Author: SANDEEP KUMAR BANGARWA
- The term heterosis was first used by Shull in 1914.
- Heterosis may be defined as the superiority of an F1, hybrid over both its parents in terms of yield or some other character.
- Heterosis, or hybrid vigour, describes the superior performance of heterozygous hybrid individuals compared with their homozygous parental inbred lines.
- Generally, heterosis is manifested as an increase in vigour, size, growth rate, yield or some other characteristic. But in some cases, the hybrid may be inferior to the weaker parent. This is also regarded as heterosis; Often the superiority of F, is estimated over the average of the two parents, or the mid -parent.
- The superiority of F1 is estimated over average of the two parents (mid parent). This practise has found some acceptance particularly in the practical studies.
- However, in practical plant breeding the superiority of F1 over mid parent is of no use since it does not offer the hybrid any advantage over the better parent. Therefore, average heterosis is of little or no use to the plant breeder.
- More generally, heterosis is estimated over the superior parent such heterosis is referred as heterosis.
- The term heteroecism is not commonly used since most breeders regard this to be only case of heterosis and referred to as such i.e Heterosis.
- However, the commercial usefulness of a hybrid would primarily opened on its performance in comparison to the best commercial variety.
- In many cases the superior parent may be inferior to the best commercial variety. In such cases, it will be desirable to estimate heterosis in relation to the best commercial variety is commonly known as economic or useful heterosis.
- Economic heterosis is the only estimate of heterosis, which is of commercial or practical value in 1944.
- Powers suggested that, the term heterosis should be used only when the hybrid is either superior or inferior to both the parents.
- Heterosis is estimated in three different ways,
2. Better parent heterosis
3. Standard heterosis
1. Mid Parent Heterosis
- When the heterosis is estimated over the mid parent i.e. mean value or average of the two parents is known as mid parent heterosis. It is also known as average heterosis or relative heterosis and calculated by using formula.
- Mid Parent Heterosis = (F1- MP) / 100 x MP ̶
Where F1 is mean of F1 and
̶ MP is mean of two parent
- When the heterosis is estimated over the better parent is known as better parent heterosis. It is also known as heterobeltiosis and calculated by using formula:
- Heterobeltiosis = (F1- BP) / 100 x BP Where BP is mean of better parents.
- The term heterobeltiosis was used by Bitzer et al (1968) to describe the improvement of heterozygote over the better parent of the cross.
- It refers to the superiority of F1 over the standard commercial check variety. It is also called as economic heterosis or useful heterosis and calculated by using formula.
- Standard Heterosis = (F1- Check) / 100 x Check
Heterosis leads to increase in yield, reproductive ability, adaptability, disease and insect resistance, general vigour, quality etc. - For most of the characters, the desirable heterosis is positive. But for some characters like earliness, height in cereals and toxic substances are negative heterosis.
- There are three possible genetic causes of heterosis viz. 1. Dominance
2. Over dominance
3. Epistasis
1. Dominance Hypothesis - This theory was proposed by Davenport (1908) Bruce (1910) and Keeble and Pellew (1910).
- This is the most widely accepted hypothesis of heterosis.
- According to this hypothesis, heterosis is the result of the superiority of dominant alleles, when recessive alleles are deleterious; here the deterious recessive genes of one parent are hidden by the dominant genes of another parent and the hybrid exhibits heterosis.
- Both the parents differ for dominant genes.
- This hypothesis suggests that at each locus the dominant allele has a favourable effect, while the recessive allele has an unfavourable effect.
- In heterozygous state, the deleterious effects of recessive alleles are masked by their dominant alleles.
- Thus heterosis results from the masking of harmful effects of recessive alleles by their dominant alleles. Inbreeding depression, on the other hand, is produced by the harmful effects of recessive alleles, which become homozygous due to inbreeding.
- Therefore, according to the dominance hypotheses, heterosis is not the result of heterozygosity; it is the result of prevention of expression of harmful recessives by their dominant alleles.
- Heterosis is directly proportional to the number of dominant genes contributed by each parent.
2. Overdominace Hypothesis
- It was independently proposed by Shull and East in 1908 and supported by Hull ( 1945).
- This theory is called by various names such as stimulation of heterozygosis, cumulative action of divergene alleles, single gene heterosis, super dominance and over dominance.
- Though this theory was proposed by Shull and East in 1908, the overdominace was coined by Hull in 1945 working on maize. This term is now in common use.
- According to this hypothesis is the result of superiority of heterozygote over its both homozygous parents.
- Thus heterosis is directly proportional to the heterozygosis.
- The superiority of heterozygote over both homozygotes may arise either due to if
1. Production of superior hybrid substances in heterozygote is completely different from either of the homozygous products or due to
2. Greater buffering capacity in the heterozygote resulting from cumulative action of divergent alleles of stimulation of divergent alleles. - Overdominace has been reported in barley.
- In maize, available evidence suggest that if overdominace occurs, it is either infrequent in occurrence or small in magnitude.
- Dominance and overdominace hypothesis have some similarities and some dissimilarity.
3. Epistasis
- Epistasis refers to interaction between alleles of two or more different loci.
- It is also known as non-allelic interaction.
- The non-allelic interaction is of three type’s viz. additive X additive, additive X dominance and dominance X dominance.
- Heterosis has positive association with the presence and magnitude of non allelic interaction.
- Epistasis, particularly that involves dominance effects (dominance X dominance) may contribute to heterosis.
- This has been observed in cotton and maize.
References:
1. Phundan Singh, 2006. Essentials of Plant Breeding . Kalyani Publishers, New Delhi.
2. Singh, B.D. 2012. Plant Breeding: Principles and Methods. Kalyani Publishers, New Delhi.
About Author / Additional Info:
I am currently pursuing Ph.D. in Plant Breeding and Genetics from MPUAT-Udaipur (Raj.)