Heterosis and inbreeding depression and causes of hybrid vigour, Production and application of hybrid vigour in plant breeding

Heterosis:-

•      The term Heterosis was first coined by Shull in 1914.

•      Definition:- When the F1 hybrid is superior from both its parents in many traits such as normal vigor, adaptation, yield etc., it is called heterosis or hybrid vigor.

•      There are 2 synonyms of Heterosis: -

i. Hybrid Vigor

ii. Out breeding Enhancement

Types of Hybrids:-

1. Single Cross Hybrids:- A x B

2. Three Way Hybrid:- (A x B) x R

3. Double Cross Hybrid:- (A x B) x (R x S)

Genetic Basis of Heterosis:- Heterosis can be explained by three hypotheses-

1.Dominance Hypothesis

2. Over dominance Hypothesis

3. Epistasis Hypothesis

1. Dominance Hypothesis:-

·     This hypothesis was given by Davenport in 1908 and supported by Bruce in 1910.

·     According to this hypothesis, heterosis develops due to the superiority of dominant allele over recessive allele.

AA = Aa > aa

BB = Bb > bb

The higher the number of dominant genes contributed by each parent, the higher is the heterosis.

Example:-

2. Overdominance Hypothesis:-

·     This hypothesis was independently given in 1908 by East and Shull.

·     According to this hypothesis, heterosis develops due to heterozygosity. In the heterozygous state, the dominant allele exhibits over-expression compared to the homozygous state.

AA < Aa >> aa

BB < Bb >> bb

The higher the number of heterozygous pairs in F1 hybrids, the higher the heterosis.

Example:- 

3. Epistasis Hypothesis:-

·     This hypothesis was given by Gowen in 1952.

·      According to this hypothesis, heterosis develops due to favorable interactions between non-allelic genes.

AA > Aa > aa

BB > Bb > bb

Estimation of Heterosis:- Heterosis can be estimated in 3 ways: -

1. Average Heterosis

2. Heterobeltiosis

3. Useful Heterosis

1. Average Heterosis:- When the F1 hybrid is superior than the mid parent, it is called average heterosis.

AH   = Average Heterosis

F1  = Average Yield of F1 Hybrid

MP  = Average of Yields of both the parents

2. Heterobeltiosis:- When the F1 hybrid is superior than the superior parent, it is called heterobeltiosis.

HB = Heterobeltiosis

F1 = Average Yield of F1 Hybrid

BP = Average Yield of superior parent

3. Useful Heterosis:-

·    The term was coined by Meredith and Bridge in 1972.

·     This word also have 2 synonyms: -

i. Economic Heterosis

ii. Standard Heterosis

·      This type of heterosis has practical use in plant breeding.

·      Definition:- When the F1 hybrid is superior than the improved variety, it is called useful heterosis.

UH  = Useful Heterosis

F1  = Average Yield of F1 Hybrid

CV = Average Yield of improved Check Variety

Luxuriance:- When the F1 hybrid, produced by interspecific hybridization, is superior than both of its parents, it is called Luxuriance. The F1 hybrid exhibits a much larger size and vigor than its parent and it is partially or completely sterile.

Commercial Application of Heterosis:- Improved hybrid varieties have been developed in many crops using heterosis.

1. Maize:- Ganga-1, Ganga-5, Ganga- 11

2. Bajra:- HB-3, BJ-104, MH-179

3. Sunflower:- BSH-1, KBSH-1, KBSH-11

4. Castor:- GCH-2, GCH-3, GCH-4

5. Pigeon pea:- ICPH-8

6. Jute:- Hybrid-C

7. Brinjal:- Vijay, Pusa Kranti

8. Water melon:- Pusa Sanyog

9. Almost all Fruit Trees

Inbreeding Depression:-

•      Definition:- When selfing is done repeatedly in cross-pollinated crops, their vigor get reduced. This is called inbreeding depression.

•     It is caused by crosses in closely related plants.

•   In this process, heterozygosity decreases and homozygosity increases.

•   This is caused by fixation of unfavorable recessive genes.