In this article we will discus about:- 1. Meaning and Features of Hybrid Varieties 2. Development of Hybrid Varieties 3. Use of Cytoplasmic Genic Male Sterility 4. Achievements 5. Merits and Demerits.
Meaning and Features of Hybrid Varieties:
The progeny of a cross between genetically different plants is called hybrid. In other words, hybrid is F1 generation of mating between genetically dissimilar plants. The F1 populations that are used for commercial cultivation are referred to as hybrid varieties. The hybrid varieties are developed to exploit the heterosis or hybrid vigour.
The main features of hybrids are given as follows:
1. Productivity:
Hybrid varieties are vigorous and highly productive because heterosis or hybrid vigour is fully exploited in such varieties.
2. Genetic Constitution:
All the individuals or plants of a hybrid variety are genetically similar. Thus hybrid varieties are heterozygous but homogeneous populations. They are highly uniform and more attractive because of homogeneous nature.
3. Adaptability:
Hybrids have wider adaptability to environmental changes than inbreeds and pure line varieties due to high inherent buffering capacity and gene combination from two divergent parents.
4. Application:
Hybrids can be developed in both cross and self-pollinated species depending upon the magnitude of heterosis. However, hybrids are more common in cross pollinated species than self-pollinated crops.
5. Resistance:
Hybrids are generally more tolerant to biotic and abiotic stresses than inbreds and pure-line varieties.
Development of Hybrid Varieties:
Development of hybrid varieties differs from species to species.
The production of hybrid varieties in maize consists of three main steps, viz.:
(1) Development of inbreds,
(2) Selection of productive inbreds, and
(3) Production of hybrid seed.
i. Development of Inbreds:
Development of inbreds is an important step in the production of hybrids. There are two methods of developing inbred lines; one by selfing of heterozygous populations and another by doubling of haploids. Various populations viz. open pollinated varieties, synthetic varieties or any other heterozygous population can be used for selfing.
Superior plants on the basis of vigour, disease resistance and yield are selected and selfed. Progeny of selected plants are grown separately from the selfed seed in the next season. Again superior plants are selected in each progeny and selfed. In this way, selfing is continued for 6-7 generations to get superior homozygous inbreds.
The main purpose of selfing is to fix desirable genes in homozygous condition and eliminate genotypes with undesirable deleterious genes. Inbreds can also be developed from haploids by doubling the chromosome number through colchicine treatment. This is the short cut method of developing inbreds. There are various ways of developing haploids. The vigour which is lost during inbreeding is regained when two unrelated or diverse inbred lines are combined to develop F1 hybrid.
ii. Evaluation of Inbred Lines:
The value of an inbred is assessed from its performance in hybrid combination with other inbreds. The inbred lines are evaluated on the basis of their general combining ability (gca) and specific combining ability (sea).
Two methods, viz.:
(i) Top cross method, and
(ii) Single crosses are commonly used to measure the combining ability of inbreds.
(i) Top Cross Method:
Top cross refers to a cross between an inbred line and an open pollinated variety. Several inbred lines say 100 are crossed to a common tester (open pollinated variety) to produce 100 single crosses. The yield performance of these crosses is evaluated in replicated trials on multi-locations.
Those lines which produce high yielding single cross with tester are selected. This method was suggested by Davis in 1927. A large number of lines can be evaluated by this method at a time. The yield is compared from the mean yield of all the crosses. Inbred lines which give high yield in top crosses generally produce high yielding single crosses.
This method is used for measuring general combining ability (gca). Here combining ability refers to yield performance and not to gca variances and effects. The top cross seed is produced by planting alternate rows of inbred and open pollinated variety and removing the tassel (male inflorescence) of inbred line.
(ii) Single Cross Method:
This method is used to measure the specific combining ability (sea) of those inbreds which are selected on the basis of top cross performance. The selected lines are crossed in all possible combinations viz, n(n – 1)/2, where n is the number of inbred lines. With 10 inbred lines, there would be 10(10 – 1)/2 = 45 single crosses excluding reciprocals.
These single crosses are evaluated in replicated trials over several locations for yield performance. The best performing single crosses are identified for release as a variety or for use in the production of double cross hybrids. This method can evaluate only limited number of inbreds at a time, because inclusion of more inbreds in crossing increases the number of single crosses in such a way that their handling becomes difficult. For example use of 50 inbreds will give rise to 50(50 – 1)/2 = 1225 single crosses.
Time of Testing:
The testing of inbreds for general combining ability should be started from 3rd, 4th and 5th generation of selfing. This will help in retaining of inbreds with good combining ability and elimination of lines with poor combining ability. Some workers suggest that visual selection is effective in improving combining ability in early inbred generations.
Predicting Double Cross Performance:
Single crosses are used to predict the performance of double cross hybrid. The yield of a double cross can accurately be predicted from the mean yield of the four non-parental single crosses. The average performance of single crosses A x C, A x D, B x C and B x D is used to predict the performance of the double cross (A x B) (C x D).
The use of single cross performance for the prediction of double cross performance has become a standard breeding procedure. Multi-location or multi-seasonal testing is desirable to predict the performance of a double cross hybrid.
The predicted yield is worked out as follows:
Predicted yield = (A x C) + (A x D) + (B x C) + (B x D)/4
iii. Production of Hybrid Seed:
After identification of superior inbred lines, the hybrid seed is produced.
Three types of intervarietal hybrids, viz.:
(i) Single cross hybrid,
(ii) Three way cross hybrid, and
(iii) Double cross hybrid are produced.
In case of single and three way cross hybrids, the rows of female and male parents are planted in 2: 1 ratio.
The tassels of female parents are removed and natural cross pollination is allowed. In case of three way cross hybrid, the single cross hybrid is used as female parent and inbred lines as male parent. In case of double cross hybrid, the rows of female and male parents are planted in 3: 1 or 4: 1 ratio.
The tassels of female single cross are removed so that it is pollinated by the pollen of single cross used as male parent. The seed production is earned out in isolation to prevent crossing with other compatible genotypes and maintain the high genetic purity.
Use of Cytoplasmic Genic Male Sterility:
The use of cytoplasmic male sterility helps in reducing the cost of hybrid seed by eliminating the process of detasseling.
In maize, the CMS can be used in three different ways in the production of double cross hybrids as given below:
1. One CMS Inbred and No Restorer Gene:
In such case the single cross hybrid involving CMS inbred will be male sterile and the other single cross involving both fertile inbreds will be fertile. For the production of double cross hybrid, the male sterile single cross will be used as female and fertile single cross as male. The double cross hybrid thus produced will be male sterile, because none of the inbreds contains pollen restoring gene.
In such case another double cross hybrid is produced using all the four normal inbreds, which will be male fertile. The seed of male sterile and fertile double crosses is mixed in 3: 1 ratio and planted in the field. This will ensure proper pollination in male sterile double cross hybrid.
2. One CMS Inbred and One Inbred with Dominant Restorer Gene:
In such situation one single cross will be male sterile and another male fertile with restorer gene. Cross between these two single crosses will produce double cross hybrid, which produces male sterile and fertile plants in 50: 50 ratio. The fertile plants will pollinate the sterile plants.
3. One CMS Inbred and Two Inbreds with Dominant Restorer Genes:
In such situation, the double cross hybrid will be male fertile given as follows:
Practical Achievements of Hybrid Varieties:
Heterosis is utilized in two ways, viz. either fully or partially. Full heterosis is exploited through the development of hybrids and partial heterosis is exploited by developing synthetics and composites. In cross pollinated crops like maize and pearl-millet, heterosis is utilized fully as well as partially. But in often cross pollinated species and self-pollinated crops heterosis is fully exploited by developing high yielding hybrids.
Heterosis has been commercially exploited in cross pollinated species like maize, pearl-millet, sunflower, castor, onion and cucurbits. It has also been used in some self-pollinated species such as Sorghum, cotton, tomato, brinjal, rice and wheat. A brief account of practical achievements made in India through heterosis breeding is presented in Table 22.3.
All the varieties of sugarcane and potato are hybrids. Several promising hybrids have been developed in mango. The important mango hybrids include, Mahmood, Bahar, Prabha Shankar, Neeluddin, Swarna, Jehangir, Neelgoa, Neleshan, Mallika, Amrapalli and Ratna. Hybrids have also been developed in chilli and bottle guard.
In China, hybrid rice has been developed for commercial cultivation through the use of cytoplasmic genie male sterility. Hybrid rice is reported to cover about 18 million hectares in China. In India, efforts are being made to develop hybrids in rice. Similarly, in Japan hybrids have been developed and released in wheat. However, production of hybrid seeds is a major problem in wheat.
In India, ICAR has launched a Hybrid Project entitled, “Promotion of research and development efforts on hybrids in selected crops” from 1989 to intensify hybrid research. The selected crops include rice, maize, sorghum, pearl-millet, pigeon-pea, cotton, sunflower, rapeseed-mustard and castor. Several high yielding superior quality hybrids have been developed in above nine crops.
Merits and Demerits of Hybrid Varieties:
Merits:
1. Hybrid varieties have higher yield potential as compared to synthetics, composites and open pollinated varieties.
2. Hybrid varieties are more uniform and attractive than synthetics, composites and open pollinated varieties.
3. Hybrid varieties can be developed in both self and cross pollinated species, whereas synthetics and composites are relevant to cross pollinated crops.
4. It is possible to reconstitute the hybrid with same genotype, which is not possible in case of composite varieties and open pollinated varieties.
Demerits:
1. Fresh seed has to be produced every year, because in F2 the hybrid produces various types due to segregation and recombination. Farmer has to purchase fresh seed every year.
2. The seed of hybrid is costlier than synthetics, composites and open pollinated varieties.
3. Cultivation of hybrids requires more input (fertilizer, water, plant protection, etc.) to exploit their full potential.
4. Production of hybrid requires more technical skill and areas as compared to synthetics, composites and open pollinated varieties.
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