The following points highlight the two main types of heteroploidy. The types are: 1. Euploidy 2. Aneuploidy.
Heteroploidy: Type # 1. Euploidy:
The change in chromosome number which involves entire set is called euploidy. In other words, euploidy refers to numerical change in entire genome. The somatic chromosome number of an euploid individual is exact multiple of basic number of that species. Euploidy includes monoploids, diploids and polyploids.
a. Monoploids and Haploids:
Monoploids contain a single chromosome set and are characteristically sterile. In other words, monoploids have the basic chromosome number (x) of a species. Monoploids differ from haploids … which carry half or gametic chromosome number (n). In a true diploid species, both monoploid and haploid chromosome number is the same (n = x).
Thus a monoploid can be a haploid but all haploids cannot be monoploids. The main differences between monoploids and haploids are presented in Table 6.1. In monoploids proper meiosis does not occur due to lack of pairing partners of chromosomes.
Types of Haploids:
Depending upon the origin, haploids are of two types, viz., euhaploids and aneuhaploids. Euhaploids develop from a euploid species and have complete chromosome set. Euhaploids are of two types, viz., mono-haploid which develop from a normal diploid species: and polyhaploids—which develop from polyploid species.
Polyhaploids are again of two types, namely allohaploids— which arise from allopolyploid species; and auto-haploids—which develop from autopolyploid species. When a haploid develops from a tetraploid species, it is called di-haploid.
Aneuhaploids develop from aneuploid species and have either one additional or missing chromosome. Aneuhaploids include disomic haploids (n + 1), nullisomic haploids (n – 1), substitution haploids (n – 1 + 1), misdivision haploids etc. Mis-division haploids have an isochromosome which is produced by vertical division of centromere. Generally centromere divides longitudinally. Aneuhaploids are generally inviable.
Origin and Production:
Haploids may originate in several ways in various crop plants . In maize, they occur spontaneously by parthenogenesis though at a very low frequency. In some members of Solanaceae, haploids develop from interspecific crosses. Such haploids have been observed especially in potato crosses differing in ploidy level.
In barley, haploids develop through chromosome elimination from interspecific crosses. In pepper, they develop as a member of a set of twins. In cotton, haploids develop by semi-gamy. In wheat, haploids are developed through the use of alien cytoplasm. In tobacco, haploids are developed by anther culture.
Now various methods are known by which haploids can be produced at will. These methods include:
(i) Pollination with foreign pollen,
(ii) Delayed pollination,
(iii) Use of X-ray irradiated pollen for pollination,
(iv) Temperature shock,
(v) Treatment with chemicals like colchicine,
(vi) Interspecific and inter-generic crosses, and
(vii) Anther and pollen culture.
The anther and pollen culture technique was first used for the production of haploids in Datura (Guha and Meheswari, 1964). Now this technique is used for the production of haploids in tobacco, rice, and several other crops.
Uses of Haploids:
Haploids have several applications in plant breeding. They are used for:
(i) Development of pure lines,
(ii) Disease resistance,
(iii) Development of inbred lines, and
(iv) Production of aneuploids.
These aspects are briefly described below.
Development of Pure Lines:
Pure lines can be obtained through chromosomal doubling of haploids. This is a short cut method of obtaining homozygous lines.
Doubled haploids can be used in two important ways, viz:
(i) As a parent in the hybridisation, and
(ii) As a cultivar.
In maize, doubled haploids have been successfully used in the development of commercial hybrids and in barley for developing new cultivars. In Brassicci napus, a doubled haploid called Maris Haplona has been released from the cultivar ‘Oro’. The new variety is superior to the parent variety in oil yield.
In tobacco, doubled haploids obtained from anther culture have been used to improve the disease resistance of the leading Japanese flue cured cultivar Mc 1610.
Production of Inbreds:
In dioecious plants, development of inbreds requires several generations (6-7) of selfings. The haploid method is a useful tool for obtaining inbred lines within two crop seasons in such crop species.
i. In wheat, a complete series of nullisomics, monosomies, and trisomies was developed from haploids. These aneuploids provided the basic material for extensive studies about breeding, cytogenetics and evolution of wheat crop.
ii. In wheat, a haploid deficient for chromosome 5B provided the evidence that homologous pairing was prevented by the activity of genes on the 5B chromosome. This concept of diploidiation created interest about pairing in other polyploids.
iii. In potato and alfalfa, haploids have provided convincing evidence for the polysomic nature of these tetraploid crops.
Normal diploids are known as disomics. They have regular bivalent pairing during meiosis. Diploids also have disomic genetics with two alleles at each locus. Pure lines or inbred lines of diploids have homozygosity at each locus. Polyploids which behave like diploids are known as disomic polyploids like wheat, cotton, etc.
An organism or individual having more than two basic or monoploid sets of chromosomes is called ployploid and such condition is known as ployploidy. It is estimated that about one third species of flowering plants are polyploids. In wild species of grass family, polyploidy has been reported up to 70%.
In animals, polyploidy is rare because of its lethal effects. It is found only in those species of animals which are hermaphrodite such as leeches and earthworms or those which develop parthenogenetically like aphids.
Polyploidy is of two types, viz:
(i) Autopolyploidy, and
Polyploids which originate by multiplication of the chromosome of a single species are known as autopolyploids or autoploids and such situation is referred to as autopolyploidy. In other words, autoploidy refers to the situation in which additional sets of chromosomes arise from the same species.
Autoploids include triploids (3x), tetraploids (4x), pentaploids (5x), hexaploids (6x), septaploids (7x), octaploids (8x), and so on. Autoploids are also known as simple polyploids or single species polyploids.
They have three sets of chromosomes of the same species. They can occur naturally or can be produced artificially by crossing between autotetraploid and diploid species. Triploids are generally highly sterile due to defective gamete formation. Triploids are useful only in those plant species which propagate asexually like banana, sugarcane, apple, etc.
A few examples of practical use of autotriploidy are given below:
Cultivated varieties of banana are triploids and seedless. Such bananas have larger fruits than diploid ones.
Some varieties of apple are triploids which are propagated asexually by budding or grafting.
Triploid sugarbeets have higher sugar contents than diploids and are generally’ resistant to moulds.
d. Water Melon:
Triploid water melons are seedless or have rudimentary seeds like cucumber. These seedless water melons are produced by crossing tetraploid female with diploid male. However, the reciprocal cross is not successful.
All other autoploids with odd chromosome sets, viz., pentaploids, septaploids, etc. also behave like triploids.
They have four copies of the genome of same species. They may arise spontaneously or can be induced artificially by doubling the chromosomes of a diploid species with colchicine treatment. Tetraploids are usually very stable and fertile because pairing partners are available during meiosis.
In such individuals diploid gametes (2n) are formed. Autotetraploids are usually larger and more vigorous than the diploid species. Rye, grapes, alfalfa, groundnut, potato and coffee are well known examples of autotetraploids.
A few cases of practical use are described below:
Autotetraploid rye is grown in Sweden and Germany. They have larger seeds and higher protein than diploids.
Tetraploid grapes have been developed in California, USA, which have larger fruits and fewer seeds per fruit than diploids.
Tetraploid varieties of alfalfa are better than diploid in yield and have better recovery after grazing.
A polyploid organism which originates by combining complete chromosome sets from two or more species is known as allopolyploid or alloploid and such condition is referred to as allopolyploidy. Alloploids are also known as hybrid polyploids or bi-species or multi species polyploids.
An allopolyploid which arises by combining genomes of two diploid species is termed as allotetraploid or amphidiploid. Allopolyploidy can be developed by interspecific crosses and fertility is restored by chromosome doubling with colchicine treatment. Alloploidy has played greater role in crop evolution than autopolyploidy, because allopolyploidy is found in about 50% of crop plants.
Some important natural allopolyploid crops are wheat, cotton, tobacco, mustard, oats, etc. Interspecific crossing followed by chromosome doubling in nature have resulted in the origin of allopolyploids (Table 6.2).
The origin of some natural allopolyploid crops is briefly presented below:
The bread wheat (Triticum aestivum) is an allopolyploid. It is believed that A genome of wheat has come from Triticum monococcum (2n = 14), D genome from Triticum tauschi (2n = 14) and B genome from unknown source probably from an extinct species (2n = 14). Thus hexaploid wheat has two copies of the genomes from three species.
First allotetraploid Triticum turgidum developed from a cross between Triticum monococcum and unknown species of B genome. Then cross between T. turgidum and T. tauschi resulted in the development of hexaploid wheat T. aestivum.
There are two cultivated species of tobacco, viz., Nicotiana tabacum and N. rustica. N. tabacum is an amphidiploid between N. sylvestris (2n = 24) and N. tomentosa (2n = 24). N. rostica is believed to be amphidiploid between N. paniculata and N. undulata. Each of these two species has 2n = 24.
The tetraploid American cotton (Gossypium hirsutum) is believed to be an amphidiploid between G. africanum, an old world species, and G. raimondii, a new world species. Both these species are diploid with 2n = 26. The chromosomes of old world species are larger than new world species.
The cultivated oat (Avena sativa, n = 21) is an allohexaploid which is considered to have originated from a cross between A. barbata (tetraploid. n = 14) and A. strigosa (a diploid, n = 7).
In brassica, there are three basic species, viz., Brassica nigra (BB, n =8) B. oleracea (CC, n = 9) and B. campestris (AA, n = 10). The cross between B. nigra and B. oleracea gave rise to B. carinata. Cross between B. campestris and B. oleracea led to the development of B. napus, and cross between B. campestris and B. nigra resulted in the development of B. juncea. All the resulting species are amphidiploids.
Artificial alloploids have been synthesized in some crops with two main objectives, viz:
(i) Either to study the origin of naturally available alloploids or
(ii) To explore the possibilities of creating new species (Table 6.3).
Some examples of artificial alloploids are given below:
This is a classic example of artificially synthesized alloploid. This was developed between radish (Raphanus sativus, n = 9) and cabbage (Brassica oleracea, n = 9) by Russian geneticist Karpechenko in 1927. He wanted to develop a fertile hybrid between these two species with roots of radish and leaves of cabbage.
But he got a fertile amphidiploid (4n = 36) by spontaneous chromosome doubling which unfortunately had roots of cabbage and leaves of radish. Thus it was of no use.
Clausen and Good speed synthesized a new hexaploid species of tobacco (Nicotiana) from a cross between Nicotiana tabacum (2n = 48) and N. glutinosa (2n = 24). The F1 was sterile with 2n = 36, which was made fertile by doubling of chromosome through colchicine treatment. The new species is known as N. digluta.
Triticale is a new crop species which has been synthesized from a cross between wheat (Triticum aestivum) and rye (Secale cereale, n = 7). Some triticales are developed from cross between tetraploid wheat (Triticum turgidum) and rye and some from cross between hexaploid wheat (T. aestivum) and rye.
The F1 was sterile, which was made fertile by colchicine treatment. Triticales produced using tetraploid and hexaploid wheat are hexaploid and octaploid, respectively. Triticale is now commonly grown in Canada, Mexico, Hungary and some other countries.
In wheat, McFadden and Sears developed hexaploid between Triticum turgidum (formerly T. dicoccum) and T. tauschi (formerly Aegilops squarrosa). This resembled T. aestivum (formerly T. spelta) and produced fertile F1 when crossed with natural T. aestivum. This suggested involvement of these two species in the evolution of T. aestivum in the long past.
The American upland cotton (Gossypium hirsutum) was synthesized from a cross between G. herbaceum and G. raimondii. Both these species are diploid with 2n = 26. The former is old world cultivated diploid and the latter, new world wild diploid. This suggested involvement of these two species in the evolution of upland cotton.
A fertile amphidiploid was synthesized from a cross between greengram (Vigna radiata) and blackgram (Vigna inongo). The F) was sterile with 2n = 22, which was made fertile by doubling of chromosomes through colchicine treatment. This cross was successful only when greengram was used as female parent and blackgram as male parent. The reciprocal cross was not successful.
Induction of Polyploidy:
Polyploidy is mainly induced by treatment with a chemical known as colchicine. This is an alkaloid which is obtained from the seeds of a plant known as Colchicum autumnale, which belongs to the family Liliaceae. Colchicine does not affect Colchicum from which it is extracted, because this plant has an anticolchicine substance.
Colchicine is applied in a very low concentration, because high concentration is highly toxic to the cells. For effective induction of polyploidy, usually concentrations of 0.01% to 0.5% are used in different plant species. The colchicine induced polyploidy is known as colchiploidy.
In plants, colchicine is applied to growing tips, meristematic cells, seeds and axillary buds in aqueous solution or mixed with lanolin. The duration of treatment varies from 24 hours to 96 hours depending upon the species of plants.
Colchicine induces polyploidy by inhibiting formation of spindle fibres. The chromosomes do not line up on the equatorial plate and divide without moving to the poles due to lack of spindle fibres. The nuclear membrane is formed around them and the cell enters interphase. Thus nucleus has double the chromosome number.
Effects of Polyploidy:
Polyploidy has marked effects on the morphology of plants. The distinct features of autoploids are increase in general vigour and size of various plant parts. Such features are generally referred to as gigantism.
Autoploids have the following important features:
i. Stems are thicker and stouter.
ii. Leaves are fleshy, thicker, larger and darker green in colour.
iii. Roofs, are stronger and longer.
iv. Flowers, pollens and seeds are larger than diploids.
v. Maturity duration is longer and growth rate is slower than diploids.
vi. Water content is higher than diploids, etc.
Applications in Crop Improvement in Polyploidy:
Polyploidy plays an important role in crop improvement both autopolyploid and allopolyploidy are useful in several ways. However, allopolyploidy has wider applications than autopolyploidy.
Applications of autopolyploidy and allopolyploidy in crop improvement are briefly presented below:
Both triploids and tetraploids have been used in crop improvement. However, their applications have been limited to few species only. Autotriploids have been developed in sugar beets and water melon only. The triploid sugar beets have larger roots and higher sugar content than diploids.
The triploid water melons are seedless or have rudimentary and soft seeds like cucumber. The triploid seed is produced by using tetraploid as female and diploid as male. The reciprocal cross is not successful. Moreover, triploid water melons have irregular shape and fresh seeds have to be made every year.
Autotetraploids have been developed in forage crops like berseem, alfalfa and rye; vegetables like radish, turnips and cabbage; and fruits like grapes. Tetraploid varieties of rye are grown in Sweden and Germany. They have larger seeds and higher protein than diploids.
Tetraploid grapes have been developed in California USA, which have larger fruits and fewer seeds per fruit than diploids. Tetraploid varieties of alfalfa are better than diploid in yield and recovery after grazing. However, tetraploid cabbage and turnips have higher water content than diploids.
Alloploidy is useful in four principal ways, viz:
(i) In tracing the origin of natural allopolyploids,
(ii) In creating new species,
(iii) In interspecific gene transfer, and
(iv) As a bridging species.
These are briefly described below:
i. Tracing the Origin of Crop Species:
Alloploidy plays an important role in tracing the origin of natural allopolyploids. Study of chromosome pairing in a cross between allopolyploid and a diploid species helps in tracing the origin of polyploid species.
The affinity in pairing indicates the involvement of diploid species in the evolution of such polyploid. Lack of pairing between the chromosomes of two species rules out the involvement of diploid species in the origin of polyploid under study.
ii. Creation of New Species:
Alloploidy sometimes leads to the creation of new crop species. Triticale is the best example, which is alloployploid between wheat and rye. It combines desirable character of both the species i.e., grain quality of wheat and hardiness of rye. Triticales are of two types, viz., primary triticales and secondary triticales.
The primary triticales are derivatives of cross involving either tetraploid wheat or hexaploid wheat with rye. The secondary triticales are derivatives of the cross either between two primary triticales or between primary triticale and wheat. These are superior to primary triticales in several aspects.
Presently cultivated strawberry originated from a cross between North and South American species in the middle of seventeenth century in Europe. Loganberry was developed from a cross between raspberry and blackberry in 1880 in California, USA.
iii. Interspecific Gene Transfer:
When the desirable character is not found within the species, it is transferred from the related species. Interspecific gene transfer is done in two ways, viz., by alien addition and alien substitution. In case of alien addition, one chromosome of wild species is added to the normal complement of a cultivated species.
In case of alien substitution, one pair of chromosome is substituted in cultivated species with those of wild donor species.
Such type of gene transfer has been achieved in crops like wheat, tobacco, cotton and oats. In cotton, lint strength has been transferred from G. thurberi to G. hirsutum. However, in such gene transfer several undesirable characters (genes) are transferred along with desirable ones. In tobacco, mosaic resistance has been transferred from N. glutinosa to N. tabacum through alien addition.
iv. Bridging Cross:
Sometimes direct cross between two species is not possible due to sterility in FI. In such case, first an amphidiploid is made between such species and then amphidiploid is crossed with the recipient species. Such types of bridging crosses have been made for transfer of genes from wild species particularly in crops like tobacco and cotton.
Besides these, alloploids have some other applications. For example, heterotic effects can be conserved more easily in allotetraploids than in diploids. Moreover, interspecific hybrids can be made fertile by artificial doubling of chromosomes.
Limitations of Polyploidy:
Polyploidy has several limitations.
Some important limitations of polyploidy in crop improvement are briefly presented below:
i. Limited Use:
The single species polyploidy has limited applications. It is generally useful in those crop species which propagate asexually like banana, potato, sugarcane, grapes, etc.
ii. Difficulty in Maintenance:
The maintenance of monoploids and triploids is not possible in case of sexually propagating crop species.
iii. Undesirable Characters:
In bispecies or multispecies polyploids characters are contributed by each of the parental species. These characters may be sometimes undesirable as in case of Raphanobrassica.
iv. Some other Defects:
Induced polyploids have several defects such as low fertility, genetic instability, slow growth rate, late maturity, etc.
v. Chances of developing new species through allopolyploidy are extremely low.
Heteroploidy: Type # 2. Aneuploidy:
The change in chromosome number which involves one or few chromosomes of the genome is called aneuploidy and such individuals are known as aneuploids. In other words, an individual with other than exact multiple of the basic chromosome number is called aneuploid.
Aneuploids are of three types, viz:
(B) Nullisomics, and
These are described below:
An individual lacking one chromosome from a diploid set (2n-1) is called monosomic and such condition is known as monosomy. In other words, monosomics contain the normal diploid chromosome set except loss of one chromosome from one pair. Double monosomics lack one chromosome each from two different pairs (2n-1-1).
Monosomics have been reported in Drosophila, Nicotiana, Triticum, cotton and several other crops. Monosomics are viable in polyploid species where loss of a chromosome is balanced by homologous or partially homologous chromosomes from other genomes. In true diploid species, monosomics are generally inviable.
Monosomics may originate in three main ways, viz:
(i) From diploids,
(ii) From nullisomics, and
(iii) From trisomics as described below.
(i) From Diploids:
Monosomics may originate spontaneously from diploids. Sometimes nondisjunction during meiosis gives rise to n-1 gamete. If this gamete is fertilized by a normal (n) gamete, a monosomic zygote (2n-1) is produced.
(ii) From Nullisomics:
Nullisomics produce n-1 gametes.
Union of such gamete with normal gamete gives rise to monosomics as shown below:
(iii) From Trisomics:
Trisomics (2n+1) also give rise to monosomics. Sometimes nondisjunction of three homologous chromosomes in a trisomic during meiosis gives rise to n-1 gametes. Union of such gametes with normal one results in the development of monosomic zygote.
Interspecific hybridization and asynaptic strain also lead to the development of monosomics by producing n-1 type of gametes. Union of n-1 gamete with normal one gives rise to monosomic individual. Monosomic individuals produce two types of gametes, viz., n-1 and n.
In most of the cases, it has been found that n-1 gametes are more frequent than-n type of gametes. The single chromosome remains as laggard and ultimately is lost.
An individual lacking one pair of chromosomes from a diploid set (2n-2) is called nullisomic and such situation is referred to as nullisomy. In other words, nullisomic plants are with one pair of chromosome less than the normal.
Nullisomics are inviable in true diploid species. Among polyploids, hexaploids can tolerate loss of one pair of chromosome more than tetraploids, because they have two other pairs of similar chromosomes in the other genome. Monosomics and nullisomics together are known as hypoploids, which refer to loss of one or two chromosomes from the diploid complements.
An individual having either single or one pair of extra chromosome in the diploid complement is known as polysomic and such condition is referred to as polysomy. Polysomics are also known as hyperploids, which refer to addition of one or two chromosomes to a single or two different pairs.
Polysomics are of two types, viz:
(i) Trisomics, and
Trisomics are more frequent than tetrasomics.
Addition of one chromosome to one pair in a diploid set is known as trisomy and such individuals are known as Trisomics (2n+1). Trisomics may be of two types, viz., simple Trisomics and double Trisomics, When increase in chromosome number is in one pair only (2n+1), it is known as simple trisomic. When there is addition of one chromosome in two different pairs, it is called double trisomic (2n+1+1).
Trisomics were first reported by Blakeslee in 1910 in Datura. Now they have been reported in tomato, Nicotiana, Secalecereale, Pisum, Oenothera and Drosophils, If a plant has n=12 chromosomes, it can form 12 different Trisomics.
Depending on the nature of extra chromosome, simple Trisomics are of three types, viz:
(a) Primary Trisomics,
(b) Secondary Trisomics, and
(c) Tertiary Trisomics.
These are briefly described below:
(a) Primary Trisomics:
Trisomics in which the additional chromosome is normal one, are called primary trisomics. Such trisomics may exhibit three types of configurations at metaphase [Fig. 6.1(a)],
(b) Secondary Trisomic:
Trisomics having additional chromosome as isochromosome are known as secondary trisomics, lsochromosomes originate by vertical division of centromere. Both the arms of such chromosomes are homogeneous (Fig. 6.1). In case of secondary trisomics, a ring of three chromosomes is possible at meiotic metaphase [Fig. 6.1(b)].
(c) Tertiary Trisomics:
When additional chromosome in a trisomic is translocated one, it is known as tertiary trisomic. In this case, the chromosomes may exhibit either end to end configuration of chain of five chromosomes at meiotic metaphase [Fig. 6.1(c)].
Trisomics may arise in two different ways. In diploid species, sometimes non-disjunction during meiosis leads to the formation of n+1 and n-1 type of gametes. Union of n+1 gamete with normal (n) gamete leads to the development of trisomic individual. A cross between tetrasomic (2n+2) and normal diploid can also give rise to trisomics.
Addition of two chromosomes to one pair or two different pairs is known as tetrasomy and such individuals are known as tetrasomics.
When there is addition of two chromosomes to one pair (2n+2), it is called simple tetrasomic and when two chromosomes are added each to two different pairs, it is called double tetrasomic. Trisomics and tetrasomics can be tolerated even by diploid species, whereas monosomics and nullisomics are inviable.
Applications in Crop Improvement in Aneuploids:
Aneuploids are useful in crop improvement in various ways.
Some of the uses of aneuploids in plant breeding are briefly presented below:
i. Locating Genes:
Aneuploids are useful tools for locating genes on a specific chromosome. Monosomics and nullisomics are used for this purpose. Monosomic analysis has been used in wheat, cotton, tobacco, oat and other crops for locating genes on specific chromosome. In case of nullisomics, loss of a pair of chromosome will affect expression of some characters.
The altered characters are considered to be associated with the missing chromosomes. Thus genes on various chromosomes can be located by developing nullisomic series. However, monosomics are better for such analysis than nullisomics, because nullisomics are less vigorous and less fertile than monosomics.
ii. Interspecific Gene Transfer:
Monosomics are also used in transferring chromosomes with desirable genes from one species to another.
iii. Aneuploids are used for developing alien addition and alien substitution lines in various crops.
iv. Primary Trisomics are useful in identification of chromosomes involved in translocations.