of granular bodies. The thick bead-like structures of the chromonema were named chromomeres by Balbiani (1875). The position of chromomeres in the chromonema is found to be fixed for a given chromosome.
The cytologists have given various interpretations regarding chromomeres. A chromomere is supposed to be formed due to dense coiling of chromonema. This view has been confirmed by the electron microscopic observations. Some other cytologists believe that chromomeres are formed due to heavy accumulation of chromatic substance.
The early workers were of the opinion that chromomeres represented the sites of genes (i.e., the units of heredity) but it is no longer supposed to be always true as the genes may be located on the extrachromomeral or interchromomeral regions of chromonema.
7. Principal Landmarks in the Chromosomes:
The principal landmarks which may be seen in the chromosomes at metaphase are:
(i) Primary constriction or centromere or kinetochore to which spindle fibres or microtubules are attached, and
(ii) In some chromosomes one or more secondary constrictions. The position of centromere is fixed in a given chromosome. The primary constriction or centromere stains lighter with feulgen stain (basic fuchsin) than the other parts of chromosome, especially during metaphase and anaphase.
It indicates that DNA extends through this region. The presence of DNA has been demonstrated in the kinetochores of several organisms, that forms centromeric heterochromatin.
The ultrastructure of centromere has been difficult to resolve because of certain limitations in the preparation techniques. Recent observations with electron microscope have revealed some structural details of centromere. There may not be a common pattern of structure in plants and animals.
In a metaphase chromosome the kinetochore consists of two or more centromeric chromomeres of varying sizes and interchromomeral fibrillae.
The fibrillae are strands of chromonemata and their number in a kinetochore varies somewhat, but usually only a few are visible. In some of the kinetochores centromeric chromomeres may not be visible and only thin fibrillae can be seen in them. In most of the kinetochores there is a definite symmetry in the arrangement of fibrillae and chromomeres.
There is a central component (the interior zone) consisting usually of one or two chromomeres and the arrangement of chromomeres and fibrillae is same on both the sides of central component. In some of the metaphase chromosomes four granules or chromomeres may be seen within the kinetochore and they are arranged in a square.
During anaphase when the two chromatids separate, each of them becomes daughter chromosome.
Previously it was believed that the centromere duplicated sometime during the metaphase but it is now clear that the centromere is actually duplicated before metaphase of mitosis and anaphase I of meioses in most of the organisms and the chromatids are held together not by a single kinetochore but by an adhesion of chromatids.
The centromere in the chromosome of Chinese hamster consists of a dense core surrounded by a wider less dense zone.
The dense core is composed of axial fibrils which are 50-80 Å thick and cohelically coiled. The less dense zone shows several microfibrils which loop out on the outer surface (pole-ward) of each chromatid at metaphase. The loops disappear after the chromosomes have reached to the spindle poles.
During cell division, kinetochore participates in the organization of microtubule units into spindle fibres and, in fact, its role in the movement of chromosomes during anaphase can be considered its principal function. Some investigators have suggested that the chromosome movement is at least partly autonomous and that the centromere simply provides force for the movement.
The position of centromere is fixed in a particular chromosome. Levan, Fredga and Sandberg (1964) have proposed a nomenclature for centromeric position on chromosomes.
Normally, the chromosomes possess a single centromere i.e., they are monocentric. But, in some instances, the chromosome may have two centromeres (i.e., dicentric chromosome) or many centromeres (i.e., polycentric chromosome). Sometimes the chromosome may be devoid of centromere. Such a chromosome is said to be acentric.
The position of centromere may vary in different chromosomes. The parts of a chromosome on each side of centromere are called arms. The centromere plays important role in directing the chromosome movement during cell division.
According to the positions of centromere and relative arm lengths, the monocentric chromosome may be designated as follows (Fig. 9.3):
1. Metacentri:
In this the kinetochore is located at or near the mid-point (median) so that the arm ratio is 1 : 1 or nearly so. Such chromosomes are V shaped at anaphase stage.
2. Submetacentric:
When the centromere is located slightly away from the mid-point so that the two arms are unequal, its position is said to be submedian and the chromosome is called submetacentric such a chromosome appears L shaped during anaphase.
3. Acrocentric:
When the centromere is located away from the mid-point and is found near one end of the chromosome, its position is said to be subterminal and the chromosome is called acrocentric. Acrocentric chromosomes appear rod shaped or j-shaped during anaphase and have one long arm and the other very short arm.
In submetacentric and acrocentric chromosomes one arm is longer than the other. The shorter arm is also designated ‘p’ arm (French word petite meaning small or short) and the longer arm is termed ‘g’ arm.
4. Telocentric:
When the centromere is located at one end of the chromosome it is said to be terminal. The chromosome with terminal centromere is called telocentric. Such a chromosome is characterised by a single arm. Telocentric chromosomes appear rod-shaped during anaphase.
Telocentric chromosomes are found only rarely. They have been reported by Marks (1957) in some species of plants, protozoa and mammals. Truly, telocentric chromosomes are sometimes unstable.
Isochromosome:
Isochromosome is a chromosome in which the two arms are identical. It arises by an error in the plane of centromere division. Occasionally one chromatid of the telocentric chromosome after duplication of arm may swing around the centromere to produce isochromosome with two identical arms.
Normally, the centromere divides in a plane parallel to the longitudinal axis of the chromosome but in some cases the division of centromere occurs at right angle to the chromosome axis (mis- division) which may result in the formation of two telocentric chromosomes (Fig. 9.3), each of which is relatively stable and functional because of its kinetochore construction.
Such mis-division of kinetochore has been observed in pisum, fritillaria, and triticum and at least in one animal, the grasshopper microstethus.
These form categories are not sharply distinct but grade imperceptibly into each other. Whatever its position, the centromere has a fixed position in each chromosome and maintains its position throughout the life time of chromosome and from generation to generation. Thus a chromosome can be recognized partly by position of its centromere.
Dicentric chromosomes:
In some instances, chromosomes with two centromeres may be formed. They are called dicentric chromosomes. Dicentric chromosome may be produced in inversion of heterozygotes as a result of pairing between structurally different homologous chromosomes and subsequent crossing over between the non-sister chromatids.
If the two centromeres of a dicentric chromosome move to the opposite poles of the spindle during anaphase, the chromosome breaks. Rarely, a new additional centromere may appear on the monocentric chromosome resulting in an abnormal dicentric chromosome. Such a centromere is called neocentromere. The occurrence of neocentromere has been reported in zea mays.
8. Chromosome with Diffuse Kinetochore and Polycentric Chromosome:
In some animals for example, coccids (homoptera) and monocotyledons (juncales and cyperales), there is no single localized centromere and the chromosome is provided with more than one centromere, i.e., the kinetochore is diffused along the entire length of the chromosome.
Such chromosomes are called holokinetic chromosomes (Fig. 9.4).
According to levan’s nomenclature diffuse kinetochore is called lateral centromere. The chromosome with diffuse kinetochore behaves as if the entire chromosome were a kinetochore.
During cell division, spindle fibres are associated all along the length of chromosome and each member chromatid moves to its respective pole in a plane perpendicular to the axis of the chromosome. The two chromatids separate from each other from their entire length at once.
The polycentric chromosome often resembles chromosome with diffuse kinetochore in behaviour as in coccids. The polycentric chromosome differs from the chromosome with diffuse kinetochore in only one respect that it regularly fragments into smaller, stable functional chromosomal units.
The polycentric chromosomes are found in parascaris univalens (ascaris megalocephala univalens) in which gametes contain only a single chromosome. After fertilization 2 chromosomes of somatic cells of embryo fragment and give rise to many small functional chromosome units. Somatic tissues in this worm, frequently have as many as 42 chromosomes per cell.
Acentric chromosome:
Sometimes chromosome may break into two pieces so that only one part retains the centromere and the other is devoid of centromere. The chromosome part lacking in centromere is said to be acentric. Since acentric chromosomes do not have centromeres, spindle fibres are not attached with them and so they are unable to move to the poles during anaphase.
Such a chromosome is usually lost from the cell and may result in deficiency and sometime lethality.
Secondary constriction:
Besides the primary constriction or centromere, the arm of one or more chromosomes may show one or more constrictions called secondary constrictions. Since such constrictions are constant in position and extent, they are useful in identifying particular chromosomes in a set.
Secondary constriction may be short or long. It is often associated with nucleolus during interphase and may take part in the reorganization of nucleolus during telophase stage of cell division.
For this reason, such a secondary constriction is called the nucleolus organizing region or secondary constriction I. In some cases, nucleolus organizer exists at the terminal position in the chromosome but go undetected cytologically because of its small size.
Under the light microscope the nucleolus organizer appears as a constriction near one end of the chromosome. Normally, in each diploid nucleus there may be two chromosomes with nucleolus organizing region but in some diploids and polyploids, the number of nucleolar organizers may be more than two. The part of chromosome extending beyond nucleolus organiser is very short and appears like a dot or satellite.
This extension is satellite body. Chromosomes bearing satellites are called SAT-Chromosomes. Satellite bodies vary in size according to the position of the secondary constriction. The satellite may be minute dot-like in many cases but in some cases large satellite may be seen as for example, in Vicia faba which may be connected to the main body of chromosome by very light-staining strands (Fig. 9.5 A).
Observations of human chromosomes have revealed that five pairs of chromosomes, numbered 13, 14, 15, 21 and 22 have nucleolus organizing regions.
In a few cases a single chromosome has two secondary constrictions. The secondary constriction other than nucleolus organizer is referred to as secondary constriction II. The location of secondary constriction II is fixed for a given chromosome and is a diagnostic characteristic of the particular genome. It has been suggested that secondary constriction represents the site of breakage and reunion in chromosome.
Telomeres:
The ends of a chromosome are called telomeres. The telomeres behave differently from the interstitial portions of chromosome. Each telomere has polarity and it prevents the ends of the chromosomes from attaching to each other. A broken or fractured chromosome will not attach to a telomere unless that telomere is also fractured.
If the ends or telomeres are broken off either spontaneously or by induction with x-rays or Gamma-rays they will not unite with each other and may disappear from the nucleus in subsequent cell divisions.
The cut end of the chromosome is unstable and may unite with broken end of other chromosome resulting in change of morphology. Thus the telomeres maintain the individual integrity of each chromosome. They are specially modified for attachment with nuclear envelope. The ends of the chromosomes are usually found associated with the nuclear wall during interphase and early prophase.
Ring chromosome:
When the two cut ends of a chromosome unite, the ring chromosome is formed (Fig. 9.5 B).
Karyotype:
The term karyotype usually refers to the general morphology of a set of chromosomes at somatic metaphase of an individual. In karyotypic studies pairs of homologous chromosomes of a cell are arranged in a decreasing series. (Fig. 9.6).
When the karyotype of an individual is represented diagrammatically in the form of histogram showing all the morphological features of chromosomes it is called an idiogram or karyogram (Fig. 9.7). It is now well recognized that most of the species possess a definite chromosome individuality.
The morphology of chromosomes of an individual includes the following:
(i) The number of chromosomes in the set,
(ii) Relative lengths of chromosomes,
(iii) Arm ratios (i.e., ratio of the lengths of long arm and short arm as determined by the position of the centromere),
(iv) The position of secondary constriction, nucleolus organisers and satellite, and
(v) the staining reaction and differentiation of bands of euchromatin and heterochromatin in the chromosomes.
In animals the, karyotypes are generally different in males and females owing to the presence of X and Y chromosomes (sex chromosomes). Excepting the cases with chromosomal aberrations, the karyotypes for the autosomes (i.e., the set of chromosomes other than sex chromosomes) are similar within the species but differ from species to species.
Since the closely related species have, more or less, similar karyotypes and distantly related species have variable karyotypes, a comparative study of karyotypes is of great value in determining the taxonomic relationship among, the species.
The differences between the absolute chromosome sizes of related species or genera probably reflect the difference in the amount of gene duplication. Until recently the chromosomes were grouped into only seven groups, designated A, B, C, D, E, F & G and ordered according to their sizes. Within each group the chromosomes were almost indistinguishable morphologically.
But now the staining techniques (called banding techniques) have been developed that give rise to specific patterns of bands along the chromosomes. Such chromosome banding studies have opened a new way that serves to differentiate each chromosome in the karyotype.
The Russian school of comparative study of karyotes under the leadership of G. Levitzky (1931) has developed the concept of karyotype Symmetry.
According to this concept, the karyotypes may be of two types:
(i) Symmetrical karyotype:
In this, all the chromosomes of a set are approximately of the same size and have median or sub-median centromeres.
(ii) Asymmetrical karyotype:
It is a heterogeneous karyotype exhibiting two main characteristics:
(a) Good degree of variation in the chromosomes size;
(b) Variation in the position of centromeres of the chromosomes i.e., asymmetry occurs through the shifting of centromere position from median to sub-median to sub-terminal to terminal. In general, symmetrical (homogeneous) karyotypes are regarded as primitive and asymmetrical (heterogeneous) karyotypes as specialized and advanced.
Supernumerary or B-Chromosomes:
Many species of plants and animals contain, in addition to normal constant complement of A-Chromosomes, a variable number of minute and usually heterochromatic chromosomes. These are called supernumerary chromosomes or extrachromosomes, or accessory chromosomes or B-chromosomes.
B-chromosomes are distinguished from the other smaller chromosomes of the normal complement in their staining properties. In some cases, however, the distinction on the basis of staining reaction seems impossible. In tradescantia and trillium, for example, supernumeraries appear to be largely euchromatic and in maize they contain both kinds of chromatin.
B-chromosomes were first discovered by Wilson (1905) in hemipteran insect and by Randolph (1927-28) in maize. Subsequently, their presence in higher plants was reported by Darlington (1937), Hakansson (1945), Fernandes (1946), Melander (1950) and many others.
The occurrence of B-chromosomes has been reported now in more than 375 families. B-chromosomes differ from normal or A-chromosomes in the following respects.
1. Morphology:
They are usually much smaller in size than the smallest A-chromosome. In gross morphology most of the accessory chromosomes are acrocentric or telocentric and those which are metacentric are usually, if not always, isochromosomes in which the two arms are genetically identical.
Accessory chromosomes are frequently and not exclusively heterochromatic. In tradescantia and trillium, B-chromosomes appear to be largely euchromatic and in maize they contain both kinds of chromatin (Figs. 9.8 A and B).
2. Genestic effectiveness:
Accessory chromosomes normally do not strongly influence the phenotype and viability of the organisms carrying them.
3. Numerical variation:
The number of accessary chromosomes may vary in different cells, tissues, individuals, and populations.
4. Meiotic behavior:
The B-chromosomes do not pair with A-chromosomes and there is no chiasma formation. They show lower degree of pairing among themselves, early separation from each other at late prophase I and metaphase I, lagging during anaphase and elimination or preferential non-disjunction during meiosis and post-meiotic divisions (Fig 9.8).
5. Mitotic behavior:
During mitotic division of cell, there may be lagging or elimination of B-chromosomes. They may also show polymitosis or preferential distribution.
According to White (1945), there are two types of B-chromosomes:
(i) First types which are mitotically stable and all the cells of carrier organisms have the same number of B-chromosomes.
(ii) Second type which are mitotically unstable, giving rise to different numbers of B-chromosomes in different cells of some individual.
Accumulation systems of B-chromosomes are different but normally fall into two categories:
1. Accumulation system in which B-chromosome undergoes distorted segregation at meiosis and is thereby preferentially transmitted to gametes.
2. Accumulation system in which B-chromosome undergoes meiotic non-disjunction in some particular direction or tissues.
Although no example is known of accessory chromosomes affecting the external morphology of carrier plants, they are known to affect the over-all vigour of the plants as well as their pollen fertility.
Vegetative vigour is usually reduced by high numbers of accessory chromosomes such as 10 or more in Zea mays, but in some species such as cultivated rye (secale cereale) a marked decrease in vigour occurs in plants which have one or two B-chromosomes.
Generally polyploids are much less likely to contain accessory chromosomes than their diploid relatives. Pollen fertility is more sensitive to the presence of accessories. In sorghum purpurea and seriseum any increase in the number of accessories causes reduction in pollen fertility.
Favourable effects of accessory chromosomes may also be seen in some cases. A special type of accumulation system of B-chromosomes in population results in a tendency for the B-chromosomes to increase in frequency in males and to decrease correspondingly in females. In festuca pratensis vigour may be increased by the presence of one or two B-chromosomes.
In diploid species of A- chillea, A.asplenifolia and A.setacea, plants having two B-chromosomes are vigorous and more fertile than plants lacking them altogether.
In cross A.asplanifolia XA setacea though not in reciprocal, F1 plants could be obtained only when the female parent contained accessories. In this case, the presence of accessories in the stylar tissue of A. asplanifolia apparently promotes the growth of the pollen tube of A setacea. Plants of these species or their hybrids which contain uneven or high number of B-chromosomes are relatively infertile.
It is generally believed that B-chromosomes have adaptive importance, since their occurrence and frequency within the species vary in the populations of different origin and habitats.
In some instances, as in festuca pratensis, centaurea scabiosa and lilium medeoloides populations containing B-chromosomes exist in restricted regions of geographic range of the species which is associated with the specific factors of environment or population structure.
In Sweden, populations of festuca pratensis having B-chromosomes occur chiefly in regions of highly clay soils.
A few workers have suggested that B-chromosomes might be parasitic and maintain more or less independently of any adaptive benefit or handicap.
The origin of B-chromosomes is not known, yet it is presumed that they represent centromere containing heterochrometic segments of normal chromosomes. This means that supernumerary chromosome might have originated from small fragments caused by the structural rearrangement and might have eventually become as large as other chromosomes by repeated duplication.