Everything you need to know about doubled haploids in crop improvement !
Q. 1. What do you mean by doubled haploids?
Ans. Doubled haploids refer to the diploid lines which are obtained by doubling the chromosome number of a haploid line by colchicine treatment. Such condition is referred to as doubled haploids.
Q. 2. What are main features doubled haploids?
Ans. Main features of doubled haploids are briefly presented below:
i. Doubled haploids are represented as DH.
ii. Double haploids are developed from haploids by doubling the chromosome number of haploids by colchicine treatment.
iii. Double haploids are completely homozygous lines.
iv. Doubled haploids can be produced in both self pollinated and cross pollinated species.
v. Double haploid method allows to fix recombinant gametes directly as fertile homozygous lines.
vi. This is a rapid method of producing pure lines. The pure lines can be produced from a heterozygous parent in one generation. The development of pure lines or inbred lines by conventional methods takes 3-5 years.
vii. Doubled haploids can be released as cultivars after testing or can be used as parents of hybrids.
viii. Doubled haploid method reduces the time needed to develop and release new cultivars by 3-5 years in comparison with conventional breeding techniques.
ix. Doubled haploids are homozygous and homogeneous populations and hence their produce is highly uniform.
x. The doubled haploids obtained from recombinant lines generally have broad genetic base due to gene combination from two different sources.
xi. The doubled haploid method opens a unique way for the fixation hybrid vigour or heterosis in homozygous lines, which would avoid all problems associated with the production of hybrid seed. For example, from a cross between AAbb x aaBB genotypes we can get a haploid with AB genotype which can be fixed as doubled haploid (AABB).
xii. Doubled haploids segregate much simpler in the progeny of crosses being true breeding individuals.
Q. 3. What are differences between doubled haploid and conventional methods of producing pure lines?
Ans. There are several differences in the doubled haploid and a conventional breeding method used for developing pure lines/inbred lines.
Q. 4. What are advantages of doubled haploid method?
Ans. The double haploid method of breeding has several advantages which are briefly discussed below:
i. A Rapid Method:
It is a rapid method of developing pure lines as compared to conventional method. It takes one year for developing pure lines and 2-3 years for developing Cultivars.
ii. Fixation of Heterosis:
The DH method permits fixation of hybrid vigour. The recombinant gametes with heterotic effects can be fixed by this method. Thus DH method is useful in conserving heterosis.
iii. Gene Mapping:
Doubled haploid populations are commonly used for gene mapping and construction of linkage maps.
iv. Identification of Mutants:
Doubled haploid method is very much useful in the instant identification of recessive mutants which is very difficult by conventional methods.
v. Instant homozygosity:
DH method is useful in obtaining instant homozygosity. The homozygosity can be obtained in a single step.
vi. Uniform Produce:
Doubled haploid cultivars are homozygous and homogeneous, hence their produce are highly uniform and attractive.
Q. 5. What are disadvantages of doubled haploid method?
Ans. The doubled haploid method as some limitations or disadvantages which are briefly presented below:
1. Difficult to Obtain:
The doubled haploids are developed from haploids. It is difficult to obtain haploid in all crops. The production of haploids varies from crop to crop. In some crops such as cereals and rap seed they can be easily obtained while in other crops they are obtained with difficulty.
2. Expensive Method:
It is an expensive method of developing pure lines. The cost is 2-3 times higher than conventional method of developing pure lines. However, high cost of this method is compensated from the results achieved. This method requires well equipped tissue culture laboratory and well trained manpower, because haploids are obtained from anther culture or interspecific crosses.
Q. 6. What do you mean by haploid?
Ans. An individual having gametic chromosome number in the somatic cells is known as haploid and such condition is referred to as haploidy.
Main features of haploids are presented below:
i. Haploids occur spontaneously as well as can be induced artificially. The spontaneous haploids have been reported in 10 families covering 26 genera and 36 species. Main crops in which spontaneous haploids have been reported include tomato, cotton, coffee, beets, barley, flax, pearl millet, rapeseed, asparagus and wheat.
ii. Haploids are sterile due to lack of pairing partners of chromosomes,
iii. Haploids are represented by n, Haploids differ from monoploids. Monoploids have single copy of the genome and are represented by x. In a true diploid species both monoploid and haploid chromosome number is the same (n = x). Thus a monoploid is haploid but all haploids cannot be monoploids.
iv. Haploids are of different types; their detailed description is beyond the scope of this discussion.
Q. 7. How would you induce haploids?
Ans. Haploids can be induced in four principal ways, viz. by:
(i) Wide crossing,
(ii) Irradiation and – chemical treatment,
(iii) Selection of twins, and
(iv) By anther and pollen culture.
These are briefly discussed below:
(i) Wide Crossing:
It includes interspecific and intergeneric hybridization and is also known as distant hybridization. Interspecific crosses have resulted in development of haploids in wheat and barley. Use of maize as pollen parent for wheat has produced high frequency of haploids in wheat.
(ii) Irradiation and Chemical Treatment:
The pollination with irradiated pollen stimulates the unfertilized eggs to parthenogenic development. Such haploids have been developed in tabacco, wheat, snapdragon and oenothera. Application of a dye toluene blue (TB) to the pistils has induced haploids in Vinca rosea, tomato, maize and poplar.
(iii) Selection of Twins:
In some species haploids can be obtained from the screening of poly-embryonic seeds. Poly-embryonic seeds can produce three types of seedlings viz. haploid-haploid; diploid-diploid and haploid-diploid. This has been reported in capsicum and in some horticultural species.
(iv) Anther and Pollen Culture:
This method is widely used for production of haploids. Haploids develop directly from pollen grains in culture, either through direct formation of embryos from pollen grains or formation of callus and subsequent plant regeneration.
Q. 8. What are advantages of haploids?
Ans. Haploids have several advantages or applications in crop improvement which are presented below:
i. Haploids are used for rapid development of homozygous diploids or polyploids. In other words, haploids are used for developing doubled haploids.
ii. Hapioids reduce the time needed for developing new cultivars.
iii. Haploids are useful in identification of recessive mutants.
iv. Haploids can be used to produce a male or female plant from a dioecious species such as asparagus.
v. Haploids can also be used for fixation of heterosis in recombinant lines.
vi. Haploids can be used for gene mapping, transformation, somatic hybridization, germplasm storage etc.
Q. 9. What are disadvantages of haploids?
Ans. There are some disadvantages of haploids which are briefly presented below:
i. The crop cultivars respond differently to nutrient or culture medium. The culture medium suitable for regeneration of one variety may not be suitable for another variety.
ii. The response to colchicine treatment also varies from genotype to genotype.
iii. Sometimes unwanted traits such as albinism may be fixed with doubled haploid method.
iv. The cost of raising doubled haploids for plant breeding is expensive.
v. Doubled haploid lines often appear inferior to inbred lines obtained by conventional method. This may be due to very high level of homozygosity.
vi. Moreover, colchicine treatment, gametoclonal and somaclonal ‘ variation may have negative effect on the agronomic performance of doubled haploid plants.
Q. 10. What is dihaploid?
Ans. A haploid individual which is produced from a tetraploid species is referred to as dihaploid. It has haploid or gametic chromosome number of a tetraploid species. In other words, it has two copies of the genome.
Q. 11. Define double haploid.
Ans. A haploid individual with haploid chromosome sets of two diploid species is referred to as double haploid. Such individuals have two sets of genome, one from each species. The term double haploid was coined by Warmke and Blakeslee in 1939.
Q. 12. What do you mean by deploidization?
Ans. The process by which a polyploid species behaves like a diploid species is referred to as diploidization. There are several polyploid species such as wheat, oats, Brassica, tobacco and cotton which behave like a diploid species. The diploidization is the result of intragenomal pairing or homologous pairing.
This pairing behaviour of chromosomes in these polyploid species is under genetic control. In wheat the homologous pairing is controlled by a mutant gene present in the long arm of chromosome 5 of B genome. This system in wheat is known as 5B system which prevents homeologus (intergenomal) pairing and permits only homologous pairing.
Q. 13. What is double reduction?
Ans. In polyploids, a condition in which a heterozygous individual (simplex-aa aA) produces homozygous gametes with dominant alleles (AA) is referred to as double reduction. It occurs when sister alleles migrate to the same gamete during meiosis. It generates variability and causes decrease in the equilibrium frequencies of deleterious alleles. Such gametes are formed under special situations.
Q. 14. What do you mean by quasi-diploid?
Ans. An individual having trisomic-monosomic constitution i.e. 2n+l-l is referred to as quasi-diploid. Such individuals originate from a cross between trisomic and nullisomic parents.
Q. 15. Describe briefly practical achievements of doubled haploid method.
Ans. Doubled haploids (DH) are presently being used in breeding of a number of crop species. Doubled haploid cultivars have been developed in different countries such as Canada, USA, UK, Spain, China, Hungry, and Italy etc. The maximum DH cultivars have been developed in Barley followed by Rapeseed, wheat, melon, pepper, Rice, Asparagus, Tobacco and Eggplant (Table 20.2). Moreover, maximum doubled haploid cultivars have been released by anther culture.
Q. 16. What are uses of doubled haploids?
Ans. The use of doubled haploid method is gaining increasing importance these days looking to its various advantages.
Doubled haploids are used in:
i. Development of purelines
ii. Development of cultivars
iii. Development of hybrids as parents
iv. Construction of genetic maps
v. Gene tagging/locating genes
vi. Identification of molecular markers for trait selection.