Here is an essay on ‘Distant Hybridization’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Distant Hybridization’ especially written for school and college students.
Essay on Distant Hybridization
Essay Contents:
- Definition of Distant Hybridization
- Types of Distant Hybridization
- Techniques Used in Obtaining Zygotes from Distant Crosses
- Roles of Distant Hybridization in Crop Improvement
- Achievements of Distant Hybridization
- Applications of Distant Hybridization
- Limitations of Distant Hybridization
Essay # 1. Definition of Distant Hybridization:
Crossing between two different species of the same genus or two different genera of the same family is called distant hybridization and such crosses are referred to as distant crosses or wide crosses. Wide crossing or distant hybridization has been used in the genetic improvement of some crop plants.
It is an effective means of transferring desirable genes into cultivated plants from related species and genera. Distant crosses are more successful in more closely related species or genera than in less closely related species or genera.
Essay # 2. Types of Distant Hybridization:
Distant hybridization is of two types, viz:
(1) Interspecific hybridization, and
(2) Intergeneric hybridization.
I. Interspecific Hybridization:
Crossing or mating between two different species of the same genus is referred to as interspecific hybridization. Because interspecific hybridization involves two species of the same genus, it is also termed as intrageneric hybridization.
Main features of interspecific hybridization are given below:
1. It is used when the desirable character is not found within the species of a crop.
2. It is an effective method of transferring desirable genes into cultivated plants from their related cultivated or wild species.
3. Interspecific hybridization is more successful in vegetatively propagated species like sugarcane and potato than in seed propagated species.
4. Interspecific hybridization leads to introgression which refers to transfer of some genes from one species into the genome of another species.
5. Interspecific hybridization gives rise to three types of crosses, viz.
(a) Fully fertile,
(b) Partially fertile, and
(c) Fully sterile in different crop species.
a. Fully Fertile Crosses:
Interspecific crosses are fully fertile between those species that have complete chromosomal homology. Chromosomes in such hybrids have normal pairing at meiosis and as a result the F1 plants are fully fertile.
Fully fertile interspecific crosses have been observed between some species in cotton, wheat, oats and soybean as given below:
i. Cotton:
There are four cultivated species of cotton viz. Gossypium, hirsutum, G. barbadense, G. arboreum and G. herbaceum. The first two New World species belong to tetraploid group (2n = 52) and the last two Old World species to the diploid group (2n – 26). Crosses between tetraploid species G. hirsutum and G. barbadense and between diploid species G. arboreum and G. herbaceum are fully fertile.
G. hirsutum (2n = 52) x G. barbadense (In = 52) → F1 plants are fully fertile.
G. arboreum (2n = 26) x G. herbaceum (2n = 26) → F1 plants are fully fertile.
ii. Wheat:
The hexaploid wheat (2n = 42) has several species. Interspecific crosses between common wheat (Triticum aestivum) and club wheat (T. compactum) are fully fertile.
Triticum aestivum (2n = 42) x T. compactum (2n = 42) → F1 plants are fully fertile.
iii. Oats:
There are two cultivated species of oat, viz. white oat (Avena sativa) and red oat (Avena byzantiana). Both these species are hexaploid (2n = 42). Crosses between these two species are fully fertile.
Avena sativa (2n = 42) x A. byzantiana (2n = 42) → F1 plants are fully fertile.
iv. Soybean:
The cultivated soybean (Glycine max) is believed to have originated from wild species G. Soja. Both these species are annual diploid (2n = 40). The other wild species are perennials. The crosses between G. max and G. Soja are fully fertile.
Glycine max (2n = 40) x G. Soja (2n = 40) → F1 plants are fully fertile.
b. Partially Fertile Crosses:
Interspecific crosses are partially fertile between those species which differ in chromosome number but have some chromosomes in common. In such situations, the F1 plants are partially fertile and partially sterile.
Partially fertile interspecific crosses have been reported in wheat, cotton and tobacco as given below:
i. Wheat:
In wheat, there are three types of species, viz. diploid (2n = 14), tetraploid (2n = 28) and hexaploid (2n = 42). The cross between common wheat (Triticum aestivum, 2n = 42) and durum wheat (T. durum, 2n = 28) are partially fertile. In both these species chromosomes of A and B genomes are common and as a result the F1 hybrids are partially fertile. In F1 there are 14 bivalents and 7 univalents during meiosis. There is occasional seed set in this cross.
ii. Cotton:
In cotton, there are two types of species, viz. diploid (2n = 26) and tetraploid (2n = 52). The cross between American cultivated cotton (G. hirsutum, 2n = 52) and American wild diploid (G. thurberi) are partially fertile, because these two species have chromosomes of D genome in common. Meiosis in F1 leads to formation of 13 bivalents and 13 univalents. There is occasional seed set in this cross.
iii. Tobacco:
In tobacco, there are three types of species, viz. diploid (2n = 24), tetraploid (2n = 48) and hexaploid (2n = 72). The cross between hexaploid wild tobacco (Nicotiana digluta) and common tetraploid tobacco (N. tabacum) shows partial fertility due to common chromosomes of T1 and T2 genomes in these two species. Meiosis in F1 leads to formation of 24 bivalents and 12 univalents. There is rarely seed set in this cross.
c. Fully Sterile Crosses:
Interspecific crosses are fully sterile between those species which do not have chromosomal homology. In such species, chromosome number may or may not be similar. The lack of chromosomal homology does not permit pairing between the chromosomes of two species during meiosis.
As a result, the F1 plants are fully self-sterile. Such hybrids can be made self-fertile by doubling of chromosomes through colchicine treatment. Fully sterile F hybrids have been reported in tobacco, wheat, cotton, Brassica, Vigna and several other crops.
i. Tobacco:
Clausen and Goodspeed (1928) made a cross between two wild diploid species of tobacco, viz. Nicotiana sylvestris (2n = 24) and N. tomentosa (2n = 24). The F1 hybrid was sterile. When the F1 plants were treated with colchicine, a fully fertile tetraploid (2n = 48) was obtained which resembled cultivated species (N. tabacum).
They made another cross between another two wild diploid species of tobacco, namely N. paniculata (2n = 24) and N. undulata (2n = 24). Again the F1 was sterile. Treatment of F1 with colchicine resulted in the production of fertile amphidiploid (2n = 48) which was similar to cultivated species N. rustica.
ii. Cotton:
Harland (1940) made a cross between Asian cultivated diploid (Gossypium arboreum, 2n = 26) and American wild diploid (G. thurberi, 2n = 26). The F1 was sterile. Treatment of F1 plants with colchicine resulted in the production of fertile amphidiploid (2n = 52) which was similar to upland cotton (G. hirsutum).
iii. Brassica:
Interspecific crosses in the genus Brassica were made by several workers. Three crosses were made among three species namely cabbage (Brassica oleracea), rapeseed (B. campestris) and black mustard (B. nigra). The F1 hybrids were sterile in all the three crosses.
The treatment of F1 plants with colchicine resulted in the production of fertile amphidiploid in each cross as given below:
iv. Vigna:
Interspecific crosses were made between green-gram (Vigna radiata, 2n = 22) and black-gram (V. mungo, 2n = 22) by Singh and Singh, 1975 and others. The F1 was sterile. The doubling of chromosome number of F1 through colchicine treatment resulted in the production of fertile amphidiploid.
II. Intergeneric Hybridization:
Intergeneric hybridization refers to crossing between two different genera of the same family. Such crosses are rarely used in crop improvement because of various problems associated with them.
The main features of intergeneric crosses are given below:
1. Intergeneric hybridization is used when the desirable genes are not found in different species of the same genus.
2. This method is rarely used in crop improvement programmes and that too for transfer of some specific characters into cultivated species from allied genera.
3. Intereneric hybridization has been generally used in asexually propagated species.
4. F1 hybrids between two genera are always sterile. The fertility has to be restored by doubling of chromosomes through colchicine treatment.
5. Intergeneric hybridization was used by some workers to develop new crop species.
Some examples of intergeneric hybridization are given below:
i. Wheat-Rye Cross:
The first intergeneric cross was made in the family Gramineae between bread wheat (Triticum aestivum, 2n = 42) and rye (Secale cereale 2n = 14) by Rimpau around 1890 in Sweden. The F1 was sterile which was made fertile through colchicine treatment. The amphidiploid (2n = 56) was named as Triticale.
This combines yield potential and grain quality of wheat and hardiness of rye. Triticale is the best example of the practical achievements of intergeneric hybridization. Now Triticale is commercially grown in countries like Canada and Argentina. Several improved varieties of Triticale have been released for commercial cultivation. Research work on Triticale is in progress at CYMMIT, Mexico.
ii. Radish Cabbage Cross:
Intergeneric cross between radish (Raphanus sativus) and caage (Bassica oleracea) of the family Cruciferae was made by Karpechenko in 1928 in Russia. The main objective was to combine root of radish with leaves of cabbage. The F1 was sterile. e doubling of chromosome number by colchicine treatment resulted in development of fertile amphidiploid which was named as Raphanobrassica by Karpechenko. But the new species thus developed had roots like cabbage and leaves like radish, which was a useless combination.
iii. Intergeneric Crosses in Sugarcane:
Several intergeneric crosses have been made in sugarcane. There are eight genera in which intergeneric hybrids have been made with sugarcane (Saccharum). These genera include Eccoilopus, Erianthus, Miscanthidium, Miscanthus, Narenga, Rapidum, Sclerostachya and Sorghum (sweet sorghums).
Many of the intergeneric hybrids are easily made with the help of male sterility. However, intergeneric hybrids with sugarcane have made little contribution to the development of modern commercial cultivars. Intergeneric hybrids have great potential for the improvement of germplasm.
iv. Maize-Tripsacum Crosses:
Intergeneric crosses between maize and Tripsacum were also attempted. The basic chromosome number is 10 in maize and 9 in Tripsacum. Crosses are successful in both directions, but hybrids can be more easily produced when Tripsacum is used as the female parent, because the maize pollens are able to produce long pollen tube to reach the ovule.
On the other hand, Tripsacum pollen are unable to produce long pollen tube to reach the ovule of maize. Hence when reciprocal cross is made, the maize styles (ear silks) have to be reduced in length by cutting so that the Tripsacum pollen tube can reach the ovule. Now hybrid derivatives of Tripsacum x diploid maize are being utilized in commercial crop improvement programmes.
v. Intergeneric Crosses in Barley:
Intergeneric crosses of barley (Hordeum) were attempted with some species of Avena, Phleum, Dactylis, Alopercunis, Triticum. Lolium, and Festuca. Seed set was a major problem in these crosses.
Application of 2, 4-D prior to pollination followed by gibbrellic acid treatment was found useful in making above intergeneric crosses successful. But none of these intergeneric crosses contributed to cultivar development in barley. They remained only of academic interest.
Essay # 3. Techniques Used in Obtaining Zygotes from Distant Crosses:
There are several techniques which are used to make wide crosses successful.
The following techniques are useful in obtaining zygotes from distant crosses:
i. Choice of Parents:
Genetic differences exist among parents in a species for cross compatibility. More compatible parents should be selected for use in wide crosses.
ii. Reciprocal Crosses:
It is better to attempt reciprocal crosses when distant crosses are not successful. Because reciprocal crosses are successful in some cases. For example, interspecific cross between Vigna radiata and V. mungo is successful only when former is used as female and later as male parent.
iii. Manipulation of Ploidy:
When two species of a cross differ in chromosome number, it is necessary to match their ploidy level by doubling the chromosome of the species with low ploidy. It may enhance the chances of obtaining a zygote.
iv. Bridge Crosses:
Sometimes, two species say A and C do not cross directly. In such case a third species say B which can cross with both A and C is chosen as a bridge species. First B is crossed with C and then the amphidiploid is crossed with A. Bridge crosses have been used in tobacco and wheat.
In tobacco, Nicotiana repanda can cross with N. sylvestris but not with N. tabacum. But N. sylvestris can cross with N. tabacum. For transfer of genes from N. repanda, N. sylvestris is used as a bridge species. It is first crossed with N. repanda and the resulting amphidiploid is crossed with N. tabacum.
Similarly, in wheat the cross between Aegilops ventricosa and Triticum aestivum is sterile. T. turgidum is used as a bridge species for transfer of genes from Aegilops ventricosa to T. aestivum. The bridge cross is a complicated procedure and is more successful for transfer of monogenic dominant characters. This may be used when other techniques do not work in interspecific or intergeneric gene transfer.
v. Use of Pollen Mixtures:
Cross incompatibility results due to unfavourable interaction between the protein of pistil and pollen which inhibits normal germination and growth of pollen tube. This problem has been overcome in certain interspecific crosses by using the mixture of pollen from compatible (self) and incompatible parents.
vi. Manipulation of Pistil:
In some cases, pollen tube is short and style is very long, due to species difference. Thus pollen tube cannot reach ovule to effect fertilization. In such situation either reciprocal cross should be made or the style should be cut to normal size before pollination. This technique is successful in maize – Tripsacum crosses, where maize style remains receptive even after cutting.
vii. Use of Growth Regulators:
Sometimes, the pollen tube growth is so slow that the egg cell dies or the flower aborts before the male gametes reach the ovary. In such cases, growth regulators should be used to accelerate the pollen tube growth or to prolong the viability of pistil. Use of growth regulators such as IAA, NAA, 24-D and gibberellic acid has helped in making wide crosses successful in some crops.
viii. Large Number of Crosses:
The success of seed set is generally very low in wide crosses. Hence, large number of crosses should be made to obtain crossed seeds.
ix. Protoplast Fusion:
The wide crosses can be obtained through protoplast fusion, when it is not possible to produce such crosses through sexual fusion. However, this technique still requires perfection and refinement for adoption in practical plant breeding.
x. Embryo Culture:
This technique is being used widely to obtain viable interspecific or intergeneric hybrids. This is used when hybrid zygote is unable to develop. This technique has been successfully used in Triticum, Hordeum, Phaseolus, Nicotiana, Gossypium, Lycopersicon, Trifolium, Cucurbita and several other species.
xi. Grafting:
Grafting of interspecific hybrid on to the cultivated species helps in making the cross successful. Grafting has helped in survival of interspecific hybrid in sugar-beet (Beta vulgaris) and Trifolium and induced flowering in interspecific Glycine hybrids.
Essay # 4. Role of Distant Hybridization in Crop Improvement:
Wild species or wild genetic resources are the potential sources of desirable genes for various characters of crop plants. Wide crossing is an effective method of exploiting desirable characters from wild species for the improvement of cultivated crop plants. Thus the significance of wild species and distant hybridization are interlinked.
Distant hybridization has played significant role in:
(1) Improving the crop plants for:
(i) Disease and insect resistance,
(ii) Quality,
(iii) Adaptation,
(iv) Yield,
(v) Mode of reproduction, and
(vi) Several other characters;
(2) Developing commercial hybrids in some crops, and
(3) Creation of new crops.
These aspects are briefly discussed below:
i. Character Improvement:
(i) Disease and Insect Resistance:
Distant hybridization has been instrumental in transferring disease resistance from wild species into cultivated ones. For example, resistance to rust and black arm in cotton; mosaic virus, wild fire, black-fire, blue mould, black root rot, and Fusarium wilt diseases in tobacco; sereh disease in sugarcane; late blight, leaf roll and virus x in potato; rust and eye spot in wheat; and yellow mosaic virus in okra have been transferred from wild species of these crops into cultivated species (Table 28.2).
In tomato, resistance to bacterial cankar, bacterial wilt, Fusarium wilt, grey leaf spot, leaf moulds Verticillium wilt, curly top virus, mosaic virus has been transferred from wild species to the commercial cultivars. Use of wild root stocks, in commonly grafted crops such as citrus, rubber, grape, pistachio and peach has eliminated many insect pests and diseases of these horticultural crops.
Less progress has been made on insect resistance. Resistance to jassids and boll weevil in cotton, leaf chewing insects in peanut, and aphids in strawberry has been transferred from their wild species to cultivars (Table 28.2).
(ii) Improvement in Quality:
In some crops, wild species have been used to improve the quality of cultivated ones. For example, protein content in rice, oats and rye; fibre length in cotton; oil quality in oil palm; carotenoid content in tomato; starch content in potato; leaf quality in tobacco; and oil per cent in oats have been improved through the use of their wild species in the hybridization programme (Table 28.3).
Teosinte has been used to improve maize for silage. Wild Sorghum has been used to improve green fodder in cultivated species. Wild tobacco has been utilised to reduce nicotin content in cultivated species and flavour of cultivated tea has been improved through the use of wild tea.
(iii) Improvement in Adaptation:
Adaptation to various environmental conditions has been improved through the use of wild species. For example, tolerance to cold in rye, wheat, onion, potato, tomato, grapes, strawberry and peppermints etc. has been transferred from wild species of these crops in Russia. In wheat, increased winter hardiness has been transferred from Agropyron.
In grape, hardier vines have been developed through the use of wild species Vitis amurensis in the breeding programme. In sugarcane, cold tolerance has been transferred from wild species in USA. Drought tolerance in peas and wheat, salt tolerance in tomato, tolerance to calcareous soils and photo insensitivity in Pennisetum have been achieved through the use of their wild species in the breeding programmes.
(iv) Improvement in Yield:
Improvement in yield has also been achieved through the use of wild species in some crops. For example, in oat yield increase of 25-30% over the recurrent parent was obtained from a cross between Avena sativa x A. sterilis. High yielding transgressive segregants were obtained after 4 backcrosses.
Increase in yield has been reported in several crops such as Vigna, Zea, Ribes, vanilla, Arachis, potato and tobacco through interspecific hybridization. In tobacco, yields were increased by the use of wild species Nicotianci debneyi. Yields of sugarcane and octaploid strawberries have been increased by the use of their wild species.
(v) Mode of Reproduction:
Use of wild species in the hybridization programmes sometimes leads to alteration in the mode of reproduction. The male sterility is the most common alteration in the mode of reproduction which results from interspecific hybridization. Cytoplasmic male sterility (CMS) is an economic device for hybrid seed production.
CMS has been discovered in crosses between wild and cultivated species in wheat, cotton, barley, tobacco, potato, sunflower and ryegrass. The CMS has been transferred to cultivated species of these crops. Apomictic genes have been transferred from maize — Tripsacum cross to maize and from wild species of Beta to cultivated species. The cleistogamy and self-fertility traits of wild Secale have been transferred to cultivated rye (secale cereale).
(vi) Other Characters:
There are several other desirable characters which have been transferred from wild species to cultivated plants. For example, wild species have been used to transfer dark green colour and excellent leaf texture in lettuce and bright red thin flesh in red peppers. Semi-dwarf wheat has obtained from Triticum x Agropyron hybrid derivatives. Short statured oil palms resulted from interspecific hybrids. Earliness has been achieved from use of wild species in soybean.
ii. Hybrid Varieties:
Improved hybrid cultivars have been developed through the use of wild species mainly in sugarcane, potato and some forage crops. Most of the modem cultivars of sugarcane and potato are the derivatives of interspecific hybridization. In cotton, commercial interspecific hybrids have been developed both at tetraploid and diploid levels but between cultivated species only.
Some of the varieties of upland cotton (MCU 2, MCU 5, Deviraj, Devitej, G 67, Khandwa 1, Khandwa 2, Badnawar 1 PKV081, Rajat and Arogya) are derivatives of interspecific hybridization. A hybrid between Pearl-millet and napier grass has been developed which has become very popular by virtue of its high fodder yield potential and superior fodder quality.
iii. New Crop Species:
Sometimes, distant hybridization and polyploidy lead to creation of new crop species. Nicotiana digluta has been synthesized from a cross between N. tabacum and N. glutinosa. Triticale is the example of new crop which has evolved from an intergeneric cross between Triticum aestivum and Secale sereale and combines good characters of both the species.
Essay # 5. Achievements of Distant Hybridization:
There are three main achievements of distant hybridization:
(1) Transfer of various characters from wild species to the cultivated species,
(2) Development of interspecific hybrids in some crops, and
(3) Creation of new crop plants (Table 28.4).
Various characters such as disease and insect resistance, improved quality and adaptation, earliness, dwarfness, tolerance to frost, drought and salinity have been transferred from wild species to the cultivated species through interspecific and intergeneric hybridization. Resistance to various diseases has been achieved in several crops like wheat, cotton, tobacco, sugarcane, potato, strawberry, okra etc. through distant hybridization.
Cytoplasmic male sterility has been transferred from wild species to cultivated ones in wheat, barley, cotton, tobacco, ryegrass and several other crops. Resistance to boll weevil and jassids in cotton and leaf chewing insects in peanut has been incorporated from wild species. Quality has also been improved in several crop plants.
Interspecific hybrids have been developed for commercial cultivation in sugarcane. Several modern cultivars of sugarcane have been developed from crosses of Saccharum officinarum with S. spontaneum or S. barberi. These crosses combine high sugar content of S. officinarum with the disease resistance, cold tolerance and vigour of S. spontaneum and S. barberi. Similarly, most of the modern cultivars of potato are derivatives of interspecific hybrids.
In India, interspecific hybrids have been developed for commercial cultivation in cotton. Interspecific hybrids have been developed between cultivated tetraploid species viz. Gossypium hirsutum and G. barbadense, and cultivated diploid species, viz. G. arboreum and G. herbacium.
The important tetraploid hybrids include Varalaxmi, JKHY 11, CBS 156, Savitri, DCH 32, HB 224, NHB 12, TCHB 213 DHB 105 and Sruthi. These hybrids are grown in south and central cotton growing zones. Four hybrids have been developed between G. arboreum and G. herbaceum (DH 7, DH 9, Pha 46 and DDH 2). The first two are grown in Gujarat State. DDH 2 in Karnataka and Pha 46 in Maharashtra.
There are two examples of new crops which have evolved through distant hybridization. The first is the Triticale which has evolved from intergeneric cross between Triticum aestivum and Secale cereale. Another example is garden strawberry which has evolved from a natural interspecific cross between American octaploids Fragaria cliloensis and F. virgineana in a botanical garden.
The resulting hybrid combines desirable character of both the parents. Triticale also combines good characters of both the parents, viz., grain quality and yield potential of wheat and winter hardiness of rye. Both these new species are grown for commercial cultivation.
Essay # 6. Applications of Distant Hybridization:
The application of distant hybridization in crop improvement is not an easy task. Several problems are associated with distant hybridization.
The main barriers to the use of distant hybridization include:
(1) Cross incompatibility,
(2) Hybrid inviability,
(3) Hybrid sterility, and
(4) Hybrid breakdown.
These problems along with their remedial measures are discussed below:
i. Cross Incompatibility:
Inability of the functional pollens of one species or genera to effect fertilization of the female gametes of another species or genera is referred to as cross incompatibility. In another words, failure of male and female gametes to unite to form zygote in interspecific and intergeneric hybrids is known as cross incompatibility.
This is a major problem in distant hybridization. There are three main reasons of cross incompatibility, viz. lack of pollen germination, insufficient growth of pollen tube to reach ovule and inability of male gamete to unite with egg cell. These barriers are known as pre-fertilization barriers.
ii. Hybrid Inviability:
In some wide crosses, fertilization occurs and zygote formation also takes place. But the zygote does not grow. This inability of a hybrid zygote to grow into a normal embryo under the usual conditions of development is referred to as hybrid inviability.
This may result due to three main factors:
(i) Unfavourable interaction between chromosomes of two species,
(ii) Disharmony between cytoplasm and nuclear genes and,
(iii) Unfavourable interaction among embryo, endosperm and maternal tissues.
The following techniques may be useful to overcome the problem of hybrid inviability:
Proper choice of parents, making reciprocal crosses and application of growth hormones increase favourable conditions for the development of zygote into viable seed. If the growth of embryo is inhibited by the endosperm the embryo can be removed and transferred to the culture medium.
The new plants can be regenerated from the embryoids in the culture medium. The embryo cultures have been identified and developed for various plant species. Thus embryo culture technique is an effective way of overcoming the problem of hybrid zygote development.
iii. Hybrid Sterility:
In most of the wide crosses, hybrid sterility is the major problem. The hybrid sterility refers to the inability of a hybrid to produce viable offspring. The problem of hybrid sterility is more acute in intergeneric crosses than in interspecific crosses. The interspecific crosses vary from complete fertility to complete sterility.
But intergeneric crosses are always sterile. The main cause of hybrid sterility is lack of structural homology between the chromosomes of two species.
This leads to non-pairing or reduced pairing of chromosomes resulting in following meiotic abnormalities:
i. Scattering of chromosomes throughout spindles during metaphase I.
ii. Extension of chromosomes into cytoplasm.
iii. Lagging of chromosomes during anaphase.
iv. Formation of Anaphase Bridge.
v. Presence of ring and chain configurations.
vi. Irregular and unequal anaphase separation of chromosomes.
All these meiotic irregularities lead to structural chromosomal changes, viz. deletions, duplications, translocations and inversions which cause absence of pollen formation or formation of nonfunctional or abortive pollens. In some cases, sterility has been found to be associated with completely normal pairing of chromosomes (genie sterility).
Sometimes, the sterility is due to small structural changes in chromosomes which is not detectable during meiosis. Stebbins termed it as criptic structural hybridity.
The sterility caused by structural differences between the chromosomes of two species can be overcome by doubling the chromosome number of the hybrid through colchicine treatment. After chromosome doubling, each chromosome will have a pairing partner at meiosis. This will lead to normal chromosome pairing and production of viable gametes.
iv. Hybrid Breakdown:
Hybrid breakdown is a major problem in interspecific crosses. When F1 plants of an interspecific cross are vigorous and fertile but their F2 progeny is week and sterile, it is known as hybrid breakdown. Hybrid breakdown hinders the progress of interspecific gene transfer.
There are two main causes of hybrid breakdown, viz.:
(i) Gene combination, and
(ii) Structural differences.
(i) Gene Combination:
Sometimes, homozygous dominant alleles on several loci prefer to remain in one species and homozygous recessive alleles at the same loci in another species. The F1 cross between such species would be heterozygous and vigorous. In F2, the favourable combination of dominant and recessive genes is broken due to segregation and recombination. The plants which do not have a dominant allele at each locus or which are not homozygous for all recessive alleles would be weak and sterile.
(ii) Structural Differences:
There may exist some small structural differences in the chromosomes of two species, which do not affect chromosome pairing in F1. In such hybrids, recombination between chromosome segments during meiosis may lead to production of gametes with deletions or duplications. The gametes with deletions and duplications result in hybrid breakdown.
Essay # 7. Limitations of Distant Hybridization:
Though distant hybridization has several useful applications in crop improvement, it has some limitations which have restricted its extensive use in crop improvement.
Some of the limitations are briefly discussed below:
1. Distant crosses are associated with problems of cross incompatibility, hybrid inviability, hybrid sterility and hybrid breakdown. These problems pose several difficulties in interspecific or intergeneric gene transfer.
2. Several special techniques, viz. ploidy manipulation pistil manipulation, chemical (growth regulator) treatment, bridge crossing, grafting, embryo culture etc. have to be adopted to make distant hybrids successful in some cases. Thus this is a combursome task.
3. Desirable characters are generally linked with some undesirable characters which pose difficulties in the use of desirable genes from wild species through distant hybridization. Several chromosome addition and substitution lines have been developed in wheat but- none of them could be used for commercial cultivation due to presence of some undesirable genes.
4. Sometimes, distant hybrids have several undesirable characters such as non-flowering, late maturity and seed dormancy and useless combinations like Raphanobrassica.
5. Transfer of characters controlled by recessive genes is very difficult in interspecific crosses.
6. In distant hybridization transfer of characters is not as simple as in intervarietal crosses.
No comments yet.