INTRODUCTION TO QUANTITATIVE GENETICS Part - 2

PART- II

ROLE OF THE ENVIRONMENT IN PLANT BREEDING

Environment affects selection and progress from selection. The genetic gain or response to

selection is the difference between the mean phenotypic value of selected offspring from parental population and that of the parental population before selection. Response to selection is used to compare selection methods and to predict environments.

Response to selection = rg[hx/hy]

Where rg =genetic correlation between trait (x) and yield (y),

hx = heritability for character x

hy = heritability for yield

The bigger the response to selection the better it is. It has no unit.

Heterosis

Heterosis, or hybrid vigor or outbreeding enhancement, is the increased function of any biological quality in a hybrid offspring. It is the occurrence of a genetically superior offspring from mixing the genes of its parents. Heterosis is the opposite of inbreeding depression, which occurs with increasing homozygosity. The term often causes controversy, particularly in terms of the selective breeding of domestic animals, because sometimes it's inaccurately claimed, that all crossbred plants are genetically superior to their parents. It's only true in certain circumstances. When a hybrid is seen to be superior to its parents, this is known as hybrid vigor. When the opposite happens, and a hybrid inherits traits from its parents that makes it unfit for survival, the result is referred to as outbreeding depression.

Hybrid vigour is measured in two ways:

(1) Mid-parent heterosis (Hmp)

=(F – mp)/mp

 (2) Hetero-betiosis ( better parent heterosis) Hbp

= (F – bp)/bp

Where F = Mean of F1

mp = mean of the two parents

bp = better parent

Example:

Giving the mean yield of two inbred strains A=80kg , B= 50 and F1 is 90kg, calculate

i. Hmp

ii. Hbp

Solution:

1. mp =(80+50)/2

= 65

Hmp = (F – mp)/mp

= (90 – 65)/65

=0.3846

This implies that the hybrid vigour is 38.46%

2. Hbp = (F – bp)/bp

= (90 – 80)/80

=0.125

Herobetiosis is 12.5%

The better parent heterosis is more significant as far as breeding is concerned because individual progenies are more superior to the better parent.

Manifestations of Heterosis

1. increased heterozygosity

2. increased size and productivity in plants

3. Greater resistance to diseases, insects and environmental factors

4. Early maturity when compared to either of the parents.

INCOMPATIBILITY SYSTEMS

There are various forms of incompatibility. Araso (1998) defined incompatibilityas the

inability of a plant to produce functional gametes or inability of a plant producing functional

gametes to set seed when self-pollinated.

Causes of Incompatibility

1. Failure of the pollen tube either to penetrate the stigma and;

2. To grow normally the full length of the style so that fertilization may occur.

In the later above, the pollen tube grows slowly that it may never get to reach the ovule and if it does, it would be so late that the ovule would have either been pollinated by compatible pollen or would have withered. Incompatibility restricts self fertilization and

inbreeding but it fosters cross fertilization.

Genesis of incompatibility

1. Protandry: Stamen maturing before the stigma

2. Protogyny: Stigma matures before the stamen

3. Hercogamy: This involves the physical arrangement of male and female organs on the

same plant preventing self pollination in the absence of an insect.

Apart from the morphological mechanism, which ensures open pollination, there are also

some genetic and physiological mechanisms which ensure incompatibility. Based on this,

incompatibility can be divided into two groups:

A. Heteromorphic Incompatibility

B. Homomorphic Incompatibility

A. Heteromorphic Incompatibility

This system is based on the difference in the length between the stamen and the style. The

flower with long style and short filament is called pin where as the flower with long filament and short style is called thrum.

                                                            In pin the pollen grains contains the gene labeled (ss) while that of thrum has heterozygous gene (Ss). Consequently, pollination is possible only between the anther and stigma of the same height i.e. between stamen of pin and stigma of thrum or between the stigma of pin and stamen of thrum.

Pin/thrum

ss Ss parents

s x Ss gametes

Ss ss F1- generation

in addition to the floral differences or floral morphology, pin and thrum plants also differ in other characteristics such as pollen size and the size of stigmatic cells. consequently a combination of pin x pin is incompatible and thrum x thrum is also incompatible. it means that homozygous SS will not exist.

B. Homomorphic Incompatibility

Under this condition, differences in floral morphology are excluded. We therefore have

gametophytic and sporophytic incompatibilities.

i. Gametophytic Incompatibility: This system is also known as the opposition factor system and it depends on a series of alleles on a single locus i.e. the ability of a pollen to fertilize the stigma depends on the type of gene in each locus. Under this system, pollen tube growth is usually very slow within a style that contains similar alleles e.g. S1S1 or S2S2 . Consequently, plants are virtually always heterozygous at this locus S1S2 or S2S3 or S1S3 for compatibility to be possible.

A situation with two alleles with gametophytic control with no dominance is impossible

because all plants could be incompatible and sterile. The phenomenon of incompatibility

gives rise to three types of pollination.

1. Fully compatible (S1S2 x S1S2)

In this case both alleles are common in the male and the female. All gametes are nonfunctional and so, no offspring is produced.

2. Half of pollen is compatible (S1S2 x S1S3)

Half of the pollen is compatible in which one allele is different in both the male and female gametes. S3 is the functional male and S1 is non-functional male.

S1S2 x S1S3

S1S1 S1S3 S1S2 S2S3 ----- F1

3. All pollen are compatible (S1S2 x S3S4)

In this case both alleles differ giving the progenies:

S1S3 S1S4 S2S3 S2S4

ii. Sporophytic Incompatibility:

This is similar to the gametophytic system in that genetic control is by a single gene with

multiple alleles. However, unlike gametophytic type, the functionality of pollen is determined by the genetic constitution of the plant producing it. It also differs from the gametophytic system in that the alleles may show dominance. Thus, individual action or

competition in either pollen or style is according to the allele combination involved. The

main feature of sporophytic system that differentiates it from the gametophytic system

are:

a. There are frequent reciprocal differences

b. Incompatibility can occur within the female parent

c. A family can consist of three incompatible groups or more

d. Homozygosity is a normal part of the system

e. An incompatible group may contain two genotypes.

By Mukesh Kumar Bhelloria