Development of Hybrid Varieties in Cross Pollinated Crops
Development of hybrid varieties differs from species to species. The production of hybrid varieties in maize consists of three main steps.
I) Development of inbreds.
II) Selection of productive inbreds and
III) Production of hybrid seed.
I) Development of Inbreds:
It is an important step in the production of hybrids. There are two methods of developing inbred lines i.e i) By selfing of heterozygous population and ii) By doubling of haploids.
Various population viz. open pollinated varieties, synthetic varieties or any other heterozygous population can be used for selfing. Superior plants on the basis of vigour, disease resistant and yield are selected and selfed. Progeny of selected plants are grown separately from the selfed seed in the next season. Again selection is made for the superior progeny and selfed. This processes is continue to get superior homozygous inbred.
II) Evaluation and Selection of Inbreds Lines:
The inbred is evaluated from its performance in hybrid combination with other inbreds. They are evaluated on the basis of their general combining ability and specific combing ability. There are two methods:
a) Top Cross Method:
It refers to a cross between an inbred line and an open pollinated variety. Single 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 trails on multi locations. The line, which produces high yielding single cross with tester, are selected. Davis suggested this method in 1927. Inbred line which give high yield in top crosses generally produce high yielding single crosses.
b) Single Cross Method:
This method is used to measure the specific combining ability ( SCA) of those inbreds, which are selected on the basis of top cross performance. The selected lines are crossed in all possible combination. 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. This method can evaluate only limited number of inbreds at a time.
Time of Testing:
The testing of inbred for general combining ability should be started from 3 rd, 4 th, and 5 th generation of selfing. This will help in retaining of inbreds with good combining ability and elimination of lines with poor combining ability.
III) Production of Hybrid Seed:
After identification of superior line of the hybrid seed is produced. There are three types of Intervarietal hybrid. a) Single cross hybrid, b) Three way cross hybrid and c) Double cross hybrid. 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 parent 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. While in case of double cross hybrid , the rows of female and male parent are planted in 3:1 or 4:1 ratio. The seed production is carried out in isolation to prevent crossing with other compatible genotypes and maintain the high genetic purity.