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Home , Heterosis

COMPILED AND CIRCULATED BY DR. PRITHWI GHOSH, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE

Inbreeding depression and Heterosis

Content: History, genetic basis of depression and heterosis: Applications

History

Heterosis has been known since the art of hybridization came into existence. Koelreuter (1763)

was the first to report 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 , 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. 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.

BOTANY: SEM-V, PAPER- DSE2T: , UNIT - 4: AND HETEROSIS

COMPILED AND CIRCULATED BY DR. PRITHWI GHOSH, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE

Inbreeding depression

Inbreeding depression is the reduced survival and fertility of offspring of related individuals. Large

effects are documented in wild animal and plant populations, as well as in humans. Intercrossing

inbred strains improves yield (heterosis).

Effects of inbreeding

1. Appearance of lethal and sub lethal.

2. Reduction in vigour: Appearance of dwarf plants.

3. Reduction in reproductive ability - Less seed set, sterility.

4. Segregation of population in distinct lines.

5. Increase in homozygosity.

6. Reduction in yield.

Heterosis

The term heterosis was first used by Shull in 1914. Heterosis may be defined as the superiority of

an over both of its parents in terms of yield or some other character. Generally, heterosis

is manifested as an increase in vigour, size, growth rate, yield or some other characteristic.

Heterosis is the superiority of a hybrid over its parents.

Two maize inbreeds and their F1 hybrid (middle) exhibiting heterosis

BOTANY: SEM-V, PAPER- DSE2T: PLANT BREEDING, UNIT - 4: INBREEDING DEPRESSION AND HETEROSIS

COMPILED AND CIRCULATED BY DR. PRITHWI GHOSH, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE

There are two main theories of heterosis and inbreeding depression. 1. Dominant hypothesis

2. Over hypothesis.

1. Dominant hypothesis:

First proposed by Davenport in 1908. It was later on expanded by Bruce, Keeble and Pellow.

According to this hypothesis at each locus the dominant has favorable effect, while the

recessive allele has unfavorable effect. In heterozygous state, the deleterious effect of recessive

are masked by their dominant alleles. Inbreeding depression is produced by the harmful

effects of recessive alleles which become homozygous due to inbreeding.

2. Over dominance hypothesis:

This hypothesis was independently proposed by East and Shull in 1908. It is also known as single

gene heterosis or super dominance theory. According to this hypothesis, heterozygotes or at least

some of the loci are superior to both the homozygotes. Thus, heterozygote Aa would be superior

to AA and aa. In 1936 East proposed that at each locus there are several alleles a1a2a3a4etc, with

increasingly different functions. Heterozygotes between more divergent alleles would be more

heterotic. E.g. a1a4will be superior to a1a2or a2a4

Application of Heterosis

1. Increased yield: Heterosis is generally expressed as an increase in the yield of hybrids.

The yield may be measured in terms of grain, fruit, seed, leaf, tubers or the whole plant.

2. Increased Reproductive Ability: More number of flowers/fruits/seeds.

3. Increase in Size and General Vigour: The hybrids are generally more vigorous, i.e.,

healthier and faster growing and larger in size than their parents.

4. Better Quality: In many cases, hybrids show improved quality. For example, many

hybrids in show better keeping quality, but not yield, than open-pollinated varieties.

BOTANY: SEM-V, PAPER- DSE2T: PLANT BREEDING, UNIT - 4: INBREEDING DEPRESSION AND HETEROSIS

COMPILED AND CIRCULATED BY DR. PRITHWI GHOSH, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE

5. Earlier Flowering and Maturity: In many cases hybrids are earlier in flowering and

maturity than the parents. But earliness is highly desirable in many situations particularly

in vegetables.

6. Greater resistance to disease and pest: Some hybrids are known to exhibit a greater

resistance to insect or diseases than their parents.

7. Greater adoptability: Hybrids are generally more adapted to environmental changes than

inbreeds.

8. Faster growth rate: In some cases, hybrids show a faster growth rate than their parents.

But the total plant size of the hybrids may be comparable to that of parents. In such cases,

a faster growth rate is not associated with a larger size.

9. Increase in the number of a plant part: In some cases, there is an increase in the number

of nodes, leaves and other plant parts, but the total plant size may not be larger.

Factors Affecting Heterosis

1. Mode of : The magnitude of heterosis is generally higher in cross pollinated

species than in self-pollinated species.

2. Genetic diversity of parents: The expression of heterosis is influenced by genetic

diversity of parents. For example, in alfalfa and cotton, greater heterosis was associated

with greater parental diversity.

3. Genetic base of parents: Higher heterosis is associated with broad genetic base of the

parents.

4. Adaptability of parents: Heterosis is associated with wider adaptability of the parents,

because there is close association between adaptability and genetic base.

BOTANY: SEM-V, PAPER- DSE2T: PLANT BREEDING, UNIT - 4: INBREEDING DEPRESSION AND HETEROSIS

COMPILED AND CIRCULATED BY DR. PRITHWI GHOSH, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE

References

http://ecoursesonline.iasri.res.in

https://en.wikipedia.org/wiki/Heterosis

www.biologydiscussion.com

[The information, including the figures, are collected from the above references and will be used

solely for academic purpose.]

BOTANY: SEM-V, PAPER- DSE2T: PLANT BREEDING, UNIT - 4: INBREEDING DEPRESSION AND HETEROSIS