What is Heterosis?

Plant breeding is the application of genetic principles in developing new plant varieties, known as cultivar development, crop improvement, and seed improvement. Heterosis in plant breeding is described as the superiority of an F₁ hybrid over both parents in terms of yield or other characteristics. Heterosis contributes to increased vigour, size, growth rate, yield, or other attributes. However, in exceptional cases, the hybrid may be inferior to the weaker parent. The methods of estimation of heterosis and the genetic basis of heterosis are described here.

Heterosis Definition

Heterosis refers to the superiority of F, hybrids over their parents in one or more characteristics. The word hybrid vigour is a synonym for heterosis. George Harrison Shull coined the term heterosis in 1914.

Some features of heterosis are described below.

• Superiority Over Parents: Heterosis results in superiority over its parents in adaptability, yield, quality, disease resistance, maturity, and general vigour. Positive heterosis is often seen as desirable. However, in some circumstances, negative heterosis is preferable. Negative heterosis for plant height, maturity time, and hazardous chemicals, for example, is beneficial in many circumstances since it demonstrates superiority over the parents. In most agricultural plants, heterosis of 40% or more over the superior parent is regarded as substantial from a practical standpoint.

• Confined to F₁: Heterosis is restricted to the F₁, resulting in the production of a cross. As a result of segregation and recombination, it diminishes and vanishes in F₁ and later generations of a cross. As a result, heterosis is linked to the F₁ generation.

• Genetic Control: Nuclear genes regulate the expression of heterosis. In certain cases, heterosis is caused by the interaction of nuclear genes and cytoplasm.

• Reproducible: Once recognised, heterosis may be easily reproduced in a specific environment. The expression of heterosis, on the other hand, is more evident in the region of hybrid adaptability.

• Relationship with SCA: Heterosis shows a positive relationship with specific combining ability (SCA) variation. The SCA is a measure of dominance variance, and having a high degree of dominance variance is required to carry out a heterosis breeding program.

• Heterozygosity Effect: The degree of heterosis is related to heterozygosity since dominance variance is related to heterozygosity. The dominance effects should be most significant in cross-pollinated species and least significant in self-pollinated species. As a result, heterosis occurs more frequently in cross-pollinated crops than in self-pollinated crops.

• Masks Recessive Genes: When there is heterosis, the beneficial influence of dominant genes masks harmful recessive genes. As a result, recessive mutant genes are concealed in heterozygous individuals.

• Low Frequency: The frequency of good heterotic pairings is quite low. Only a few good heterotic pairings are discovered after screening thousands of F1 crosses. All of the F1 crosses lack desired heterosis.

Genetic Basis of Heterosis

To explain the mechanism of heterosis, two significant theories have been suggested. The first is the dominance theory, while the second is the overdominance hypothesis. Epistasis is also probably related to heterosis. As a result, there are three potential genetic origins of heterosis, which are:

• Dominance

• Overdominance

• Epistasis

Dominance Hypothesis

Davenport (1908), Bruce (1910), and Keeble and Pellew (1910) proposed this hypothesis. This is the most commonly accepted explanation for heterosis. According to this theory, heterosis is caused by the superiority of dominant alleles when recessive alleles are harmful. The hybrid shows heterosis because the deleterious recessive genes of one parent are concealed by the dominant genes of the other parent. Both parents have different dominant genes.

Assume one parent's genetic make-up is AABBccdd and the other's is aabbCCDD. A hybrid of these two parents will have four dominant genes, giving it superiority over both parents having two dominant genes. Thus, heterosis is proportional to the number of dominant genes contributed by each parent.

Dominance Hypothesis

Overdominance Hypothesis

Shull and East separately presented this hypothesis in 1908. This hypothesis is known as stimulation of heterozygosis, cumulative action of divergent alleles, single-gene heterosis, super-dominance, and overdominance. Even though Shull and East proposed this hypothesis in 1908, Hull used the word overdominance in 1945 when working on maize. According to this theory, heterosis is caused by the heterozygote's superiority over both of its homozygous parents. Thus, heterosis is proportional to heterozygosity.

The superiority of the heterozygote over both homozygotes may result from:

• The production of a superior hybrid substance in the heterozygote that is entirely different from either of the homozygous.

• Greater buffering capacity in the heterozygote due to cumulative action of divergent alleles or stimulation of divergent alleles. East explained this theory in 1936, suggesting a set of alleles a1, a2, a3, and a4 with steadily increasing divergence in function. As a result, a combination of more divergent alleles will have more heterosis than a combination of less divergent alleles. Combinations of a1a4, for example, demonstrate more heterosis than combinations of a1a2, a2a3, and a3a4. Overdominance has been reported in barley.
  
  

Epistasis

The interaction of alleles from two or more distinct loci is referred to as epistasis. It is sometimes referred to as nonallelic interaction. Non-allelic interactions are classified into three types: additive x additive, dominance x dominance, and additive x dominance. It is widely documented that the presence and size of non-interaction have a positive relationship with the incidence and magnitude of heterosis. Epistasis, especially dominance effects (dominance x dominance), may lead to heterosis. Cotton and maize have both shown this (Moll and Stuber 1974). Various biometrical models can detect or estimate epistasis.

Methods of Estimation of Heterosis

Heterosis is estimated in three ways:

1. Over mid parent

2. Over better parent

3. Over a commercial hybrid

Thus, based on estimation, heterosis is classified into three types, as shown below.

• Average Heterosis: When the heterosis is estimated over the mid parent, i.e., the average value of the two parents, it is known as average heterosis, which is calculated as Average Heterosis= {(F₁-MP)/MP} X 100

Where F₁ is the mean value of F₁ and MP is the mean value of the two parents involved in the cross.

• Heterobeltiosis: It occurs when the heterosis is estimated to be superior or better than the superior or a better parent. It is known as heterobeltiosis. It is calculated as follows:

Heterobeltiosis= {((F₁-BP)/BP) X 100}

BP is the mean value (across replications) of the cross's better parents.

• Useful Heterosis: Meredith and Bridge coined the term useful heterosis in 1972. It is also known as economic heterosis and refers to F₁'s superiority over the normal commercial check type. This sort of heterosis has direct use in plant breeding. It is calculated as follows.

Useful heterosis= {((F₁-CC)/CC) X 100}

Where CC is the mean value (across replications) of the local commercial hybrid. Over the conventional commercial hybrid, heterosis is sometimes worked out.

• Standard Heterosis: Heterosis is estimated in crops where hybrids are already available for comparison. Standard heterosis refers to this sort of heterosis. This is also directly applicable in plant breeding. It is calculated as follows.

{(F₁-SH)/SH} X 100 = Standard heterosis

Where SH is the mean value of the standard hybrid.

Conclusion

Heterosis refers to the superiority of F₁ hybrids in one or more characteristics over their parents. The term hybrid vigour is used interchangeably with heterosis. Dr. G. H. Shull coined the term "heterosis" in 1914. Heterosis is the process by which a less vigorous organism is turned into a more vigorous organism by absorbing DNA from the media.