The commercially cultivated hybrids are of two types, viz.:
(1) Intraspecific hybrids, and
(2) Interspecific hybrids.
These are briefly discussed below:
Type # 1. Intraspecific Hybrid:
A hybrid between genetically different genotypes of the same species is known as intraspecific hybrid. It is also known as inter-varietal hybrid. These types of hybrids are commonly used for commercial cultivation in several cross pollinated crops and some self-pollinated crops. Intraspecific hybrids are always fertile.
Based on type of cross, intraspecific hybrids are of three types, viz.:
(a) Single cross hybrid,
(b) Three way cross hybrid, and
(c) Double cross hybrid.
(a) Single Cross Hybrid:
The hybrid progeny from a cross between two inbreds or varieties is referred to as single cross hybrid. In cross pollinated species, such hybrids are developed from a cross between two inbreds (viz. A x B), whereas in self-pollinated species they are developed from a cross between two homozygous varieties.
Main features of single cross hybrids are given below:
i. They are developed in cross pollinated crops and also in those self-pollinated crops where heterosis is exploitable.
ii. The total number of single crosses is n(n – 1)/2, where n is the number of inbred lines.
iii. Single cross hybrids are more common in some self-pollinated species than in cross pollinated species. For example, in maize single cross hybrids have small and poorly developed kernels and hence are not desirable.
iv. Single cross hybrids are useful in the development of three way cross hybrids and double cross hybrids.
v. Single cross hybrids are also used to predict the performance of double cross hybrids.
vi. Single crosses give maximum degree of heterosis and produce uniform plants.
(b) Three Way Cross Hybrid:
The hybrid progeny between a single cross and an inbred lines is referred to as three way cross hybrid. Such hybrids are sometimes used in maize. The single cross is used as female and inbred as male, viz. (A x B) x C.
Main features of three way cross hybrids are given below:
i. Such hybrids are developed when only three good inbreds are available. The rows of female and male parents are planted in 2: 1 ratio in maize.
ii. Such hybrids produce seed of normal shape and size.
iii. The main drawback of three way cross is the low pollen production efficiency of the male inbred parent.
(c) Double Cross Hybrid:
The hybrid progeny from a cross between two single crosses is known as double cross hybrid. Such hybrids are commonly used in maize and sugar-beet.
Main features of double cross hybrids are given below:
i. Double cross hybrid involves four different inbred lines, viz. (A x B) x (C x D).
ii. Double cross seed is more uniform in shape and size and is produced more economically due to high productivity of F1 used as female and high pollen production efficiency of F1 used as male parent.
iii. The rows of female and male parents are planted in 4: 1 ratio in maize. Thus 80% of the area is covered by the female parent, whereas this ratio is 2: 1 in case of single cross and three way cross hybrids.
iv. The number of all possible double crosses among selected inbreds is n (n – 1) (n – 2) (n – 3)/8, where n is the number of inbreds involved.
Some other types of crosses are briefly discussed below:
(i) Double Top Cross:
In maize, sometimes double top cross hybrids are also made. A double top cross hybrid refers to the hybrid progeny between a single cross and an open pollinated variety, i.e. (A x B) x open pollinated variety. A cross between an inbred line and an open pollinated variety is known as top cross or inbred variety cross. The top crosses are usually made for testing the combining ability of inbreds and not for commercial hybrid seed production.
(ii) Multiple Cross:
A cross involving more than four inbred lines is referred to as multiple crosses. Such crosses are generally lesser productive than the best double cross combination. Multiple cross is also known as composite cross and is used to combine monogenic characters from different sources into a single genotype. Multiple crosses generally have wider adaptation.
(iii) Poly Cross:
Open pollination of a group of selected genotypes in isolation from other compatible genotypes to promote random mating among selected genotypes is referred to as poly cross.
Type # 2. Interspecific Hybrid:
The F1 progeny between two different species of the same genus is known as interspecific hybrid. This is also referred to as intrageneric hybrid. Interspecific hybrids are rarely used for commercial cultivation, because such hybrids are fertile only in few cases. In cotton, interspecific hybrids between tetraploid cultivated species (Gossypium hirsutum x G. barbadense) and diploid cultivated species (G. arboreum x G. herbaceum) are fully fertile.
Several interspecific hybrids at tetraploid level (Varalaxmi, DCH 32, DHB 105, Sruthi, HB 224, NHB12, TCHB 213) and some hybrids at diploid level (DH7, DH9, Pha 46 and DDH2) have been released for commercial cultivation in India, besides large number of intraspecific hirsutum hybrids.
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