Heterosis or Hybrid Vigour: Types, Causes and Effects

In this article we will discuss about Heterosis or Hybrid Vigour:- 1. Meaning of Heterosis 2. History of Heterosis 3. Types of Heterosis 4. Causes 5. Effects.

Meaning of Heterosis:

When two homozygous inbreeds (a true breeding line obtained by continuous inbreeding) of genetically unlike constituents are crossed together, the resulting hybrids obtained from the crossed seeds are usually robust, vigorous, productive and taller than the either parents.

This increased productivity or superiority over the parents is known as heterosis or hybrid vigour. Heterosis can be defined as the superiority of F₁ hybrid over both the parents in terms of yield or some other character.

History of Heterosis:

Heterosis has been known since the art of hybridization came into existence. Koelreuter (1763) was the first to report hybrid vigour in the hybrids of tobacco, Datura etc. Mendel (1865) observed this in pea crosses.

Darwin (1876) also reported that inbreeding in plants results in deterioration of vigour and the crossing in hybrid vigour. On the basis of his experiments Beal (1877-1882) concluded that F₁ hybrids yield as much as 40 percent more of the parental varieties.

From subsequent studies on inter-varietal crosses in maize, it was observed that some of the hybrids show heterosis.

While discussing the work on maize during a lecture at Gottingen (West Germany), Dr. G.H. ShuII (1914) proposed the term heterosis (Gr. heteros different and osis = condition). Poweri (1944, 45) reported that the crossing, however, may result in either weak or vigorous hybrids as compared to parental inbreeds.

Hybrid vigour is used as synonym of heterosis. It is generally agreed that hybrid vigour describes only superiority of the hybrid over the parents while heterosis describes the other situation as well i.e., crossing over may result in weak hybrids e.g., many hybrids in tomato are earlier (vegetative phase is replaced by reproductive phase).

Earliness in many crops is agriculturally desirable so, it is argued that F, shows faster development in which vegetative phase is replaced by the reproductive phase more quickly than in the parents.

On the basis of this explanation it was justified to use the term hybrid vigour as synonym of heterosis.

However, Whaley (1944) was of the opinion that it would be more appropriate to term the developed superiority of the hybrids as hybrid vigour and to refer to the mechanism by which the superiority is developed as heterosis. Smith (1955) opined that the use of heterosis and hybrid vigour as synonyms is highly desirable on the basis of their long usage.

Types of Heterosis:

Heterosis is of two types:

True heterosis (euheterosis) and pseudo-heterosis.

1. True heterosis:

It is inherited.

It can be further divided into two types:

(a) Mutational true heterosis:

It is the sheltering or shadowing of the deleterious, un-favourable, often lethal, recessive mutant genes by their adaptively superior dominant alleles.

(b) Balanced true heterosis:

It arises out of balanced gene combinations with better adaptive value and agricultural usefulness.

2. Pseudo-heterosis:

Crossing of the two parental forms brings in an accidental, excessive and un-adaptable expression of temporary vigour and vegetative overgrowth. It is also called luxuriance.

Causes of Heterosis:

The phenomenon of heterosis can be explained on the basis of the causes: Genetic causes and Physiological causes.

A. Genetic Causes:

There are two possible causes of heterosis viz.;

(i) Dominance

(ii) Over-dominance

(i) Dominance hypothesis:

This theory was proposed by Davenport (1910), Bruce (1910) and Keable and Pellew (1910).This theory is based on the assumption that hybrid vigour results from bringing together female dominant genes. According to this theory, genes that are favourable for vigour and growth are dominant, and genes that are harmful to the individual are recessive.

The dominant genes contributed by one parent may complement the dominant genes contributed by the other parent, so that F₁ will have the more favourable combination of dominant genes, than either parent e.g., Dominant genes ABCDE are favourable for good yield. Inbred A has the genotype AA BB cc dd (AB dominant) and inbred B has the genotye aa bb CC DD (CD dominant).

The genotype of the F₁ hybrid is as follows:

The F₁ hybrid contains dominant genes at all the loci represented here (ABCD) and exhibits more vigour than either of the parent inbred lines.

Objections:

There are two main objections:

1. Failure in the isolation of inbreeds as vigorous as hybrids:

If the above hypothesis is true, it should be possible to isolate inbreeds with all dominant genes. Such inbreeds would be as vigorous as the F₁ hybrids. However, such hybrids have not been isolated. Jones (1917) in this modified theory entitled “Dominance of Linked Genes Hypothesis” provided explanation for this.

He suggested that there may be a linkage between some favourable dominant genes and some un-favourable recessive genes and as a result it is not possible to obtain true breeding homozygous individual for all dominant genes in F₂ generation.

2. Symmetrical distribution in F₂:

It is already studied that in F₂ dominant and recessive characters segregate in the ratio of 3:1. If heterosis is due to dominance of independent factors, the F₂ distribution curve for heterotic character should be skewed (slanting) rather than smooth and symmetrical.

But the curve of F₂ is always, smooth and symmetrical and not skewed. This objection was removed by Collins (1921). According to him trait like yield is governed by large number of genes or polygenes, which exhibit continuous variation resulting in symmetrical distribution of genes.

(ii) Over dominance hypothesis:

This hypothesis was given by Shull (1903) and East (1908) independently. According to the supposition hybrid vigour on the basis of heteozygosity is superior to homozygosity. According to this hypothesis there are contrasting alleles for example a₁ and a₂, for a single locus. Each allele produces favourable yet different effects in the plant.

In a heterozygous plant (a₁, a₂) a combination of the effects is produced which is more favourable in the plant than the effect produced by either of the alleles alone. This phenomenon of heterozygote(a₁ a₂) being superior to the homozygotes (a₁a₁ or a₂a₂) is termed over dominance.

Various names have been given to this idea e.g., super dominance (Fisher 1930), interaction of alleles at a single locus (East, 1930) over-dominance (Hull, 1945) etc., but the term over-dominance is widely accepted.

B. Physiological Causes:

(i) Greater initial capital hypothesis:

This hypothesis was proposed by Ashby (1930). He studied the physiology of inbreeds and hybrids of maize and tomato and concluded that hybrid vigour is due to an increased initial embryo size. He termed it as ‘Greater initial capital.’

(ii) Cytoplasmic-nuclear interactions:

Michelis, Shull, Lewis, and others suggested that hybrid vigour is the interaction of cytoplasmic and nuclear systems. Cytoplasm is a transparent fluid rich in RNA and mitochondria, which is usually transmitted through the female parent to the offspring.

Effects or Manifestations of Heterosis:

Whatever may be the cause (genetical or physiological), heterosis is a well known phenomenon.

It is basically the result of the increased metabolic activity of the heterozygote Its effects are well established or manifested in the following tree ways:

1. Quantitative Effects:

(a) Increase in size and genetic vigour:

Hybrids are generally more vigorous I;e larger, healthier and faster growing than the parents e.g., head size in cabbage jowar cob size in maize, fruit size in tomato etc.

(b) Increase in yield:

Yield may be measured in terms of grain, fruit, seed, leaf tuber or the whole plant. Hybrids usually have increased yield.

(c) Better quality:

Hybrids show improved quality e.g., hybrids in onion show better keeping quality.

2. Physiological Effects:

(a) Greater resistance to diseases and pests:

Some hybrids show greater resistance to insects or diseases than parents.

(b) Greater flowering and maturity:

Earliness is highly desirable in vegetables In many cases, hybrids are earlier in flowering and maturity than the parents, e.g. tomato hybrids are earlier than their parents.

(c) Greater Adaptability:

Hybrids are usually less susceptible to adverse environmental conditions.

3. Biological Effects:

Hybrids exhibiting heterosis show an increase in biological efficiency i.e., an increase in fertility (reproduction ability) and survival ability.

Heterosis in animals:

(i) Mule is a hybrid from a cross between Jack (Equus hemicus) and Mare (Earns equus) which has been known since ancient times for its well-known qualities of strength and stubbornness.

(ii) Cross between red Sindhi breed of Indian Cattle and Jersey breed of America contams 30% more butter fat in milk.

(iii) Increased pork yield in pigs, more egg laying hens, silk production in silk worms etc.