Most of our cultivated crops are generally cross pollinated in nature. Many of the cereals (Particularly maize), forage crops, vegetables and fruit crops belong to this category.
These plants are heterozygous and posses a highly complex genotype and carry harmful recessive genes in the genome pool. In such plants, inbreeding is deleterious, as it brings together recessive genes in the homozygous condition.
The methods employed in breeding of cross pollinated plants are diverse and many a time cannot be clearly defined as in self pollinated crops. There are three important steps in the breeding of these plants. These are Selection, hybridization and production of composite varieties.
Image Source: newyorker.com
1. Selection:
ADVERTISEMENTS:
Mass selection is usually followed. In addition to this, progeny selection and recurrent selection is also done. In progeny selection, selected offspring are grown in separate plots to ascertain the breeding behavior of these plants.
Seeds from many of these plants are grouped together and progenies are selected from these. In recurrent selection, plants with desired traits are selected and selfed. The seeds from these are sown in rows and are intercrossed in all combinations.
Hybrid seeds obtained from these are put in a bulk to produce a mixed population. Seeds from suitable plants from these are again sown in rows (seeds from a single plant are sown in arrow).
ADVERTISEMENTS:
Cross pollination is allowed and seeds obtained. In this way, interbreeding and cross breeding are alternatively followed to finally select a progeny for further breeding.
2. Hybridization:
This is done in cross pollinated crops as follows –
i. Single cross method:
Two inbred lines A and B are crossed and the hybrid is used for raising the crop.
ADVERTISEMENTS:
ii. Three way cross method:
Three inbred lines are involved in this (A, B and C). At the first stage, the stocks A x B are crossed and the progeny of this is crossed with C. (A x B) x C. In the second cross the F, of the first cross is usually used as the female parent.
iii. Double cross method:
Four inbred lines A, B, C, and D are used in this. At the first stage crosses are made between A x B and C x D. The F, of each of these crosses is crossed to obtain the progeny used for further selection. The crosses may be symbolically represented as follows.
Double cross method has been widely exploited in the case of maize which has resulted in a very high yield. This method has been widely employed in USA with phenomenal success.
In our country also work on these lines has been going on at IARI, Delhi and Punjab Agricultural University, Ludhiana. Some of the well known double cross varieties are –
Ganga 101, Ganga 2, Ganga 3 etc., in maize HBI variety and NB 21 of Bajra
CHS1 and CHS2 of Sorghum
Top cross method:
In this method widely used in maize, many inbred lines are chosen and back crossed with the original variety there are two types of top crosses viz, single top cross and double top cross.
In single top cross method, two different inbred lines are crossed with each other and the F, crossed with the original variety.
Individuals with desired traits are selected. In double top cross method F hybrids of two different lines are crossed and the product crossed with one of the parents.
3. Synthetic or composite varieties:
A number of selected genotypes are crossed and a synthetic variety is produced after a series of crosses. Only the desirable traits from a large number of plants are put together in the synthetic variety.
A synthetic variety may be defined as a strain developed as a result of crossing and combining together many clones, in which seeds are harvested in bulk and replanted in successive generations. Synthetic varieties of maize have been produced in our country.
Some of these are – Jawahar, Amber, Vijay, Vikram, Kissan, Sona etc. Synthetic varieties grown away from local varieties could be used again and again to produce seeds for the next season. In the case of hybrid varieties, seeds have to be produced afresh (after crossing) for every crop.