Method of Plant Breeding in Cross Pollinated Plants – Hybridization
Definition of Hybridization:
The mating or crossing of two plants of dissimilar genotype is known as hybridization. In plant crossing is done by placing pollen grain from one genotype (male parent) on to the stigma of flower of another genotype (female parents).
The seed as well as the progeny resulting from the hybridization are known as hybrid of F1. The progeny of F1 obtained by selfing or intermating of F1 and the subsequent generation are termed as segregating generation. Hybrid and synthetic variety have been highly successful in many cross – pollinated species. E.g. Maize, Jowar, and Bajara and even in some self – pollinated crops. E.g. Rice, tomato, etc. In India almost all the recommended varieties of maize are either hybrid or composite varieties.
Definition of Hybrid:
The progeny of a cross between genetically different plants is called hybrid. In other word hybrid is F1 generation of mating between genetically dissimilar plants. Most of the hybrid varieties are F1 from two or more purelines (Tomato, L esculentum) or inbreds (Maize, Zeamays).
An inbred is a nearly homozygous line obtained through continuous inbreeding of cross – pollinated species. When F1 generation from a cross between two or more purelines inbreds or other genetically dissimilar population is used for commercial cultivation is called as hybrid variety. Hybrid varieties are the most potent means for the exploitation of heterosis.
Type of Hybrid:
The commercially cultivated hybrids are of two types, i.e
A) Intraspecific hybrid and
B) Interspecific hybrid
A) Intraspecific Hybrid:
A hybrid between generally different genotypes of the same species. It is also known as Intervarietal hybrid. Intraspecific hybrids are always fertile. Based on type of cross , there are three types viz. ( a. Single cross hybrid, b. Double cross hybrid c) Three way cross hybrid.
a) Single Cross Hybrid:
A cross between two inbreds or varieties is referred as single cross. E.g (A X B) and the hybrid progeny obtained from a cross between two inbreds or varieties are referred as single cross hybrid.
In cross pollinated crops such hybrids are developed from a cross between two inbreds, whereas in self pollinated crop, they developed from a cross between two homozygous varieties.
Merits Features of Single Cross Hybrid:
1. They are developed in both self and cross- pollinated crop, where heterosis is exploitable.
2. The total number of single crosses is n (n-1) /2 where ‘n’ is number of inbred line.
3. It is more common in some self –pollinated species than cross – pollinated species.
4. It is used for the development of double cross and three way cross hybrid.
5. It is also used to predict the performance of double cross hybrid.
6. It gives maximum degree of heterosis and produce uniform plant.
b) Double Cross Hybrid:
A cross between two single crosses is referred as cross between two single crosses is known as double cross hybrid.
It is more commonly used in maize and Sugarbeet and it involves for different inbred line viz. (A X B) X ( C X D). The rows of female and male parent are planted in the ratio 4:1 in maize. The number of all possible double crosses among selected inbred is calculated by n ( n-1) ( n-2) (n-3) /8 where ‘n’ is number of inbreds involved.
Double Top Cross Hybrid:
It refers to the hybrid progeny between a simple cross and an open pollinated variety. i.e (A X B) X Open pollinated variety. It is used in maize.
A cross between an inbred line and an open pollinated variety is known as top cross. It is also known as inbred variety cross, and is used as testing the combining ability of inbreds and not for commercial hybrid seed production.
A cross involving more than four inbred line is referred as multiple crosses.
Open pollination of a group of selected genotypes in isolation from other compatible genotype to promote random mating among selected genotype. It was proposed by Tysdal et. Al. (1942).
Three Way Cross Hybrid:
The hybrid progeny by crossing of gene and inbred lines is referred as three way cross hybrid. E.g (A X B) X C. These hybrids sometimes used in maize, in which single cross is used as female and inbred as male and are planted in the ratio of 2:1. These hybrids produced seed of normal shape and size but the main drawback is the low pollen production efficiency of the male inbred parent.
A) Interspecific Hybrid:
The F1 progeny between two different progeny of the same genus is known as interspecific hybrid. It is also referred as intrageneric hybrid. These hybrids are rarely used for commercial cultivation because such hybrids are fertile only in few cases. In cotton interspecific hybrids between tetraploid cultivated sp. (G. hirsutum X G. barbadense) and diploid cultivated species (G. arboretum X G. herbaceum). E .g. A tetraploid level: Var.Laxmi, Surti, HB224, etc. At diploid level: DH-7, DH-9, and Pha46, etc.