Heterosis breeding and inbreeding depression.pdf

This presentation ppt gives information about heterosis breeding, genetic basis and physiological basis of heterosis. It explains about inbreeding depression and effect of inbreeding depression and production of hybrid seed production in some crops.

1. DEPARTMENT OF GENETICS AND
PLANT BREEDING
FACULTY OF AGRICULTURE
ANNAMALAI UNIVERSITY
GPB 502-PRINCIPLES OF PLANT
BREEDING
SUBMITTED BY
A.VIKRAMAN
1
ST
YEAR M.SC (AGRI) GPB
2251150005

2. HETEROSIS BREEDING
B73
Mo17
F1 (B73 X Mo17 )
Definition: Superiority of F1 hybrids over the parents

3. AN INTRODUCTION TO HETEROSIS
HybridvigourinplantswasfirstdiscoveredbyKolreuterin1766.
ThetermheterosiswasfirstcoinedbyShullin1914
Heterosis/HybridvigourreferstothesuperiorityofF1hybridsin
oneormorecharactersoverbothofitsparents.
HeterosisreferstoincreaseofF1overparentsingeneralvigour,
yieldandadaptation,resultingfromcrossingofgeneticallyunlike
organisms.
Heterosisdiffersfromluxuriance.Luxuriancereferstoincreaseof
F1overparentsinvegetativegrowthnotinyieldandadaptation.
INTRODUCTION TO HETEROSIS

4. SUPERIORITY OVER PARENTS :
Heterosis leads to superiority in adaptation, yield, quality,
resistance and general vigour over its parents. But in some cases, hybrid
may be inferior to the weaker parents is also regarded as heterosis.
CONFINED TO F1:
Heterosis is confined to F1 generation of a cross. It declines
and disappears in F2 and subsequent generations due to consequence of
segregation and recombination.
GENETIC CONTROL :
The expression of heterosis is governed by nuclear genes. In
some cases due to interaction between nuclear genes and cytoplasm.

5. REPRODUCIBLE:Heterosis once identified can be easily
reproducedin a definite environment.
EFFECT OF HETEROZYGOSITY :The magnitude of heterosis is
associated with heterozygosity. The dominance effects are
expected to be maximum in cross pollinated species and
minimum in self pollinated species.
CONCEALS RECESSIVE GENES :In case of heterosis,deleterious
recessive genes are covered by the favorable effect of dominant
genes.

6. GENETIC BASIS OF HETEROSIS
There are two main theories which have been advanced to explain the
mechanism of heterosis .
One is the dominance hypothesis and the second is overdominance
hypothesis.
The epistasisis also considered to be associated with heterosis.
Thus there are three possible genetic causes of heterosis.viz,
1) Dominance
2) Overdominance
3) Epistasis
1)Dominance
2)Overdominance
3)Epistasis
GENETIC BASIS OF HETEROSIS

7. 1. DOMINANCE HYPOTHESIS
This theory was proposed by Davenport(1908), Bruce(1910) and
Keeble and Pellew(1910).
This is the most accepted explanation of heterosis.
According to the hypothesis heterosis is the result of the
superiority of dominant alleles when recessive alleles are
deleterious.
Here the deleterious recessive genes of one parent are hiddenby
the dominant gene of another parent and the hybrid exhibits
heterosis.

8. Suppose genetic constitution of one parent is AABBccddand that
of another as aabbCCDD.
A hybrid between these two parents will have four dominant genes
and exhibit superiority over both the parents which have two
dominant genes each.
Thus heterosisisdirectly proportional to the number of dominant
genescontributed by each eachparent.

9. OBJECTIONS TO DOMINANCE HYPOTHESIS
First objection, If the hypothesis is true, it should possible to obtain pure
homozygous individuals (AABBCCDD) in F2, which are all homozygous for
all the dominant genes which should have same vigouras AaBbCcDd.
Second objectionis that if the heterosis is due to dominance, the F2
curve should be skewed towards dominant genes, but the curve of F2 is
found always smooth and symmetrical not skewed.

10. EXPLANATION FOR OBJECTIONS
Jones (1917) and Collins (1921) helped to remove these objections to
some extent :
Jones explained for first objection with the presence of linkage
hypothesis, He suggested that there may be linkage between some
favourable genes and unfavourable genes and as a result it is not
possible to obtain true breeding homozygous individual for all
dominant gene in F2.
Collins provided explanation for second objection. He suggested
that trait like yield is governed by large number of genes or
polygenes which exhibit continuous variation resulting in
symmetrical distribution of genes.

11. 2. OVERDOMINANCE HYPOTHESIS
This hypothesis was independently proposed by Shull and East in
1908 and supported by Hull in 1945. It is also known as single gene
heterosis and super dominance theory.
The term overdominance was coined by Hull in 1945 working on
maize
According to this hypothesis, heterosis is the result of superiority of
heterozygote over its both homozygote parents. Thus heterozygote
Aawould be superior to AAand aa.

12. East in 1936 further elaborated this theory by proposing a series of
alleles a1,a2,a3,a4etc, are gradually increasing divergence in
functions. Thus the combinatonof a1a4will exhibit higher heterosis
as compared to combinations of a1a2,a2a3 and a2a4.

13. 3.EPISTASIS (NON-ALLELIC GENE INTERACTION)
Epistasis refers to interaction between alleles of two or more
different loci. It is also known as non allelic interaction.
The non allelic interaction is of three types viz. additive xadditive,
dominance x dominance and additive x dominanceand heterosis
has positive association with the presence and magnitude of non
allelic interaction.
Epistasis, particularly that involves dominanceeffects(dominance x
dominance) may contribute to heterosis. This has been observed in
cotton and maize.

14. PHYSIOLOGICAL BASIS OF HETEROSIS
Early stages on physiological basis of heterosis focussed on embryo
and seed size, growth rates.
Heterozygosity results from the greater initial weight of the embryo.
Hybrid vigour is nothing more than the maintenance of initial
advantages in embryo size.
Hybrids do not differ from their parents in relative growth rate.

15. STAGES OF PHYSIOLOGICAL BASIS OF
HETEROSIS
1.Seed and embryo development
High positive correlation was recorded between hybrid and embryo
weight.
Copland noted that hybrid embryo exhibited greater vigour in the
earlier stages of development.
They are found significant differences between embryo weight of
reciprocal crosses.
Early differences in embryo size in favour of hybrids resulted from
rapid growth rate of multi cellular zygote.

16. 2. Early seedling growth
Hybrids shows higher growth rate than inbred immediately
after germination but not later.
Hybrids do have a larger maximum growth rate period.
They are says embryo size is more significant than reserve food
material.
3. Later seedling growth
Hoffer found root dry matter content of hybrids to be
intermediate between parents.
Hybrids found to absorband utilize more amount of N,Pthan
the inbred lines.
Catalase activity was higher in heterosis hybrids.

17. FIXATION OF HETEROSIS
1. Asexual Reproduction : Heterosis can be easily conserved in
vegetatively propagated crops.
2. Apomixis : In apomixis, the seed develops without fertilization.
Apomictic seeds generally developed from maternal diploid cells and
they are identical to mother plants.
3. Balanced Lethal System : Balanced lethal system also leads to
fixation of heterosis in some plants.
4. Polyploidy : Heterosis can also fixed by chromosome doubling or
polyploidy especially in interspecific and intergeneric hybrids.
FIXATION OF HETEROSIS

18. FACTORS AFFECTING HETEROSIS
1.Mode of Pollination : The level of heterosis higher in cross pollinated
species than self pollinated species.
2.Genetic Diversity of Parents : In wheat higher heterosis is associated
with crosses of distantly related plants.
3.Genetic Base of Parents : In cotton higher heterosis is associated
with broad genetic base of the parnts.
4.Adaptability of Parents : In cotton and many other crops, heterosis is
associated with wider adaptability of the parents, because there is
closer association between adaptability and genetic base.
FACTORS AFFECTING HETEROSIS

19. ESTIMATION OF HETEROSIS
Heterosis can be classified into three on the basis of estimation.
1.Relative heterosis over mid parent/ Average heterosis
2.Over better parent/ Heterobeltiosis
3.Standard heterosis/ economic heterosis over
commercial hybrid or variety /Useful heterosis
ESTIMATION OF HETEROSIS

20. 1.MID PARENT HETEROSIS / AVERAGE HETEROSIS
When the heterosis is estimated over the mid parent,i.e, mean value or
average of the two parents, it is also known as average heterosis or
relative heterosis and calculated by using formula
Mid parent heterosis = F1 –MP / MP X 100
Where, F1 is the mean value of F1 and
MP is the mean value of two parents
involved in the cross .

21. 2.BETTER PARENT HETEROSIS /
HETEROBELTIOSIS
When the heterosis is estimated over the superior or better parent, it is
referred to as heterobeltiosisand calculated by using formula
Heterobeltiosis= F1 –BP / BP X 100
Where BP is the mean value of the better parent of the particular cross.
The term heterobeltiosiswas used by
Bitzer et al (1968)to describe the improvement
of heterozygote over the better parent.

22. 3.STANDARD HETEROSIS /USEFUL HETEROSIS
It refers to the superiority of F1 over the standard commercial check
variety. It is also called as economic heterosis or usefulheterosisand
calculated by using formula.
Standard Heterosis = F1 –check / check x 100
Heterobeltiosisleads to increase in yield, reproductive ability,
adaptability, disease and insect resistanc, general vigour, quality etc. For
most of the characters, the desirable heterosis is positive. But for some
characters like earliness, height in cereals and toxic substances are
negative heterosis.

23. INBREEDING AND INBREEDING DEPRESSION
INBREEDING :
Inbreeding is a form of mating system in sexual organism.
It implies mating together of individual that are close to each
other by ancestral or pedigree relationship. E.g. Full sib and half sib
mating.
The highest degree of inbreeding is achieved by selfing.
The chief effect of inbreeding is to increase homozygosity in
progeny ,which is proportionate to inbreeding.

24. INBREEDING DEPRESSION :
Inbreeding depression refers to decrease in fitness and vigour
due to inbreeding in cross pollinated cross.
The impact of inbreeding is the loss of vigourand the
physiological efficiencyof an plant characterized by size and
fertility.
Inbreeding depression results due to fixation of unfavourable
recessive genes in F2, while in heterosis
the unfavourablegene of one parent
is coveredby favourabledominant
gene of another parent.

25. In fact heterosis and inbreeding depression may be considered
as the two opposite sides of the same coin.
Selfingreduces heterozygosity by a factor 1 / 2 in each
generation.
Inbreeding reduces
the level of
heterozygosity to half
after each generation.

26. Man has recognized inbreeding depression for a long time. Marriage
between closely related ancestries have been prohibited.

27. EFFECTS OF INBREEDING
Selfing, full / half –sib pollination

28. EFFECTS OF INBREEDING DEPRESSION
Inbreeding causes reduction in vigourand reproductive capacity
that is fertility.
There is a general reduction in size of various plant parts and in
yield.
Some of the effects of inbreeding may be summarisedas under :
a) Appearance of lethal and sub lethal alleles:
Inbreeding to the appearance of lethal, sublethal and subvital
characteristics.

29. b) Reduction in vigour:
Plant becomes shorter and weaker because of general reduction in
the size of various plant parts.
c) Reduction in Reproductive ability :
The reproductive ability of population decreases rapidly.
d) Separation of the population into Distinct lines:
The population rapidly separates into phenotypically distinct lines.
c) Increase in homozygosity :
Inbreeding increases homozygosity ,ultimately after 7 to 8
generations of selfingall the lines are became uniform.

30. DEGREE OF INBREEDING DEPRESSION
The various plant species differ considerably in their responses to
inbreeding.
Inbreeding depression may range from very high to very low or may
even absentinto the following four broad categories.
1)High inbreeding depression
2)Moderate inbreeding depression
3)Low inbreeding depression and
4)Absence of inbreeding depression.

31. 1.HIGH INBREEDING DEPRESSION :
A large population of plants produced by selfingshows lethal
characteristics and do not survive.
The lines show greater reduced yield, generally less than 25
percent of the yield of open pollinated varities.
Example : Alfalfa, Carrot etc.,
2.MODERATE INBREEDING DEPRESSION :
Many lethal and sub lethal types appear in selfedprogeny, but
a substantial proportion of the population can be maintained
under self pollination.
Inbredsyield upto50 percentof open pollinated variety
Example : Maize, Jowar, Bajra etc,.

32. 3. LOW INBREEDING DEPRESSION :
Only a small proportion of the plants show lethal or sub vital
characteristics.
The lines show less or same as open pollinated variety.
Example : Onion, many cucurbits, Rye, Sunflower.
4. NO INBREEDING DEPRESSION :
The self pollinated species do not show inbreeding
depression although they do not show heterosis.
It is because their species reproduce by self fertilization and
as a result, have developed homozygous balance.
In cost cross pollinated species exhibit heterozygous balance.

33. INBRED LINES
DEFINITION :
Inbred lines are homozygous lines are obtained through
continuous selfingwith selection over several generations.
INTRODUCTION :
Inbred lines are developed from a variable source population. The
source population is generally an open pollinated variety ( First
cycle inbreds) or it may be synthetic, a single cross or a backcross
(Second, third or four cycle inbreds).
Inbredsare usually developed through a suitable system of close
inbreeding ( 6-7 generation of selfing) .

34. DEVELOPMENT OF HYBRID VARITIES
Development of hybrid varitiesdiffers from species to species.
The production of hybrid varitiesconsists of three main,
1) Development of inbreds
2) Evaluation of Inbred lines
Phenotypic evaluation
Top Cross Method
Single Cross Method
3) Production of hybrid seed

35. 1. DEVELOPMENT OF INBREDS
Development of inbredsis an important step in the production of
hybrids.
An inbred line can be maintained by selfingor sibling.
The purpose of inbreeding is to fix the desirable characters in
homozygous condition in order to maintain them without any genetic
change.
CM-100-199-Yellowflint CM-400-499-WhiteDent
CM-200-299-YellowDent CM-500—599M-Yellow
CM-300-399–WhiteFlint CM-600-699–White

36. There are two methods of developing inbred lines.
1.Selfing of heterozygous populations (Pedigree method, Bulk
method, Single seed descent method, Back cross method).
After each selfingdesirable plants are selected and self
pollinated or sib pollinated.
Usually it takes 6 –7
generations to attain
near homozygosity.

37. 2. Haploid plant or Doubled haploid method
a. Production of haploid plant
1. Anther culture / Pollen culture : Culturing of anthers
containing microspores in nutrient medium
2. Interspecific hybridization : Crossing of two species from
the same genus.
b. Chromosomal doubling
1. By colchicine treatment :
Doubling of chromosomes in plant
Using Colchicine.

38. 2. EVALUATION OF INBRED LINES
After an inbred line is developed, it is crossed with other inbreds
and its productiveness in single and double cross combination is
evaluated.
The ability of an inbred is evaluated on the basis of an inbred to
transmit their desirable performance to its hybrid progeniesis
referred as combining ability ( GCA & SCA ).
GENERAL COMBINING ABILITY SPECIFIC COMBINING ABILITY
General combining ability is directly
related to the breeding value of the
parent and is associated with additive
genetic effects.
Specific combining ability is the
performance of a cross that is associated
with Non additive gene
action.(Dominance and epistasis).

39. The inbredsare evaluated in following way :
1. Phenotypic evaluation : It is based on phenotypic perfomances
of inbredsthemselves. It is effective for characters which are highly
heritable i.ehigh GCA. The performance of inbredsare tested in RYT
and Poor performing inbredsare rejected.
2. Top Cross Method : Top cross refers to a cross between an
inbred line and an open pollinated variety. The inbredswhich are
selected on phenotypic evaluation, are crossed to a tester with wide
genetic base. The top cross seed is produced by planting alternate
rows of inbred and open pollinated variety and removing tassel.

40. 3. Single Cross Evaluation:
Out standing single cross combinations can be identified only by
testing the performance of single cross. This method is used to
measure the specific combining ability of those inbredswhich are
selected on the basis of top cross performance.
TIME OF TESTING : The testing of inbredsfor general combining
ability should be started from3
rd
, 4
th
, and 5
th
generation of selfing.
This will help in retaining inbredswith good combining ability .
No. of single crosses with reciprocals = n(n-1)
No. of single crosses without reciprocals = n(n-1)/2

41. 3. PRODUCTION OF HYBRIDS
After identification of superior line of the hybrid seed is produced.
There are three types of intervarietalhybrid.
a) Single cross hybrid
b) Three way cross hybrid and
c) Double cross hybrid
In case of single and three way cross hybrids, the rows of female and
male parents are planted in 2 : 1 ratio. The tassels of female parent
are removed and natural cross pollination is allowed.

42. In case of three way cross hybrid, the single cross hybrid is used
as female parent and inbred lines as male parent.
While in case of double cross hybrid, the rows of female and
male parent are planted in 3 : 1 or 4 : 1 ratio.
The seed production is carried out in isolation to prevent
crossing with other compatible genotypes and maintain the high
genetic purity.

43. HYBRID SEED PRODUCTION
TECHNIQUES IN HYBRID SEED PRODUCTION
USE OF MALE STERILITY AND HAND EMASCULATION IN FIELD
CROPS :
1) Cytoplasmic genic male sterility ( CGMS Or CMS )
Rice, sorghum, bajra, sunflower.
2) Genetic male sterility ( GMS )
Red gram

44. 3. Detasseling and pollination
Maize
4. Emasculation and dusting / pollination
Cotton
USE OF MALE STERILITY AND HAND EMASCULATION IN
HORTICULTURAL CROPS :
1. Hand emasculation and manual pollination
Tomato, brinjal, sweet pepper, okra, hot pepper
2. Pinching of staminateflowers and hand pollination
Bitter gourd, bottle gourd, pumpkin

45. 3. Removal of staminate flowers + emasculation + hand pollination
Water melon and muskmelon
4. Functional male sterility and hand pollination
Tomato and brinjal
5. GMS + bee pollination
Chilli
6. CMS + Natural pollination
Capsicum, onion, cabbage, carrot, radish
7. Self incompatabilityand natural pollination
Cauliflower, broccoli .

46. References :
Essentials of Plant Breeding –PhundanSingh
Plant Breeding –B.D. Singh
Seed science and technology –K. Vanangamudi
Steps in development of inbred and hybrid varities–
Dr. Prabhat KR. Singh
Inbreeding and Inbreeding depression in plants –Tulshiram
Rathod.