The following points highlight the five main types of trisomics which is a cell tissue or individual possessing the extra chromosome. The types are: 1. Primary Trisomics 2. Secondary Trisomics 3. Telocentric or Telosomic or Telotrisomics 4. Tertiary Trisomics 5. Compensating Trisomics.
Contents
Type # 1. Primary Trisomic:
In this type of trisomic, the extra chromosome is normal and completely homologous to one pair of homologues in the chromosome complement. Each chromosome exerts a separate effect on the phenotype of the plant and therefore, trisomics for different chromosomes can be identified.
In Daturastramonium, primary trisomics (2n + 1 = 25) for each chromosome were distinguished according to the differences in capsule size and shape; size and length of spines; size of plants; growth habit; size, shape and forms of leaf, flower and stigma. Trisomic for each chromosome in Datura was given a separate name by Blakeslee.
They are named as rolled:
(1) Glossy
(2) Buckling
(3) Elongate
(4) Echinus
(5) Cocklebur
(6) Microcarpic
(7) Reduced
(8) Poinsettia
(9) Spinach
(10) Globe
(11) Ilex
In barley (Hordeumvalgare), primary trisomics (In + 1 = 15) for different chromosomes were named as bush (1) slender (2) pale (3) robust (4), Pseudo-normal (5) purple (6), and semi-erect (7).
Primary trisomic series has been established in other crops also such as, Pennisetum, tomato, and rye. Morphologically, primary trisomics can be identified in several plant species. However, in certain plant species, differences between trisomics for different chromosomes are not clearly distinguishable from each other, e.g., in Clarkia, Triticum and maize.
Origin and Sources of Primary Trisomics:
Trisomics are produced when a gamete containing an extra chromosome (n + 1) is fertilized by a normal (n) gamete. Followings are the kinds of origin and sources of primary trisomics.
(1) They occur in the progeny of auto-polyploids, such as, triploids, tetraploids etc. Auto-triploids are the best source of trisomics which may be produced in selfed auto- 3x or in auto-3x x 2x crosses.
(2) They may occur in the progeny of normal diploids. Gametes containing an extra chromosome (n + 1) may be produced due to nondisjunction, and their fertilization with normal gamete will produce primary trisomic. Due to nondisjunction, n + 1 type of gametes are formed which produce trisomics after fertilization with n type of gametes.
(3) Asynaptic and de-synaptic mutants are good sources of trisomics.
(4) They also occur in the progeny of tetrasomics (2n +2). The egg with n + 1 chromosomes is viable in many species but the pollen with extra chromosome is generally nonfunctional. In this case, the egg with n +1 chromosomes produces trisomic after fertilization with n type of male gamete. High frequency of trisomics may be produced by crossing of tetrasomic plant with a diploid plant.
(5) Trisomics have been obtained by using ionizing radiations.
(6) Primary trisomics have been reported to be produced after treatment with colchicine and other chemical mutagens.
(7) Primary trisomics may also be produced in the progeny of interchange heterozygotes. They may be called as “primary trisomic interchange homozygotes” and “primary trisomic interchange heterozygote”.
Meiosis in Primary Trisomics:
There are three homologous chromosomes in a trisomic and they synapse during meiosis. Different pairing configurations are observed based on points of initiation of pairing (Fig. 16.11).
(a) Single point pairing initiation results into one bivalent and a univalent; partner exchange is not possible.
(b) Pairing may initiate at two points far from each other; partner exchange may occur and a trivalent may be formed.
(c) There may be more points of initiation of pairing and partner exchange may occur at several points leading to a high frequency of trivaients.
Trivalents form different shapes depending upon position of chiasmata. They may take rod shape, frying pan shape, Y-shape, J-shape, V-shape and zigzag shape, and others (Fig. 16.11).
At AI, the extra chromosome may go to one pole producing (/i + 1) and (n) types of gamete. In barely trisomics (2n + 1 = 15), several kinds of separation, such as, 8-7, 9-6, 8-1-6, 7-1- 7 etc. have been reported. The Y-, V- and zigzag shaped trivalents segregate in 2 : 1, while the middle chromosome in rod shape and the chromosome projecting outside in the frying pan shape lag and remain at the equatorial plate.
The univalents also may lag behind.
Misdivision of centromere:
Sometimes the lagging chromosome behaves like a mitotic chromosome at AI and the spindle fibre is attached on both sides of the centromere. It divides into two chromatids at AI (precocious centromere division), and both poles receive one chromatid in addition to other chromosomes. However, this type of division takes more time as compared to division of the bivalents into chromosomes.
In several organisms, the centromere of the univalent orients and misdivision of centromere occurs resulting into two chromosomes with one arm only. The misdivision of centromere may be in such a way that one arm receives full or the major part of the centromere and the other arm receives a very small or no part of the centromere and it is nonviable.
The viable chromosome part becomes a telocentric which divides at All normally, and the chromatids are included in the daughter nuclei. The two chromatids may not separate but unite at the centromere and a chromosome may be produced with two similar arms; such a chromosome is called iso-chromosome (Fig. 16.2).
In case of precocious centromere division, the single chromatid at All may be lost in cytoplasm. But sometimes the centromere of the single chromatid at All may divide into two telocentric chromosomes which are included into the daughter nuclei.
Transmission and Breeding behavior:
The gamete containing an extra chromosome (n + 1) is sterile or nonfunctional due to chromosomal dis-balance. In some cases, they may be functional but pollen competition restricts the transmission of the extra chromosome. In barley, the extra chromosome is not transmitted through male, but in several species, such as, maize, Daturastramonium, they are occasionally transmitted through pollen.
There is frequent transmission of the extra chromosome through female. The expected frequency of trisomics in the selfed progeny is 50%, but the observed frequency is quite low. The frequency of trisomics varies in different species. Within the same species, it varies according to the chromosome (Table 16.6).
In Daturastramonium, it was the lowest (2.96%) in trisomic for chromosome 19.20 and the highest (32.53%) for the chromosome 23.24. In tomato, the frequency of trisomics in the selfed progeny of trisomics ranged from 8.3% (for chromosome 2) to 25.6% (for chromosome 5) with an average of 20.4%.
In barley, Tsuchiya in 1960 found the highest frequency of 2x + 1 types for chromosome 1 (31.4%) and the lowest frequency for chromosome 6 (19.3%) in the selfed progeny of trisomics.
In general, the rate of transmission of extra chromosome in the selfed progeny of trisomics varies between 20 to 30% for different chromosomes of different plant species. In the progeny of primary trisomics, secondary trisomics and unrelated trisomcs (trisomics for other chromosomes) also appear.
Unrelated trisomics are produced due to the effect of the trisomic on the separation of the other chromosomes leading to nondisjunction. Secondary trisomic is produced due to misdivision of centromere of the lagging extra chromosome.
Genetic Segregation:
For a particular locus, say Aa, there are four genotypes possible in a trisomic:
(i) AAA, triplex,
(ii) AAa, duplex,
(iii) Aaa, simplex and
(iv) aaa, nulliplex.
The gametes and their ratios in a duplex individual (AAa) are 1AA : 2Aa : 2A : la. If all the gametes function on both sides, the constitution of the progeny will be as shown in the Fig. 16.12.
(i) If all the gametes function, the ratio of the phenotypes A : a will be 35 : 1, the recessive phenotype (a) being only 2.86%.
(ii) If n + 1 type gametes do not function on male side the phenotypic ratio (A : a) will be 17 : 1, the recessive phenotype being 5.88%.
(iii) If n +1 type gametes do not function on both sides, the ratio will be 8 : 1, the recessive being 12.5%.
The test cross ratio is 5A : la, and the recessives are 16.6%. Data on trisomic segregation in barley are presented in Table 16.7. In the progeny of trisomics, there is a low frequency of recessives. But the disomics produce 25% recessive plants for the particular gene.
Using the above principles, trisomics have been utilized for locating genes on particular chromosomes and assigning linkage groups in several plant species, such as Datura, Antirrhinum, maize, barley, tomato and Petunia.
Method of gene location:
(1) The mutant is crossed to trisomics for all the chromosomes.
(2) In F1, trisomic plants are selected through cytology and morphology, and they are back crossed to homozygous recessive plants.
(3) The test cross progeny are analysed genetically for segregation of the character. A deviation from the 1A : la ratio will indicate that the mutant gene was located on that particular chromosome for which it was trisomic.
Double reduction:
Occurrence of sister alleles in the same gamete is called double reduction. It causes an increase in the frequency of recessive gametes. The process of double reduction is shown in the Fig. 16.13. The figure shows a duplex (AAa) cell, and crossing over has occurred between the centromere and the locus in question (Aa).
Due to crossing over, both the chromosomes have one dominant allele and one recessive allele on the chromatids. For 1/3 times, these two chromosomes move to the same pole, the other pole will receive one chromosome carrying dominant alleles on its chromatids in the first meiotic division.
At All, there are two possibilities:
(i) Sister alleles go to different poles and are included in different gametes, in half of the cases.
(ii) (In half of the cases), sister alleles aa and AA will go to same pole and are included in the same gamete (one gamete).
Type # 2. Secondary Trisomic:
When the extra chromosome is an iso-chromosome, the aneuploid is called secondary trisomic; its formula is “2n + iso”. One chromosome arm is represented four times in the secondary trisomic (Fig. 16.10, 16.14).
Origin:
Isochromosome is produced by misdivision of centromere (Fig. 16.2).
Secondary trisomics are obtained in two ways:
(i) In normal diploid plants, occasionally secondary trisomics arise from the occasional univalents. They are more frequent in the progeny of the plants with one or more univalent chromosome,
(ii) They occur in the progeny of primary trisomics.
Univalent chromosome may produce iso-chromosome by misdivision of centromere. Each chromosome may produce two iso-chromosome, one for each arm; thus for each primary trisomic, two types of secondary trisomics are possible (Fig. 16.10). Thus in Datura (2n – 24), 12 primary and 24 secondary trisomics are possible. The Datura secondary trisomics mare morphologically different from each other as well as from normal disomics and primary trisomics.
Blakeslee and Avery identified 14 secondary trisomics in Datura and named them as, polycarpic (1.1), leaf (2.2), smooth (3.3), strawberry (5.5), areolate (6.6), undulate (7.7), mutilated (9.9), thistle (10.10), wedge (11.11), marbled (13.13), mealy (14.14), acalloped (15.15), dwarf (17.17) and di-regent (14.14).
The numbers represent the iso-chromosomes of normal chromosomes given in Table 16.6. Secondary trisomics have also been reported in other plants such as, maize, tomato, wheat, oats and barley.
Chromosome Pairing:
In secondary trisomics, chromosome configuration may be a trivalent or “bivalent + univalent” based on chromosome pairing and chiasma formation,
(i) Both the normal homologues pair to form a bivalent, while both arms of the iso-chromosome pair together to form a U-shaped univalent,
(ii) Each arm of the iso-chromosome pairs with a normal homologue to form a trivalent. Trivalents are of different shapes depending on the number and position of chiasmata. Six different configurations are possible in a secondary trisomic (Fig. 16.14). However, the diagnostic configurations are,
(a) ring of 3 chromosomes
(b) U-shaped ring formed by the univalent.
Transmission and Breeding Behavior:
Progeny of secondary trisomics consist of normal (2n), secondary trisomic and primary trisomic individuals. Unrelated primary and secondary trisomics may also occur, but with a low frequency. Transmission of iso-chromosome varies according to the chromosome arm involved. In Datura, Blakeslee and Avery found that the transmission rate of the secondary trisomic in the selfed progeny ranged from 2.45% for (2n + 1.1) to 31.12% for (2n + 5.5) secondary trisomics. The average transmission rate of the 14 secondary trisomics was 18.28%.
Segregation:
Secondary trisomics possess four homologous arms, two arms of normal homologues and two arms of the iso-chromosome. Thus it becomes a tetrasomic for the particular arm. Therefore, the segregation ratio is different from that of primary trisomics.
There are two situations regarding the alleles in the secondary trisomic:
(i) The iso-chromosome carries the dominant alleles (A), while the normal chromosomes carry the recessive allele (a). In case of random chromosome assortment, the gametes will be 2AAa : 1AA : 1 aa : 2a. and the test cross ratio will be 1 : 1. In the absence of crossing over, the diploid progeny will show recessive phenotype. If crossing over occurs, the dominant allele (A) will be transferred to normal chromosome and the diploid progeny will show dominant phenotype (Fig. 16.15).
(ii) The recessive allele (a) is located on the iso-chromosomes, while the dominant allele (A) is located on the normal chromosomes. In this case, no recessive plant will be produced in the progeny. The dominant allele may be transferred to iso-chromosome by crossing over (Fig. 16.15).
Uses of Secondary Trisomics:
1. Secondary trisomics can be used in chromosome mapping. They will enable to determine the location of gene on the particular arm of the chromosome and location of centromere position.
2. They may be used to know the method of action of certain genes.
Type # 3. Telocentric Trisomic:
An individual with a normal chromosome complement plus an extra telocentric chromosome is called telotrisomic or telosomic trisomic or telocentric trisomic (Fig. 16.10); the formula is “2n + t”. Thus a telocentric fragment chromosome is homologous to one arm of a chromosome pair in the standard complement.
In wheat nomenclature, it is called mono-telotrisomic. In barley, the telotrisomics are designated by the number of chromosomes involved, followed by the letter S or L to indicate the short or long arm involved. If the long arm of chromosome 2 is involved, it will be written as “telosomic 2L”. Such trisomics are designated as Triplo 1L, Triplo 2L, Triplo 2S, and so on.
Origin:
Telocentric trisomics are produced by misdivision of centromere (Fig. 16.2). Such trisomics occur occasionally in the progeny of normal plants, but they occur more frequently in the progeny of plants with one or more univalent chromosomes. In barley, telocentric plants have been isolated in the progenies of auto-triploids, primary trisomics and other trisomic types. They have been reported in several plant species such as, barley, maize, Datura, tomato, and wheat.
Phenotypic Effect:
Phenotypic effect of telocentric trisomics is less pronounced than that of primary and secondary trisomics. In barley, telotrisomics of long arm usually show the characteristics similar to those of primary trisomics for the same chromosome, but the telotrisomics for the short arm resemble normal diploids or show less pronounced morphological characteristics.
Cytology:
Telocentric trisomics are identified by Karyotypic analysis and by studying Giemsa-C banding pattern or Giemsa-N banding pattern. Singh and Tsuchiya found that telocentric chromosomes in barley contain half of the centromere. Thus stability of telocentric chromosome in barley does not depend upon completeness of the centromere. Certain telocentric chromosomes are not stable in the somatic tissues and they produce diploid tillers occasionally.
Telocentric chromosome fragment can pair with the homologous arm of the normal chromosome and may form trivalent (Fig. 16.16). If unpaired or if there is no chiasma formation after pairing, the telocentric remains as a univalent. Trivalents are of different shapes such as, Y- shaped, rod shaped, frying pan shaped and zigzag at MI. Ring trivalent is not possible in telotrisomics.
Transmission and Breeding Behavior:
Telocentric fragment chromosomes have less deleterious effects as compared to entire chromosome or iso-chromosome. The small size of telocentrics reduces the chance of chiasma formation and therefore, there is less recovery of these chromosomes in the progeny.
In barley, average transmission rate of the extra telosome is about 31% which is higher than the rates reported for primary trisomics (Table 16.6). The rate of transmission of the telocentric through male is low ranging from 0.0 to 3.2%, with an average of 1.2%.
Theoretical ratios in telptrisomic analysis are different from primary trisomics. The genes located on the disomic portion will show disomic segregation ration (3 : 1). In the trisomic portion, the genes will show trisomic ratios, based on random chromosome and random chromatid segregations.
In case of a duplex genotype (AAa), where the telosome and one normal chromosome carry the dominant allele (A), while the other normal chromosome carries the recessive allele (a), random chromosome assortment will show the segregation ratio typical for a primary trisomic, i.e., all A phenotypes in the trisomic portion and 3A : 1a in the diploid portion in F2.
In case of random chromatid assortment, the gametic ratio in the AAa genotype will be 11AA : 12Aa : 1aa : 7A : 5a. If the hyperploid gametes do not function on male side, the segregation ratios will show 17.4% recessives in disomic portion and 1.74% recessives in trisomic portion. Telotrisomic analysis will provide information on the order of genes in the linkage map of the particular arm. Multiple marker stock is required in such studies.
When the dominant allele (A) is located in the telosome (extra chromosome), and its recessive allele (a) is located in the homologous normal chromosomes (simplex, Aaagenotye), the test cross progeny will show recessive phenotype in the diploid portion, while dominant phenotype in the trisomic portion.
In the other situation, i.e., telosome carries the recessive allele, and the normal chromosomes carry dominant allele, the test cross progeny will show dominant phenotype in both the diploid and trisomic portions. An example of test cross in maize telocentric trisomic studied by Rhoades in 1936 is presented here.
The trisomic was telotrisomic for short arm of chromosome 5 and could be identified on the basis of morphological characteristics with short and broad leaves. The trisomic carried the recessive allele bm (brown midrib) in the two normal chromosomes and the dominant allele Bm in the telosome.
On the disomic part of the normal chromosome, gene Prpr was present in heterozygous condition (pr for purple aleurone). When this trisomic was crossed to a recessive plant (bmbmprpr), the progeny consisted of 94 Pr and 99 pr plants, showing a segregation ratio of 1: 1.
Segregation for the other gene was 1 Bm : 171 bm in diploid portion, while 85 Bm : 0 bm in telotrisomic (2n + telo) portion, the overall segregation being 86 Bm : 171 bm, i.e., 1 : 2 ratio. In primary trisomics, this type of cross is expected to produce 1 : 1 ratio. Transmission of telotrisomic in maize is about 30%. The ratio of 1 Bm : 2 bm is expected when two normal chromosomes always paired and telocentric lagged in 1/3 of the meiocytes. Crossing over between bm and the centromere is expected to produce the plants with Bm gene in the diploid progeny.
Uses of Telocentric Trisomics:
(1) Telotrisomics can be used to determine the order of genes in a linkage map. The recessive stocks are crossed to the elotrisomic. The number of recessive homozygotes in the F2 population reflects the order of genes in the telocentric arm. An example of telotrisomic analysis for two genes cu2 and uz in the long arm of chromosome 3 of barley was presented by Tsuchiya and Singh in 1981. In this example, the number of homozygotes in the diploid portion of the population was 10 for uz and 18 for cu2. Thus according to the theoretical segregation ratios, it is obvious that gene cu2 is closer to the centromere than the gene uz.
(2) Chromosome arm-gene association can be established using telotrisomics. The genes present in the trisomic arm will show trisomic ratio while those present in the disomic arm will show normal disomic ratio (3 : 1).
(3) Centromere position can be located through genetic analysis of tplotrisomics. Based the genetic analysis in telotrisomics, the centromere position was located for the time in linkage map of chromosomes 1 to 5 in barley by Tsuchiya.
Type # 4. Tertiary Trisomic:
The cell or individual carrying a trans-located extra chromosome is called tertiary trisomic. The ends of the extra chromosome are homologous to the ends of two different chromosomes that are non-homologous. In a reciprocal translocation, there are two trans-located chromosomes and thus there are two possible types of tertiary trisomy (Fig. 16.10).
(i) The extra chromosome carries the centromere of one chromosome and trans-located segment of the other chromosome (12).
(ii) The extra chromosome carries the centromere of the other non-homologous chromosome carries and the trans-located segment of the first (21).
Origin:
Tertiary trisomics occur in the progeny of interchange heterozygotes. Due to non-co-orientation of the translocation quadrivalent, 3 : 1 segregation occurs leading to the formation of ‘n + 1’ type of gametes. These gametes produce trisomics after fertilization with a normal (n) gamete. There are four possible ways in which 3 : 1 segregation may occur to produce n + 1 type of spore (Fig. 16.17).
The hyperploid gamete containing 2 normal and one trans-located chromosome will produce tertiary trisomic whereas the gamete containing 2 trans-located and one normal chromosome will produce a primary trisomic. The selfed progeny of interchange heterozygote includes 8 trisomic types, and 4 more types may appear in the progeny of selfed tertiary trisomics making a total of 12 types based on the chromosome constitution.
Of these, 6 are tertiaries and 6 are primaries. Chromosome constitution and the nomenclature of these trisomics are given in Fig. 16.18. Tertiary trisomics have been produced in several plant species, such as, Datura maize, rye, peas, barley and tomato etc.
Phenotypic effect:
Phenotypic effect of tertiary trisomics is like that of primary trisomics. Tertiary trisomic has the effect of two chromosomes which are interchanged.
Chromosome Pairing:
Tertiary trisomics show a chain of 5 chromosomes (lv), bivalents plus univalent (2II + 1I) or trivalent plus bivalent (1III + 1II) at MI. Tertiary trisomic interchange heterozygote may also show a quadrivalent plus univalent (1IV + 1I) Fig. 16-19). This configuration may also be observed in a primary trisomic interchange heterozygote. At MI, various configurations of the pentavalent chain may be observed such as, V-shaped, rod shaped (straight chain), J-shaped, zigzag and a chain of 3 attached to a bivalent.
Breeding Behaviour of Trisomics:
Tertiary trisomics producen +1 and n types of gamete. Generally, the transmission rate of extra chromosome through male is nil or very- low. The selfed progeny consists of tertiary trisomic, primary trisomic and diploid. The extra Chromosome may become shorter by deletion of both ends to such an extent that it can be transmitted through pollen. This will result into the increased number of chromosomes in the complement. Wiebe in 1976, produced 16 chromosome barley through this method.
Uses of Tertiary Trisomics:
Tertiary trisomics may be used to:
(i) Locate genes on chromosomes,
(ii) Maintain genes for lethality and male sterility, and
(iii) Produce female parent for hybrid seed production.
Balanced Tertiary Trisomics (BTT):
The term balanced tertiary trisomic has three words of which (1) “trisomic” indicates the presence of extra chromosome, (2) “tertiary” indicates that the extra chromosome is a trans-located chromosome, and (3) “balanced” refers to the breeding behaviour of the trisomic.
Ramage defined the BTT as a tertiary trisomic constructed in such a way that the dominant allele of a marker gene, closely linked with the translocation breakpoint of the extra chromosome is carried on the extra chromosome, and the recessive allele is carried on the two normal chromosomes that constitute the diploid complement.
The dominant marker gene may be located on the centromere segment or the trans-located segment of the extra chromosome (Fig. 16.20).
Breeding behaviour of BTT:
BTT produces the following 3 types of functional gametes:
(i) ‘n’ Type which consists of both the normal chromosomes and is functional on both male and female sides
(ii) ‘n + V type consisting of 2 normal and one trans-located chromosomes, functional on female side only
(iii) ‘n + 1’ type consisting of normal chromosomes (Fig. 16.21)
Selfed progeny of a BTT will produce 3 types of plants : diploid, primary trisomic and BTT. The BTT will be identified by the presence of the dominant marker (Fig. 16.21). Transmission of the extra chromosome is nil through pollen, while through the egg, it is transmitted, but with a low rate.
The progeny of BTT consists of 30% BTT and 70% diploid, and a very low frequency of primary trisomics. Diploid and the primary trisomics in the progeny possess recessive alleles of the marker gene.
Use of BTT:
Balanced tertiary trisomic is utilized for the production of male sterile female parent to be used in hybrid seed production. The BTT is constructed in such a way that the gene for male sterility (ms) is located on the normal chromosome, while its dominant allele (Ms) is located on the extra trans-located chromosome.
In this case, the selfed progeny will consist of BTT which carries the Ms gene and are male fertile, while the diploids will carry gene for male sterility (ms) (Fig. 16.21).
Production of Hybrid Seed:
Crossing block consists of alternate strips of female and male rows. The male parent (pollinator) is normal diploid, a commercial cultivar, whereas the female parent is the diploid progeny of BTT (Fig. 16.22). Trisomics are weaker and late flowering and they may be rouged to produce pure stand of male sterile diploids.
In barley, seeds at a rate of 25-30 kg hectare are sown. At this seed rate, the trisomic plants are almost completely eliminated by competition. The seeds produced in female rows are hybrid seeds.
Maintenance and Production of BTT:
Seeds of BTT are produced in separate field, in the absence of competition from male sterile diploids. For commercial production, in barley, the selfed seeds from BTT are sown at a rate of 5-7 kg per hectare. The population consists of 30% BTT. Diploids are rouged at seedling stage on the basis of their leaf characteristics (leaves of diploids are normal, while the trisomics have long and narrow leaves).
A large number of BTTs have been produced in barley possessing the male sterile genes such asmsg 1, msg 4,msg 6 and rn.sg 24 etc. In this crop, the first hybrid variety named “Hembar” was produced in U.S.A. by Ramage and Wiebe in 1969.
Type # 5. Compensating Trisomics:
It is the type of trisomic in which one chromosome of the diploid standard complement is missing but is compensated for by the presence of two other chromosomes which together are equivalent to the missing chromosome.
The missing chromosome in the compensating trisomic may be compensated for by any of following:
(i) Two tertiary (trans-located) chromosomes,
(ii) Two iso-chromosomes,
(iii) One tertiary and one telocentric chromosomes,
(iv) One iso- and one telocentric chromosomes or
(v) One iso- and one tertiary chromosome.
Origin of Compensating Trisomics:
Compensating trisomics occur in the progeny of plants showing association of 6 chromosomes obtained by crossing two interchanges which have one chromosome in common. As shown in the Fig. 16.23, there are three chromosomes a.b., c.d. and 1, m, of which one chromosome c.d. is common in two interchanges a.c-b.d and (d.1-m.c.).
The Fa between these translocations will produce an association of six chromosomes (ring or chain) during meiosis. Hyperploid gamete (,n +1) containing a.b, b.d, 1.m and m.c may be formed in the When fertilized with a gamete containing normal chromosomes, the n + 1 gamete will produce compensating trisomic for the regions (.b) and (.m) (Fig. 16.23).
In this trisomic, one chromosome c.d. is missing but it is compensated for by two trans-located chromosomes b.d and m.c. Several different compensating trisomics can be derived from the association of 6 chromosomes. The distinguishing meiotic configuration in the compensating trisomic is a chain of 7 chromosomes (Fig. 16.23 D).
Other Types of Trisomics:
Centric fragments of chromosomes may also be present as an extra chromosome in a trisomic. These fragments may be of various types, such as, acrocentric and metacentric fragments. Trisomics possessing these fragments as extra chromosome are designated asacrosomictrisomics or, acrotrisomics and metatrisomics (Fig. 16.10). Acrocentric chromosomes are produced due to terminal deletion of one arm.
The deleted arm may be of varying lengths in different acrocentrics. The designation of the acrotrisomic is made as the intact long (L)/short (S) arm with the superscript of deleted arm. For example, acrocentric chromosome produced by deletion of short arm of chromosome 3 is written as (3L3S) and the trisomic for this fragment is written as (Triple 3L3S). Acrotrisomic plants are obtained in the progeny of lelotrisomics, primary trisomics and triploids.
Uses of Acrotrisomics:
Acrotrisomics are useful in physical location of genes in a linkage group because the breakpoint is measurable in the acrocentric chromosome. Cross is made between the acrotrisomic and the genetic marker stocks, and F2 segregation is observed. The genes located in the deleted arm segment show a disomic ratio, while the genes located in the intact arm show a trisomic ratio.
An example of acrotrisomic analysis in barley using acrotrisomic 5S5L is presented in Table 16.8. This acrocentric chromosome has 70% deficiency in the long arm of chromosome 5. Physical location of three mapped genes fs2 (fragile stern), f7 (chlorina 7) and trd (third outer glume) in the long arm of chromosome 5, and three unmapped genes associated with the same arm, g (golden), f3 (chlorina 3) andint-a (intermediate) was made through this method. The genes f7, trd andint-a showed disomic ratios indicating that they are located in the 70% deficient distal segment of long arm (Table 16.8). The genes fs2, g and fi showed trisomic ratios indicating that they are located in the intact 30% proximal segment of the long arm of chromosome 5.
