Ploidy Breeding
Oct 29, 2015
PBG 607. PLOIDY BREEDING (1+1)
Course teacher : Dr. S. Rajeswari, Assistant Professor (PBG)
Theory
The significance of diploidy in evolution Origin of aneuploids production, identification and maintenance monosomics, nullisomics, trisomics, tetrasomics, monotelosomics, ditelosomics Aneuploids in genetics studies: chromosome mapping by aneuploids - use of aneuploids in locating genes Aneuploids in centromere mapping Aneuploid analysis: primary trisomic, secondary trisomic, tertiary trisomic and telotrisomic analyses Balanced Tertiary Trisomoics and hybrid seed production - Synthesis of chromosome addition lines Monosomic Alien Addition Lines (MAALs) MAALs in gene transfer Chromosome substitution lines Pseudoaneuploids and karyotype reconstruction Distribution of polyploidy Factors determining the origin and spread of ployploidy Direct effects of polyploidy: Origin of new species Polyploidy and Hybridization Types of polyploids and their origin perenniality and polyploids spontaneous and induced polypoids Criteria for establishing nature of polyploids: external morphology, chromosome pairing, secondary pairing and character segregation Autopolyploids: in seed crops, forage grasses, root crops, fruit crops and ornamental crops Breeding procedures Reversion of autopolyploids to diploids Interspecific hybridisation and allopolyploids hybrids between species with same chromosome number Hybrids between species with different chromosome number Gene transfer using amphidiploids Bridge species Synthesis of new crops and their breeding value.
Practical
Induction and identification of polyploids by different methods morphological observations study of stomata pollen diameter chromosomal behaviour in meiosis pollen fertility correlation of meiotic aberrations with fertility cytogenetic analysis of interspecific and intergeneric crosses, autopolyploid and allopolyploid with diploids genome analysis.
References
Clark, M.S and W.J. Wall. 1996. Chromosomes: The complex code. Chapman and Hall London, 345p.Gupta, P.K and T. Tsuchiya. 1991. Chromosome Engineering in Plants: Genetics, Breeding and Evolution. Part A. Elsevier. Amsterdam. 639p.Gupta, P.K and T. Tsuchiya. 1991. Chromosome Engineering in Plants: Genetics, Breeding and Evolution. Part B. Elsevier. Amsterdam. 630p.Kallo, G and J.B. Chowdhury (Eds). 1992. Distant Hybridization of Crop Plants. Springer Verlag. Berlin. 271p.Schultz-Schaeffer, J. 1981. Cytogenetics: Plants, animals and human. Springer-verlag, Berlin.Stebbins Jr, G.L. 1964. Variation and Evolution in Plants. Oxford Book Company, Calcutta. 643p.Sybenga, 1985. Cytogenetics. Oxford University Press, London.
Class schedule (Theory)
ClassTopic
1The significance of diploidy in evolution Origin of aneuploids production, identification and maintenance monosomics, nullisomics, trisomics, tetrasomics, monotelosomics, ditelosomics
2Aneuploids in genetics studies: chromosome mapping by aneuploids
3Use of aneuploids in locating genes Aneuploids in centromere mapping
4Aneuploid analysis: primary trisomic, secondary trisomic, tertiary trisomic and telotrisomic analyses
5Balanced Tertiary Trisomics and hybrid seed production
6Synthesis of chromosome addition lines Monosomic and Monosomic Alien addition lines
7Chromosome substitution lines Pseudoaneuploids and karyotype reconstruction
8Distribution of polyploidy Factors determining the origin and spread of polyploidy Direct effects of polyploidy: Origin of new species Polyploidy and Hybridization
9Types of polyploids and their characteristics Polyploid complex and polyploid permanent hybrids
10Autopolyploids and their origin perenniality and polyploids spontaneous and induced polyploids
11Criteria for establishing nature of polyploids: external morphology, chromosome pairing, secondary pairing and character segregation
12Autopolyploids: in seed crops, forage grasses root crops, fruit crops and ornamental crops Breeding procedures
13Reversion of autopolyploids to diploids
14Interspecific hybridisation and allopolyploids Hybrids between species with same chromosome number
15Hybrids between species with different chromosome number
16Gene transfer using amphidiploids
17Bridge species Synthesis of new crops and their breeding value
Class schedule (Practical)
ClassTopic
1Agents for the induction of various ploidy levels
2Identification of polyploids in different crops
3Induction and identification of haploids: Anther culture
4Induction and identification of haploids : Ovule culture
5Morphological observations on autopolyploids
6Morphological observations on allopolyploids
7Morphological observations on aneuploids
8Chromosomal behaviour during meiosis in euploids
9Chromosomal behaviour during meiosis in autopolyploids
10Chromosomal behaviour during meiosis in allopolyploids
11Chromosomal behaviour during meiosis in aneuploids
12Pollen fertility correlation of meiotic aberrations with fertility
13Cytogenetic analysis of interspecific crosses
14Cytogenetic analysis of intergeneric crosses
15Maintenance of cytogenetic stocks and their importance in crop breeding
16Various ploidy levels due to somaclonal variation
17Polyploidy in ornamental crops
PBG 607. PLOIDY BREEDING (1+1)
Significance of diploidy in evolution
The most significant event in the entire history of evaluation of living organisms has been the emergence of the sexual mode of reproduction associated with the diploid condition. Most of the plant and animal species are diploid and chromosomes are found in pairs in their somatic tissues. The number of chromosomes in a species normally remains constant through successive generations and this results in constancy of characters. Gametes are produced as a result of meiosis and it possesses each chromosome represented only once. Fusion of gametes brings together, homologous chromosomes from two parents restores the diploid number. Alternation of haploid and diploid generation occur in all sexual organisms. There is great diversity in the number of chromosomes in different species. But the number is fixed in a particular species. Chromosome number is one of the characters that differentiate one species from another. In plants somatic chromosome number ranges from 4 to 1260 and in animals it ranges from 2 to 104. Consistency of genetic material through successive generations is essential for maintaining the identity of a species. But variation is necessary if evolution has to occur. Sometimes, such variations involve whole chromosomes or chromosome sets and can be seen cytologically. The diploid state also provides the population possibilities of conserving genetic variability, which can be exploited for evolutionary advance. When the environment is no longer favourable gene mutations arising in a haploid organism are immediately exposed to natural selection. In a diploid state, on the other hand, the recessive mutations remain unexpressed in a heterozygous condition. The evolutionary potential of the diploid species is, therefore much greater than that of the monoploids.
History
1907The first variation in chromosome number was discovered in gigas mutant in Oenothera by Lutz. Gigas was an autotetraploid (4n)
1910Blackeslee discovered the globe mutant of Datura stramonium which was subsequently demonstrated by Belling (1920) to be a trisomic
1916The first autotetraploid was experimentally induced by Winkler, 1916 in Solanum nigrum
1917Winge suggested interspecific hybridisation followed by chromosome doubling for evolution of new species
1890The first species synthesized experimentally was Triticale by Rimpau from a cross between Triticum and Secale. Its chromosome number was not studied till 1936
First synthetic species about which full information was furnished was Nicotiana digluta by spontaneous chromosome doubling of F1 hybrid from interspecific cross Nicotiana glutinosa (n = 12 ) N. tabacum (n = 24) by Clausen and Goodspeed.
1937The chromosome doubling action of colchicine was first described by Blakeslee and Nebel independently of each other
1955Effect of colchicine on mitosis was discovered
Aneuploids
Aneuploidy is any deviation from a euploid condition which can be expressed either as an addition of one or more entire chromosomes or chromosome segments to a genomic number or as a loss of such chromosome material. Origin and production Aneuploidy can be caused be any of the four following disturbances (Rieger et al., 1976). 1. Loss of chromosomes in mitotic or meiotic cells, often caused by lagging chromosomes or laggards, which are characterized by retarded movement during anaphase. This respects in hypoploid chromosome number (Eg. 4x-1, 4x-2, 2x-1, 2x-2 etc.)2. Non-disjunction of chromosomes or chromatids during mitosis and meiosis. This is a failure of such genetic units of separate properly and results in their not being distribution to opposite cell poles. It can cause hypo-or hyperploid chromosome number (eg. 4x-1, 4x+1, etc.)3. Irregularities of chromosome distribution during the meiosis of polyploids with uneven number of basic genomes such as in triploids, pentaploids etc. (eg. 3x, 5x). Such polyploids, some chromosomes are often present as univalents. They are distributed randomly to either pole or may be lost in anaphase I or anaphase II. 4. The occurrence of multipolar mitosis, resulting in irregular chromosome distribution is anaphase; such multiform aneuploidy can result in cells with different aneuploid chromosome numbers, causing the formation of tissue with chromosome mosaicism.
Origin and production The method of origin of aneuploids may be spontaneous or induced. 1. Spontaneous: Aneuploids arise spontaneously at a low frequency. Meiotic irregularities lead to the formation of n+1 and n-1 gametes. Eg. In Datura atleast 0.4% of pollen in likely to be n+1. 2. Triploid (3x) or pentaploid (5x) plants: The best source of aneuploids are triploid plants. Distribution of chromosomes at the first meiotic metaphase is irregular leading to production of a whole range of aneuploids in the progeny.3. Asynaptic and Desynaptic plants: In these plants, few to all chromosomes are present as univalents at metaphase I of meiosis. In the progeny of such plants, a relatively high frequency of aneuploids occur. 4. Translocation heterozygotes: The chain of four chromosomes in a translocation heterozygote would produce one n+1 and n-1 gamete. As a result in the progeny of translocation heterozygote, a variable frequency of aneuploids are found. 5. Tetrasomic plants: (2n+2) plants would produce n+1 gametes in considerable frequencies. Therefore, when they produce a high frequency of trisomics; where possible tetrasomic plants may be maintained for production of trisomics. Morphological features 1. They are weaker than diploids.2. Monosomic and nullisomic will survive in diploid species. e.g. Tobacco 3. Trisomics will survive.But they vary in vigors.4. In wheat, monosomics are equal to normal plants.
Cytological implications 1. Nullisomics generally show bivalent formation. 2. Tetrasomics form quadrivalent which involves four chromosomes and separate 2:2 at anaphase I. But sometimes it is not regular. 3. In monosomic one chromosome does not have a pair and remain as univalents. At anaphase the univalents move to any one of the poles. 4. Trisomics 2n+1. The extra chromosome forms a trivalent (or) remain as univalents which behaves irregularly. Aneuploidy in manAneuploids are seen cytologically which result in to physical, mental and reproductive abnormalities. Some of the aneuploids in man are tabulated below.
Sex chromosomePhenotypeFrequencies
Female xo (monosomic)xx (disomic)xxx (trisomic)
xxxx (tetrasomic)Turners syndrome (mentally retarded, stele)NormalSuper female with mental abnormalitiesSuper female with mental abnormalities1 in 3000
1 in 750
1 in 750
Male xy (disomic)xyy (trisomic)xxy (trisomic)xxyy (double trisomic)
xxxy (tetrasomic)NormalNormalKline felters syndromeKline felters syndrome (Body hair scanty poorly developed reproductive system)Extreme kline felters syndrome (more of 7 characters)
1 in 5001 in 500
In man aneuploidy for autosomal chromosome results in to various phenotypic abnormalities. Sometimes it is retrial. HypoploidsMonosomy (2n-1)Monosomics are organisms with one missing chromosome (6x-1, 4x-1 etc). Monosomics have been discovered in human, animals and plants. Monosomics were first found in tobacco (Clausen and Goodspeed, 1926). Monosomics were also were detected in wheat, oats, tomato, maize and cotton. Monosomics are of three types.
1. Primary monosomy (20~ + 1, 2n = 41): One chromosome is missing and the remaining homologue to the missing chromosome is a structurally normal chromosome. E.g. in wheat. 2. Secondary monosomy (20~ + 1, 2n = 41): One homeologous chromosome pair is missing and is replaced by a secondary chromosomes or isochromosome for one arm of the missing pair. Kimber and Riley (1968) and Khush (!973) called this as monoisosomy. 3. Teritary monosomy (19~ + 1 + 1 + TC, 2n = 41): As a result of pollen irradiation, two nonhomologous chromosomes are broken in the centromere region. Two arms of these nonhomeologous unite to form tertiary chromosomes with a functional centromere. The other two arms are lost. A plant fertilized with such pollen becomes a tertiary monosomic (Khush and Rick, 1966). Such a plant is a double monosomic with a tertiary chromosome (TC) addition. Monosomics have been used extensively in wheat breeding for the purpose of chromosome substitution. The first monosomic series in wheat was established by Sears (1954) in Chinese spring. The sources of such monosomics are;
1. Asynapsis as caused by nullisomy: This was the major source in what. Of 212 monosomics recovered 114 (53.8%) were obtained from progeny of asynaptic nullisome 3B (Sears, 1954). 2. Polyhaploid progeny: Of the 212 monosomics obtained by Sears (1954), 66 (31.3%) were observed from two different polyhaploid individuals. 3. Chromosome loss: As a result of non disjunction during meiosis or during the early mitotic divisions of a diploid zygote. 4. Unequal chromosome distribution: (non co-orientation) during meiosis of translocation heterozygote. 5. Irradiation treatment: Irradiation treatment has also been used as a method for production of monosomics. For instance, few monosomics in cotton (Gossypim hirsutum 2n = 52) and in oats (Avena sativa 2n = 42) have been successfully produced through irradiation of inflorescence. The irradiation presumably leads to nondisjunction of a normal bivalent leading to the production of gametes with n = (2x or 3x) -1 or (2n or3n) 1. First monosomic series are available in wheat chromosome substitution lines. The purpose of chromosome substitution lines is, if a line has disease resistance conditioned by genes carried a specific chromosomes, that desirable chromosome can be substituted to an acceptable variety and this can be monosomic female recipient for repeated backcrossing until the desirable chromosome of male donor variety is transferred. Monosomics have transmission through males than female gametes. Eg. Chinese spring wheat (monosomic), used for backcrossing with desirable cultivars. Monosomics were found in tobacco, oats, maize, tomato and cotton. In tobacco, since there is some homeologous pairing between genomes, trivalents occur in 25% of monosomics. In wheat there are no trivalents. In human, turners syndrome occurs in 0.03% of all the female births. This is monosomy of x chromosomes. Autosomal monosomic are rare in humans. But in Drosophila, autosomal monosomics are seen. Monosomic for sex chromosome XO is seen in Drosophila. They are male, but sterile. In mice XO is female bur fertile. Monosomy in diploidsIn maize monosomics were isolated by Mc Clintock 1929 and it was useful to study the fatty acid composition in embryo of maize. Monosomics in diploids can be obtained by the following methods. 1. As rare monosomics in the progeny of normal diploids.2. By treatment with mutagens.3. In the progeny of aneuploids, haploids and polyploids.4. By interspecific crosses. 5. In the progeny of plants with specific gene system (as in case of maize). Cytology of monosomicsThe monosome in monosomics usually appears as a univalent in meiosis. The univalent often shows retarded movement which first become evident at metaphase I. While all the bivalents move to the equatorial plate, the univalent lies away from the plate. When the bivalent starts undergoing disjunction, the univalent starts to move to the plate. The later behaviour of the univalent is quite variable.1. It may pass to one of the poles undivided where it may or may not be included in the telophase I nucleus.2. It may divide equationally with sister chromatids going to opposite poles of the same pole or3. It may divideDuring the second division, the univalent may divide normally if it has passed to one of the poles undivided during the first division. However, in those cells where it underwent equational division or misdivision during the first division, its movement is characteristically retarded. Thus, telocentric chromosomes and iso chromosomes are often found in the progenies of monosomics.Breeding behaviour of monosomics Theoritically one expects an individual with 2n-1 chromosomes to produce n and n-1 gametes in equal frequency. Likewise, one should be able to obtain 2n and 2n-1 individuals in equal frequency from (2n-1)2n crosses. Furthermore 2n 1 individuals when selfed should yield 2n, 2n-1 and 2n-2 progeny in the ratio of 1:2:1. However, these expectations are never realized and the departure from this idealized ratio are indeed varied. The causes of the departure are1. Production of n and n-1 spores in unequal frequency (because of the lagging of the univalent)2. Reduced variability of n-1 spores3. Competition between n and n-1 microspores4. Reduced viability of 2n 1 zygotes5. Reduced viability or inviability of nullisomic zygotes.Productionof monosomicsIn maize monosomics can be produced by(i). Chromosomal loss due to B chromosomeRhodes et al (1967) reported that the presence of B chromosome in male eliminates the knobbed chromosome 3. Non disjunction of chromosome 3 was seen during the second microspore division.(ii) Chromosome loss due to r xi (anthocyanine) deficiencyIrradiation induce a deficiency that is associated with the R locus present in chromosome 10. The gene R produces anthocyanine in the aleurone of the endosperm, while its recessive allele rgives rise to colourless aleurone when coloured and colourless are test crossed and due to nondisjunction of chromosome no 10 monosomics and trisomics are produced in equal sequence.Mono-isosomy A misdivision of the centromere of the univalent chromosome in the monosomic gives rise to isochromosomes or telocentric chromosomes. A monoisosomic plant contains an iso chromosome in the place of missing pair and thus the monoisosomy is symbolized as (2n-2+1 iso)MonoisodisomicOne chromosome is missing but is replaced by an isochromosome for one of the arms of its homologue. MonotelosomyThe condition where in one homeologous chromosome pair is missing, and a telocentric derived from that chromosome is present is referred to as monotelosomy.Uses of MonosomicsMonosomics are used for the following purposes1. Locating genes on particular chromosome2. Gene mapping1.Locating genes on particular chromosomeGenes on particular chromosome can be located either by induction of monosomics or using the already existing monosomic stocks.a.Locating genes by induction of monosomyPollen from plants having the dominant character (to be located) is irradiated by using U.V. rays and dusted to pollinate those plants that carry the corresponding recessive trait. Due to irradiation some pollen might have become n-1 hence, if the F1 is a monosomic plant for the concerned chromosome it will show recessive phenotypes due to pseudodominance and to confirm whether the recessive phenotypes in F1 are really monosomic cytological analysis can be made. The monosomic chromosome with the particular gene of interest can then be identified.b.Locating genes using monosomic stocks
The genes to be located by using monosomic stocks may be dominant or recessive. If recessive, then monosomics for the different chromosomes are crossed to recessive males, and the monosomic chromosome in the monosomic F1 plants will be obtained from the male parents. All the recessive plants in the F1 population will be monosomic if the gene is present in the monosomic chromosome and the dominant plants will be disomic. If not then the F1 population show all dominant plants. If the gene to be located is dominant, different monosomics are crossed to the dominant males. Cytological analysis were carried out to identify the monosomic plants in the F1 population, and the identified monosomic lines are selfed. The F2 population consists of disomic, monosomic and nullisomic plants. If the gene to be located is present in the monosomic chromosome, the disomic and monosomic plants will be recessive. But if the gene is not located in the monosomic chromosome then the F2 progenies will be segregated in the ratio 3:1 whatever the chromosome constitution be. Even the disomic plants will show recessive phenotype but in the former case, only nullisomics will show recessive phenotype.Polyploids consists of duplicate and triplicate genes and many also involve polysomic inheritance. Hence the constitution of linkage map in polyploids is a tough task. Monosomic analysis is used to prepare linkage maps or linkage groups which accompany the chromosome maps.Locating genes through chromosome substitutionLocating genes on particular chromosome can also be made by chromosome substitution (ie) substituting individual specific chromosome for the corresponding chromosomes of another variety having contrasting character. Kuspira and Inrau (1957) made extensive studies in hexaploid wheat. Here, each of the monosomic series of the recipient variety is crossed with the donor variety for making 21 individual chromosome substitutions. In F1, the monosomics are identified through cytological investigations, selfed to get disomic progenies which are backcrossed with the original monosomic and the cycle is repeated to reconstitute derived the genotype of the recipient variety in which one pair of chromosomes is now devised from the donor variety. If a major change happens to take place in the morphology of the character under study, then this indicates that one or more genes for this character are located on the specific chromosome induced in the substitution.Utilizing the above technique in oats, a gene for chlorophyll production was located on arm 14L; two genes (one for kinky neck and other for fatuoid suppression) were located on 20L and two genes (one for curled leaves and other for regular synapsis) were located on 20S.2.Gene MappingSears(1962) suggested the use of monotelodisomics for gene mappinig and actually utilized them for later for mapping several genes. Here the male parent is a monotelodisomic plant which carries a recessive gene in the concerned normal chromosome and its dominant allele in the telosome and it is test crossed with a disomic strains. The dominant allele is transferred from the telosome to the normal chromosome by the crossing over between the gene locus and the centromere. Hence in the test cross progeny, the frequency of plants with the dominant character is a direct measure of the frequency of recombination between the centromere and the gene in question.NULLISOMYThe organisms in which a pair of homoleogous chromosomes is missing from the normal chromosomal complement is called nullisomic and the condition is called nullisomy (2n-2). If two pairs of homeologous chromosomes are lost then the condition is said to be double nullisomy (2n-2-2). Usually nullisomics are found in polyploid population and not in diploids because in diploid organisms, loss of one pair of chromosome cannot be tolerated.The extensive studies are made on the nullisomics in wheat, where in a complete set of nullisomics are produced and also other polyploid crops like oats.SourcesThe main source of production of nullisomics is monosomics, which on selfing give a small frequency in addition to disomics and monosomics. This can occur by, the fusion of two gametes that are lacking the same chromosome. Sears (1953) reported that the monosomic 3B yields upto 10% nullisomics in wheat and several other monosomics yield only 1% after selfing.Nullisomics are generally weak. They show reduced fertility, size and vigour and their maintenance is also difficult.Meiotic and breeding behaviourIn wheat, the nullisomics should have only 20 bivalents and no univalents at metaphase I of meiosis I, but meiotic irregularities have been observed. There are atleast 10 chromosomes, that carry the genes, which affect meiotic pairing (either promote or suppress). But in nullisomics, since a pair of chromosomes is lacking that leads to loss of several such genes. Hence the meiosis is disturbed. For example, in nulli 5B, multivalents are formed due to the absence of Ph 1 locus.The nullisomics show reduced fertility in wheat. About half of the 21 nullisomics are either male sterile or female sterile. Therefore, all nullisomics cannot be maintained by selfing. Out of 21 nullisomics, eight are reasonably stable but about 8 10 % aberrant plants are produced in the selfed progenies of the 8 stable lines.Identification of nullisomicsCan be done by using morphological traits such as seedling morphology, inflorescence characters etc., Most nullisomics have reduced size and vigour. In oats also individual nullisomics can be identified by spike characters and vigour at seedling stage.UsesNullisomic analysis is used to assign dominant genes to specific chromosome. Disomic dominant character A can be crossed with nullisomic series, which show recessive character. Offsprings of these will be heterozygous (Aa) or hemizygous dominant (AO). If the heterozygotes are selfed 3:1 segregation occurs. The AO on selfing results in dominant AA+ AO in major portion and a small portion of recessive character being nullisomic. The F2 will be the carrier for the dominant gave A.The genes, which are dominant, can be located by their absence in nullisomics. For instance, Chinese spring wheat has red seed, but nulli -3D has white seeds suggesting that gene for red grain is located on 3D.
TRISOMYThe term Trisomy was coined by Blackslee in 1921. Trisomy is a condition in which an extra chromosome is present in a diploid (disomic) chromosome complement (2n+1 or 2x+1). The cell, tissue or organism possessing such a condition is referred to as trisomic. The term Trisomy signifies that a particular chromosome is present in three doses. If there are two extra chromosomes, one each for two different homologues, it is then called as double trisomic (2n+1+1 or 2x+1=1).Types of Trisomy1. Primary trisomic: Here, the extra chromosome is normal.1. Secondary trisomic: Here, the extra chromosomes is an isochromosome (2n+iso).1. Tertiary trisomic: Here, the extra chromosome is a translocated chromosome. 1. Telocentric trisomic: The extra chromosome is a telocentric chromosome.1. Compensating trisomics: It is the type of trisomic in which one chromosome of the diploid standard complement is missing but is compensated by the presence of two other chromosomes, which together are equivalent to the missing chromosome.1. Acrosomic trisomics: Trisomics possessing acrocentric fragments as an chromosome are designated as acrosomic trisomic or acrotrisomic.1. Metasomic trisomics: Trisomics possessing metacentric fragments as an extra chromosome are designated as metasomic trisomics or metatrisomics. Primary trisomyIf the extra chromosome of a trisomic is normal, then the condition is primary trisomy. In Datura stramonium primary trisomics (2n+1 =25) for each chromosome were distinguished and also were established in other crops such as Pennisetum, tomato and rye. Origin and sourcesPrimary trisomics are produced when a gamete containing an extra chromosome (n+1) fertilized by a normal.1. Asynaptic and desynaptic mutants are good sources of primary trisomics. Meiotic failure occurs due to asynapsis and desynapsis that leads to the formation of varying number of univalents at telophase I. Due to the random segregation of these univalents to the poles, gametes with extra chromosomes are produced that give rise to primary trisomics.1. Normal diploids may undergo unequal disjunction of chromosomes either due to failure of chromosomes of a bivalent to pass on to metaphase plate (non congression) or due to failure of chromosomes a bivalent to separate at anaphase (non-disjunction). That produces (x+1) and (x-1) gametes. The former produces primary trisomics.1. Autoployploids Autotriplois are the major and best source of production of primary trisomics. Triploids will form a number of trivalents or bivalents and univalents so that chromosomes in excess distributed randomly that produce (x+1) gametes. Therefore when autotriploids are selfed or backcrossed to diploids primary trisomics are produced. 1. Haploids are good source of trisomics produced by selfing or crossing with normal diploid plants. Haploids with adequate fertility are more successfully used in polyploids than in diploids.1. Tetrasomics and other aneuploids Tetrasomics crossed with a disomic give rise to primary trisomic as in Datura. Primary trisomics may also be obtained in meager number in the progenies of other aneuploids such as multiple trisomics, primary trisomics, seconday trisomics, tertiary trisomics, compensating trisomics and to a very little extent monosomics.1. Translocation heterozygotes forms a ring of four chromosomes during meiosis, which may undergo 3:1 disjunction giving rise to gametes with x+1 chromosomes which produce primary trisomics.1. Besides these primary trisomics may also be produced by using ionizing irradiations and by colchicines and other chemical mutagens treatment. Meiotic and breeding behaviourThe most important characteristic feature of primary trisomic is the formation of a trivalent at meiosis these trivalents form different shapes depending upon the extent of pachytene pairing and the number and position of chiasmata viz., rod shape, frying pan shape, Y shape, J shape, V shape, zig zag shape and in chain configuration.The chromosomal imbalance makes the gamete carrying the extra chromosome (n+1) sterile or non functional. The transmission of extra chromosome is more frequent through female and rarely through male (eg) Barley. The frequency of trisomics in the selfed progenies varies from 20 to 30 % for different chromosomes of different plant species. UsesTrisomics 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. The methodology of this kind of gene mapping is as follows.1. A plant homozygous recessive for a given gene (a), the linkage group of which is to be determined, is crossed to all plants of the primary trisomic series.1. The trisomic F1 plants are identified by cytological analysis or selected by morphological characteristics if that is possible. They are then backcrossed to the homozygous recessive.1. The F1 test cross plants are analysed genetically for their segregation. If there is a striking deviation from the normal phenotypic 1A: 1a test cross ratio, then the linkage group in question (marked by the gene a) can be assigned to the primary trisome that produced in this ratio.The trisomic here, carrying AAA produced AAa (duplex) trisomics in the F1 which produces 1 AA: 1a:2A:2Aa. The test cross ratio is 5A: 1a. This ratio may be modified depending on the distance of the a locus from the centromere permitting double reduction. (Occurrence of sister alleles in the same gamete is called double reduction). Extensive studies are made in maize, tomato, barley, Datura, Antirrhinum and Petunia.
Lecture 5Tertiary trisomyThe cell or individual which carries a translocated extra chromosome is called tertiary tisomic and the condition is said to be tertiary trisomic. Here the extra chromosome consists of segments from two non homeologous chromosome.Sources 1. Translocation heterozygotes Occasionally, in the interchange ring 3:1 disjunction is formed that produced x+1 gamete. The tertiary trisomics have been produced in several plant species such as Datura, maize, rye, peas, barley and tomato.2. Secondary trisomicsSecondary trisomics when crossed with a translocation homozygote followed by regular backcrossing with the later give rise to a tertiary trisomic interchange homozygote due to 2:1 segregation of the trivalent which on continued backcrossing to the normal give rise to tertiary trisomic.3. Primary trisomic Primary trisomic when crossed with a translocation homozygote produce primary trisomic interchange heterozygote that on usual disjunction give rise to tertiary trisomic.
4. Tertiary monosomic Due to non-disjunction of the trivalent, a tertiary monosomic gives rise to a tertiary trisomic.Meiotic and breeding behaviour In tertiary trisomics, the five chromosome involved may form (1) 1v (2) 1iii + 1ii (3) 2ii + 1i (4) 1iv + 1i (5) 1iii + 2 (6) 1ii + 3i or (7) 5i in various configuration. The pentavalent which forms dumb bell shaped is the characteristic for a tertiary trisomic. Other configurations are v shaped, rod shaped, J shaped, zigzag and a chain 3 attached to a bivalent.Tertiary trisomics produce n+1 and n types of gamete. Generate generally the transmission rate of extra chromosome through male is very low. The selfed progeny consists of tertiary trisomic, primary trisomic and diploid. Uses Tertiary trisomics are used to 1. Locate genes on chromosomes (.to determine arm location and approximate distance from the centromere)1. Maintain genes for lethality and male sterility 1. Produce female parent for hybrid seed production Balanced tertiary trisomics for hybrid seed productionRamage (1965) suggested a method for the use of tertiary trisomics to supply a male source to be used as female parent for hybrid seed production. It should be recognized that for hybrid seed production the female parent should be either hand emasculated or should be male sterile to eliminate the possibility of selfing during the process of hybrid seed production. Hand emasculation may not be feasible at a commercial scale and therefore a male sterile source is always welcome. Further, the male sterile line has to be maintained for further use. In case of cytoplasmic male sterility restoration system we use three lines.1. male sterile line to be used as female parent1. restorer line to be used as male parent1. a maintainer to be used as pollen source for maintenance of male sterile line.
When male sterile and restorer are crossed, the F1 hybrid is fertile due to restorer gene and therefore can be used for commercial cultivation, but the F1 hybrid between male sterile and maintainer (lacking a restorer gene) is male sterile and therefore can be used as female (male sterile) parent again for hybrid seed production.Balanced tertiary trisomics (BTT) described by Ramage (1965) provide an alternative method for the continuous availability of male sterile line to be used as female parent for commercial hybrid seed production. Balnced tertiary trisomics are tertiary trisomics set up in such a way that the dominant allele of a marker gene, closely linked with the interchange break point, is carried on the extra chromosome and the recessive allele is carried on the two normal chromosomes that constitute the diploid complement.A balanced tertiary trisomics, as the name indicates is so designed that despite itself being heterozygous (aaA with dominant allele on interchanged chromosome) its trisomic progeny obtained on selfing is genetically similar to the parent. These trisomics will produce three kinds functional female gametes and only one type of functional male gametes (because extra chromosome is rarely transmitted through the pollen). These will thus produce following three kinds of plants in the progeny of a selfed balanced tertiary trisomics.1. 70 % (range 60-70%) homozygous recessive diploids1. 1.0% homozygous recessive primary trisomics and1. 30 % heterozygous (aaA) balanced tertiary trisomicsFurthermore, the proportion of primary trisomics is very low, since a pair of homozygous chromosome belonging to trisomic condition, disjoins giving majority of x+ 1 gametes with translocated extra chromosome rather than the normal extra chromosome.In the above scheme, if a gene (ms) for male sterility (recessive) is used, which is closely linked to interchange breakpoint, the diploid plants in the progeny of BTT will be all male sterile, and BTT will be male fertile (due to dominant allele). The BTT should also carry an additional dominant marker on the tertiary chromosome which will allow identification of diploid plants in a mixture of diploids and balanced tertiary trisomics. One such marker could be red plant colour which will permit identification at seedling stage. Another marker could be plant height,that will allow harvesting at diploids and balanced tertiary trisomics separately using mechanical harvesting at different heights.1. an extra interchanged chromosome 1. a nuclear gene for male sterility located very close to the breakpoint and 1. one or more marker genes to be used as informational genes. The extra chromosome should not be transmitted through the pollen, but only through the eggs. The genotype of the BTT and the extra chromosome associated with it should be such that the BTT should be vigorous and competitive and should also produce abundant pollen for effective wind or insect pollination. The nuclear gene for male sterility should be expressed in all environments and for this purpose, large collections of suitable male sterility genes are available now in barley, with atleast one of them on each of the seven chromosomes. Several marker informational genes are also available.Commercial hybrid seed production schemeUtilizing the BTTs, each with the above characteristics as female parent, commercial hybrid seeds can be produced. Two types of plots are involved in this production.1. a trisomic seed production plot: here the seeds harvested from trisomic plants consist of trisomics and diploids1. a hybrid seed production plot: Male lines sown in this field are those desirable inbred lines that have a good combining ability with the female (male sterile) line and the female lines are arise a part of the seed harvested from the trisomic seed production field.The seeds harvested from BTTs are sown in the trisomic seed production plot at a rate of 5-7 kg/ha. After 3-6 weeks, the diploid seedlings are rogued out using the information genes, thus leaving a pure stand of trisomic plants. The seeds harvested from these trisomic plants is divided into two parts, one part is used for the production of trisomic seeds in the next cycle and other part is used as the female rows for the hybrid seed production.The diploid seedling need not to be rogued out instead the seeds from the diploid lines can be harvested separately using the marker genes and can be used as female rows (male sterile) for hybrid seed production. The trisomics seed production plot can also be used as crossing block with male and female rows. The seeds harvested from BTTs are used to plant male rows and harvested from diploid are used to plant female rows. The diploid seedlings from the male row are rogued out leaving a pure stand of trisomic plants in the male rows. The half of the seeds harvested from the female rows is recycled to plant female rows in the trisomic seed production plot and other half is used to plant female rows in the hybrid seed production plot. Additionally, BTTs which are balanced not only for a male sterility gene but also balanced for a rows in hybrid seed production plot. The methodology of seed production is same as before except that there is no need of rogueing in the male rows. Since the diploids will die due to the presence of seedling lethality gene in the homozygous condition, a pure stand of trisomics will be produced which is more advantageous. The only disadvantage in this method is the seed rate is doubled to provide for the 50% mortality of the seedlings.ExampleThe first commercial hybrid barely be using BTTs was produced by the name Hembar which is a spring type in Arizona state of USA.Haplo viable mutants (a substitute for BTTs) for hybrid seed productionHaplo viable mutants,that present normal functioning of pollen but are transmitted through the eggs carrying them are called male gamete eliminator. If closely linked to a male sterility gene (ms), such a male gamete eliminator can be used to produce male sterile female parents of hybrids in a manner analogous to the BTT with male sterlity and haplo viability gene to make it a balanced lethal system. The male gamete eliminator, when present in heterozygous condition, produces 50% healthy pollen carrying the normal allele of the haplo-viable mutant in a scheme suggest for the use of the haplo viable mutants for hybrid seed production, a plant that is heterozygous for three closely linked gene 9male fertility vs male sterility, Msms; green vs albino seedlings, Aa; normal vs haplo viable; 1-1h), carrying all dominant alleles on the same chromosome, will produce two types of plants on selfing.1. Male sterile albinos that will die.1. Male fertile green haplo viables, which are recycled and used as a pollinator for female rows consisting of plants that are male sterile and heterozygous for seedling lethality gene (msmsAa).
All viable pollen will carry recessive alleles for all the three (mash) and will produce seed an female rows, which will give rise two types of plants, heterozygous (aa) having albino seedlings that will die and heterozygous (Aa) having green seedlings. These seed harvested from the female rows is partly recycled and partly used for planting female rows in the hybrid seed production crossing block. A number of haplo-viable mutants, some of them closely linked to male sterlity low msg 2 an chromosome to have actually been isolated. Upto 99% male sterile progenies were obtained using haplo-viable mutants. Development of commercially useful seeds material using these haplo-viable mutants is under study.Telocentric trisomicAn individual with a normal chromosome complement plus an extra telocentric chromosome is called telotrisomic or telocentric trisomic and the condition is called as telocentric trisomy (2n+ t). The telocentric fragment chromosome is homeologous to one arm of a chromosome pair in the standard complement.SourcesTelotrisomics were isolated in the progenies of triploids, primary trisomics, asynaptic plants, compensating trisomics tertiary monosomics, monoisodisomics and monotelodisomics which are produced by mis division of centromere resulting in formation of telocentric chromosomes. They have been reported in several plant species such as wheat, barley, maize, Datura and tomato. Meiotic and Breeding behaviourTelocentric chromosome fragment can pair homoeologous arm of the normal chromosome and may form trivalent. If unpaired or if there is no chisma formation after pairing telocentric remains as univalent. Trivalents of different shapes such as Y shaped, rod shape, frying pan and zig zag at metaphase I.The small size of the telocentrics reduces the chance of chisma formation and therefore, there is less recovery of these chromosomes in the progeny. The transmission rate of telocentric through male is low ranging from 0.0 to 3.2 %.UsesThe telotrisomics can be used1. to determine the order of genes in a linkage1. to establish chromosome arm gene association1. to locate the centromere position Compensating trisomicTrisomic 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 is called as compensating trisomics.In a compensating trisomics, the missing chromosome may be compensated by two tertiary chromosmes are iso and are telocentric chromosomes.SourceCompensating trisomics are found in the progeny of plants showing association of 6 chromosomes obtained by crossing two interchanges which have one chromosome in common. The first compensating trisomics obtained in the progeny of a radium treated plant in Datura in 1921 was a double tertiary trisomic, where a missing chromosome 1.2 was compensated by two tertiary chromosomes. Such double tertiary compensating trisomics will result from plants which have rings of six, eight or more chromosomes due to induced interchanges. If more number of chromosomes are involved in the interchange ring, the more types of double tertiary trisomics can be expected in the progeny.Meiotic and breeding behaviourIn a compensating trisomic, where missing chromosome is translocated to two different non-homeologous chromosome (eg in Datura, 11 pairs + 1.2+1.9+2.5), chains of seven chromosome is formed. Two types of gametes are formed, are with x+1, and other with x chromosomes. These two types of gametes will be formed in 1:1 ratio, if no lagging of chromosomes occurs, but if lagging occurs, the frequency of gametes with extra chromosome may be reduced. The transmission of gametes with extra chromosomes in the progeny depend on the compensating ability and the viability of these gametes.UsesCompensating trisomics have been utilized in Triticum monococcum for locating eleven genes belonging to seven linkage groups. Acrotrisomics and metatrisomicsCentric 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. Trisomics having these fragments as extra chromosome are designated as acrosomic trisomics or acrotrisomics and metasomic trisomics or metatrisomics. Acrocentric trisomics are produced due to terminal deletion of one arm. Acrotrisomics are obtained from the progenies of telotrisomics, primary trisomics and triploids.In metacentric trisomics, the extra chromosome is metacentric with large segments of chromosome missing but the centromere unchanged. Sometimes, the extra chromosome becomes a pseudo iso chromosome and forms a ring trivalent with two other chromosome at meiosis. Such metatrisomics are available in barley.The autotrisomics and metatrisomics are useful in physical mapping of genes, which is not possible through the use of other trisomics, as the breakpoint is measurable in these. TetrasomyThe condition where in one chromosome is present four times instead of twice in addition to the normal chromosomal complement is said to be tetrasomy and the individual, cell to tissue carrying is said to be tetrasomic (2n+2).OriginTetrasomics are obtained in the progeny of trisomics (2n+1), by selfing or intercrosssing, when a (n+1) type female gamete is fertilized by a (n+1) male gamete. Depending upon the frequency of transmission of the extra chromosomes through male or female, the frequency of obtaining tetrasomics may be low or high. The tetrasomics have been reported in several plant species, such as Datura and wheat. Partial tetrasomics are reported in Barley that contains two fragment chromosomes that are transmitted through both egg and pollen.Meiotic behaviourTetrsomics show the presence of a quadrivalent during meiosis by four homologues, which distinguishes a tertasomic from a double trisomic easily.Uses1. Used for genome analysis is segmental allopolyploids.1. Used to determine the relative of genes from the centromere are measured by the frequency of double reduction.1. Used to establish homeologous groups in allopolyploids such as wheat.
Nulli-tetra compensation in wheatSears used the compensating nullisome tetrasome combinations where particular tetrasome compensates more or less completely for a certain nullisome. By using this technique in wheat, it has also been demonstrated that the presence of an extra chromosome may separately compensate phenotypically for the absence of each of the two other specific chromosome. This was interpreted as an evidence for partial homology ie., homology among such chromosomes.In order to establish and confirm these relationships among all the 21 chromosome and to arrange them in seven sets of hree chromosomes each, nullisomic tetrasomic lines (19II+1IV) were derived from crosses between monosomics (20 II+1I) and tetrasomics (20 II+1IV). These nullisomic-tetrasomic whenever compared well with normal wheat plants, demonstrated compensation by the chromosome in tetrasomic state for the loss of one in nullisomic state. Hence these are called as Compensating nullisomic-tetrasomic lines and the condition is called as Nulli-tetra compensation.Sears concluded that there are seven chromosome groups of three homeologous chromosomes. These groups are called as homeologous groups and are designated as 1(A,B,D) to 7(A,B,D). here the nullisomic condition for any one of the three can be compensated by the tetrasomic condition of either of the other two.TelosomyA telosomic is an individual that has one or more telocentric chromosome as part of its chromosome complement and they are otherwise called as telosomes (Endizizi and Kotel, 1966).MonotelosomicsAre monosomics, that base unpaired univalent chromosome or telosome. In wheat the monotelosomy produce 20 bivalent + unpaired telosome t. montelosomy used for mapping the genes. If a dominant marker gene (A) is located on the any arm of telosome, in a montelodisomic gene it can be recovered. By test cross after crossing over. Telosomes have been used for determining the chromosome arm location in polyploid species such as cotton and oats. The distance of a gene from the centromere can be determined by the use of telosomic because centromere becomes the marker. Telosome is transmitted usually partly.Mapping of genes in wheat by monotelosomicIn wheat the telosomics available are maintained as ditelosomics (20 II + t II) (Sears, 1966).
Alien addition lines (MAAL)Transfer of individual whole chromosomes
Whole genomes can be added for the production of many amphiploids as in the case of triticales. But except in triticale, results are limited. Hence addition or substitution of whole alien chromosome may give desired results. The technique for the production of addition and substitution lines are given below.Alien addition lines An amphiploid (2n=56) between wheat and rye is first produced following normal method of crossing the two species followed by doubling of the chromosome number in F1 hybrid. The amphiploid is crossed back to wheat giving a heptaploid with 2111 + 71, where the bivalents belong to wheat and univalents belong to rye. On selfing these heptaploids (2n=7x), monosomic (2111 + 11 rye) and disomic addition lines (2111 + 111 rye) are obtained. Whenever monosomic additions are available, these may be selfed to get the disomic addition linesThe identification of addition lines for different alien chromosomes is achieved by any one of the following techniques.1. Morphology1. Intercrossing1. Karyotype
1. Addition of different chromosomes, sometimes leads to the modification in the morphology, so that different addition lines can be distinguished from each other. 1. Addition lines also provide an opportunity to study the effect of individual alien chromosomes in the uniform wheat background. 1. Wheat rye addition lines were produced at different places using different varieties of wheat and rye, namely Chinese spring wheat and Imperial rye (Missouri, USA), Hold fast wheat and king II rye (Cambride UK) and kharkov wheat and Dakold rye (Winnipeg, Manitoba, Canada). Individual addition lines in these cases could be identified morphologically, although at Manitoba, the identification was also done on the basis of chromosome morphology i.e karyotype.1. Intercrossing between addition lines may help to find out if the plants have received same or different chromosomes through a study of meiosis in F1 hybrid, which exhibit 2111 + 21 if they differ.2. Addition of chromosomes from Aegilops, Agropyron and Haynaldia to wheat The addition lines were also produced in wheat for a number of diploid Aegilops species like Aegilops umbellulata, Ae. longissima, Ae bicornis, Ae. comosa etc., Recently, individual chromosome additions from Aegilops squarrosa (D genome) to tetraploid (4x) wheat have also been produced. In some cases, when a cross between hexaploid wheat and an alien species is not successful tetraploid wheat is used as bridging species, so that a hexaploid (2n-42) amphiploid is first produced, which is then crossed to hexaploid wheat to produce a hexaploid hybrid (2n=6x=1411+141). These hexaploid hybrids, on selfing may produce heptaploids (211+71) carrying seven alien chromosomes as univalents. On selfing this may then produce addition lines. This technique has been successfully utilized for the production of addition lines of Haynaldia villosa and Ae. umbellulata.Alien addition lines of Agropyron junceum, A. intermedium and A elongatum on wheat have also been produced. A junceum carries genes for salt tolerance, while other Agropyron species carry other desirable traits including genes for resistance against various diseases. Several of these alien addition lines were utilized for transfer of alien chromosome segments for wheat improvement.Barley chromosome additions to wheat Among the different wheat alien addition lines, production of wheat barley addition lines by Islam (of Bangladesh origin) in Australia is considered to be a very significant achievement. The success in wheat barley crosses was achieved for the first time by Kruse (1973) followed by Islam et al., (1975). Viable hybrid plants could be easily obtained, when barley was used as female parent and when the developing embryos were treated with gibberellic acid and transferred to artificial culture medium. The success was 5.8% over all crosses made by Islam et al., (1975), although upto 15.9% success could be achieved when Chinese spring (wheat) was crossed with Botzes (a barley cultivars).Since morphological characters were not helpful in the identification of specific barley chromosomes, isozyme markers were used whenever possible. Subsequently barley chromosomes in the addition lines were identified through n-banding technique. Addition of wheat chromosomes to ryeMost of the additions of individual alien chromosomes have been successfully achieved in polyploid crops, since polyploids can easily tolerate additions, deletions and substitutions. However, successful additions of individual alien chromosomes from wheat to rye have also been made recent pas (Schlegel et al., 1986). The F1 hybrids between wheat and rye were obtained using wheat as female parent and were backcrossed to diploid rye. In BC4 generation the first alloplasmic monosomic rye-wheat addition lines (2n=15=14R + 1W) were isolated and identified using N-banding and isozyme markers. Monosomic addition were achieved for ten different wheat chromosomes (3A, 4A, 5A, 7A, 1B, 5B, 6B, 2D, 3D, 7D). these monosomic addition were subsequently used for transfer of small alien segment to rye chromosomes through irradiation of premeiotic spikes.Alien addition lines in riceMAAL A complete series of monosomic aliens addition lines with O.sativa genome and a single chromosome of O.offcinalis were also produced. First O.sativa is crossed with O.offcianlis triploid is obtained and F1 is backcrossed with O.sativa to traanfer whole genome of O. sativa.In the second backcross (BC2)plants with 2x+1 were selected on the basis of morphology; these alien addition lines exhibited homeologous relationship between chromosomes of O.sativa and O.offcinalis. O.officinalis carries useful traits like resistant to brown plant hopper and MAAL 6 carries this resistance. BPH resistant plants were available in dosomic progenies produced by selfing MAAL 6 (or) directly in BC2 progenies.In addition, MAALs of O.officinalis, O.latifolia, O.brachyantha were also used for location of isozyme loci. Thus the MAALs (2n+1) also have one extra chromosome in addition to the normal chromosome, but this chromosome from the alien species.Twelve morphologically distinct types of MAALs were isolated. These MAALs expressed striking resemblance to the O.sativa primary trisomics. This suggests that the O.officinalis and O.sativa chromosomes have similar gene content and have homologous genomes.Alien addition lines in cottonIndividual lien chromosome addition in monosomic cotton have also been obtained in tetraploid cotton, Gosssypium hirsutum (2n = 4x = 52 = AADD), using the alien species G.sturtianum (2n = 2x = CC). These traits include (i) glandless seed, (ii) cold tolerance and (iii) disease resistance. Pentaploid BC1 BC4 hybrids (2n = 5x = AADDC) were screened and a number of monosomic adddition (MA) lines (with 2n= 53= AADD+1C) were isolated (Altman et al., 1987). These alien addition lines in cotton are being used for further studies and will certainly be used in future for transfer of desirable traits from G.sturtianum to G.hirsutum.The disomic addition lines can not be produced from the above monosomic addition (MA) lines, since none of the alien chromosomes from G.sturtianum in these MA lines is pollen transmitted. The female transmission from G.sturtianum In these MA lines is pollen transmitted. The female transmission varied from 0 to 100 % for chromosome C1 A and averaged to 23 % in other monosomic additions (C1 B,C1 C,C1 D; Rooney and Stelly,1991).Alien sustitution linesAfter achieving the objective of producing lien addition lines in wheat, their evaluation suggested that, they were invariably unstable, and at meiosis, they exhibited a higher frequency of univalents than in the normal wheat. Consequently, the addition lines had a tendency to e revert back to normal wheat with 2n= 21II. Therefore, regular cytological checking was required to maintain them. In view of this problem with addition lines and due to other undesirable effects on the phenotype, the addition lines could not be considered suitable for commercial use. It was argued that if chromosome number is maintained at normal euploid (2n = 42) level by substituting a pair of alien chromosomes for a pair of wheat chromosomes, then the product may be more desirable and therefore, acceptable for cultivation.Alien substitution in tobaccoThe first alien substitution lines were produced, while attempting transfer of necrotic type of mosaic resistance from Nicotiana glutinosa (n= 12) to N. tabacum (n = 24). The amphiploid (2n = 72) was crossed to N. tabacum and subsequent selection for resistance in the successive backcrosses, led to the development of a resistant variety Samsoun. An analysis of this variety showed that a pair of N.glutinosa chromosomes had been substituted for a pair of N. tabacum chromosomes. Monosomic analysis showed that the replaced chromosome was h chromosome .Alien substitutions in wheatThe first report of alien substitution in wheat involved rye chromosome 1, which was spontaneously for a wheat chromosome during wheat x rye crosses. However, for a systematic production of alien substitution lines, the addition lines, each with 22II, were backcrossed to different monosomics used as female parent. The F1 plants with 20II + 2I (1W + 1R) were selected, which either on selfing or on backcrossing to the same addition line gave monosomic or disomic substitution lines.
Monosomic x alien addition line(20II + 1I ) {21II (W)+ 1II(R)}
20II + 2I x 21IIW+1II R(W) (1W+1R)select 21II21II + 1I (21IIW or 20 IIw+1 IIR) (21 IIW + 1I R) or (20 IIW+ 1 II R + 1IW)
20 IIW+ 1 II RUtilizing that the above method, a number of alien substitution lines in wheat were produced initially for rye (Secale cereale) chromosomes and later using other species. In all these cases of alien substitutions, it was demonstrated that an alien chromosome can effectively substitute and compensate only for a corresponding homeologous chromosme. Therefore, these substitutions can not be non specific, but rather specific. However, one should recognize the fact that non-compensating substitutions which are comparable in growth and vigour to corresponding nullisomics (or even worse), have actually been produced in a non specific manner, but these substitutions were not useful, the plants being very weak and non maintainable.
