Inbreeding Depression in Plant Breeding
Authors: Pokar Scachin B. and Jadhav Shrinivas Shripatrao

Introduction

Inbreeding is matting between individuals related by descent or ancestry. When the individuals are closely related. e.g. In brother-sister mating or sib mating, the degree of inbreeding is high.

The highest degree of inbreeding due to selfing.

The chief effect of inbreeding increase in homozygosity in the progeny, which is proportionate to the degree of inbreeding. The degree of inbreeding of an individual is expressed as inbreeding coefficient (F).The value of F for an individual is the probability of the two alleles of a gene present in that individual to have been derived from a single allele of a common ancestor.

Inbreeding depression also known as the reduction or loss in vigour and fertility. The degree of inbreeding depression depends on the plant species. But within a species, the extent of depression is related to the value of F and the relative fitness of the trait.

Research Achievements and History-

A systematic observation on the effects of inbreeding date back to about 1700 A.D.

In 1876, Darwin published his book cross and self-fertilization in vegetable kingdom; Darwin concluded that progeny obtained from self-fertilization were weaker than those derived from out crossing.

Darwin also reported the result from his experiments on self-and cross fertilization in Maize; these are the first published account of inbreeding depression.

Detail information on inbreeding in maize was published independently by East in 1908 and shull in 1909.

Effects of inbreeding -

Generally inbreeding is concerned with a reduction in vigour and reproductive capacity that is fertility.

1. Reduction in vigour.
2. Increase in homozygosity.
3. Reduction in reproductive ability.
4. Appearence of Lethal and sublethal alleles.
5. Reduction in yield.

Degrees of inbreeding Depression

The inbreeding depression may range from very high to very low or it may be absent.
The inbreeding depression observed in various plant species may be grouped in to the following four broad categories

1. High inbreeding depression - Inbreeding leads to severe depression and exhibit lethal effects. After 3 or 4 generations of selfing it is hard to maintain lines. e.g. Lucerne, Carrot.

2. Moderate inbreeding depression - Though lethal effects are there, lines can be separated and maintained. e.g. Maize, Jowar, Bajra.

3. Low inbreeding depression - Only a small degree of inbreeding depression is observed. e.g. Cucurbits, Sunflower.

4. No inbreeding depression - The self-pollinated crops do not show inbreeding depression. e.g. Wheat, potato, tomato

Homozygous and heterozygous

The population develops genetic information, which favours heterozygosity; as a result, homozygosity leads to detrimental effects. This type of organisation known as heterozygous balance.

The self-fertilized species are naturally homozygous. They have no genetic load because unfavorable recessive genes become homozygous and are eliminated from the population. These species therefore develop a genetic organisation, which is adapted to homozygosity. This type of organisation known as a homozygous balance.

Homogeneous - The whole population having a same Genotype and Phenotype is known as homogeneous population

Heterogeneous - The whole population having a different Genotype and Phenotype is known as a Heterogeneous population

Conclusion -

Inbreeding result from mating between closely related individuals.
Inbreeding depression is the decline in fitness and vigour with decreased heterozygosity.
Inbreeding also reduce the reproductive ability.
Self-pollinated crops Show no inbreeding depression, while cross-pollinated crop show variable degree of inbreeding depression.

About Author / Additional Info:
I am M.Sc (Agri) in Genetics and Plant Breeding, Studding in C. P. Collage of Agriculture, Sardarkrushinagr Dantiwada Ahricultural University