A homozygous and homogeneous line that is produced by inbreeding and selection is known as inbred line. Inbred lines are produced by continued inbreeding. Inbreeding refers to mating between closely related individuals. In plant breeding, selfing and sib mating are the common forms of inbreeding.
Sib mating refers to crossing between individuals (plants) of the same family or line. Depending upon the source population, inbred lines are designated as first, second and third cycle inbreds. Inbred lines that are developed form open pollinated variety are referred to as first cycle inbreds and those which are improved and developed from the pre-existing inbreds are known as second cycle inbreds.
Earlier inbreds used to be developed from a heterozygous source. Now it is preferred to improve the existing inbreds rather than isolating new inbreds from a heterozygous population.
There are four breeding methods that are commonly used for the improvement of inbred lines:
(1) Pedigree selection,
(2) Backcross method,
(3) Convergent improvement, and
(4) Gamete selection.
These methods are briefly discussed below:
Method # 1. Pedigree Selection:
This method is used to combine desirable genes into a single genotype from two different inbreds. Suppose one inbred is resistant to a disease and another to lodging, a cross is made between these two inbreds and in segregating generations selection is made for those plants that have desirable combination of disease and lodging resistance.
This method was found to be very effective in improving the inbred for smut and lodging resistance in corn. The improved inbreds resulted in the production of high yielding hybrids with improved resistance to lodging and smut. Inbreds developed by this method are known as second cycle inbreds, because they are improved version of pre-existing inbreds.
Method # 2. Backcross Method:
Backcross method is widely used for the improvement of inbreds for various characters such as disease, lodging and insect resistance. This method is more effective in transfer of those characters which have high heritability. Two inbreds, one as donor parent and the other as recipient parent are selected from the pre-existing inbreds.
A cross is made between these two inbreds. The F1 is repeatedly backcrossed to the recipient or recurrent inbred to retain the genotype of recipient parent with desirable character under transfer. Backcross method has been used in maize in improving inbreds for resistance to smut, leaf blight caused by Helminthosporium, and corn borer. This is also used to transfer the male sterility and restorer genes into the background of more productive inbred lines.
Method # 3. Convergent Improvement:
This is a modified form of backcross method. It was proposed by Richey in 1927. This is a system of double backcrossing that is used to improve each of the two inbred lines simultaneously. Two inbred lines say A and B each with some desirable characters are selected. A cross is made between A and B inbred lines.
The F1 is backcrossed to both A and B lines. Backcross of with B line will retain some desirable genes of A line in B lines and backcross of F1 with A line will retain some desirable genes of B line in A line. Thus improvement is achieved in both the inbreds. Three to four backcrosses followed by 2-3 generations of selfing produced inbreds that were somewhat superior to the original inbreds in maize. However, the improvement was not appreciable and probably due to this reason this method is not used widely in improving inbreds.
Method # 4. Gamete Selection:
A selection procedure which is used for deteching desirable gamete from a heterozygous population and transferring the same into the background of an inbred line of known performance and combining ability is referred to as gamete selection. This was proposed by Stadler in 1944 as a method of improving inbred line.
This method consists of following main steps:
i. A good inbred line is crossed with a random sample of pollen from an open pollinated variety.
ii. All the F1 plants of above cross are selfed as well as crossed with a suitable tester that may be an inbred or some other population.
iii. The performance of test cross progeny is evaluated in replicated trials. The progenies with superior performance are identified and continued in the next season from the selfed seeds off, (item 2 above). Improved inbreds are developed from these superior progeny through selfing and selection.
The differences in the performance of test cross progeny result from the gametes received from the open pollinated variety, because inbred line will produce only one type of gamete. On the other hand open pollinated variety, being heterozygous produces several types of gametes which lead to differences in the performance of test cross progeny.
In this method, only those progeny are carried further that has received superior gamete from the open pollinated variety. Due to this reason, this method is called as gamete selection. This method has also not been used widely for the improvement of inbreds.
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