Exploitation of Heterosis

1. COURSE TITLE: Plant Breeding
COURSE NO.: PBG-122 (HO)
ADMISSION : 30HO/16
SUBMITTED BY: Rosaline Singh
SUBMITTED TO: Dr. Kaushik Kumar Panigrahi

2.  Heterosis, hybrid vigour, or out
breeding enhancement, is the
improved or increased function of any
biological quality in a hybrid offspring.
The adjective derived from heterosis is
heterotic.
 An offspring exhibits heterosis if its
traits are enhanced as a result of
mixing the genetic contributions of its
parents. These effects can be due to
Mendelian or non-Mendelian
inheritance

3.  In proposing the term heterosis to replace the
older term heterozygosis, G.H. Shull aimed to
avoid limiting the term to the effects that can be
explained by heterozygosity in Mendelian
inheritance.
 The physiological vigour of an organism as
manifested in its rapidity of growth, its height and
general robustness, is positively correlated with
the degree of dissimilarity in the gametes by
whose union the organism was formed . The
more numerous the differences between the
uniting gametes — at least within certain limits —
the greater on the whole is the amount of
stimulation . These differences need not be
Mendelian in their inheritance. To avoid the
implication that all the genotypic differences
which stimulate cell-division, growth and other

4.  Heterosis is often discussed as the
opposite of inbreeding depression
although differences in these two
concepts can be seen in evolutionary
considerations such as the role of
genetic variation or the effects of genetic
drift in small populations on these
concepts. Inbreeding depression occurs
when related parents have children with
traits that negatively influence their
fitness largely due to homozygosity. In
such instances, outcrossing should result
in heterosis.
 Not all outcrosses result in heterosis. For
example, when a hybrid inherits traits
from its parents that are not fully

5. HISTORY OF HYBRID
VARIETIES
 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.

6.  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

7. Types of 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.

8.  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.

9. 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

10. 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.

11. 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.

12.  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

13. Double-cross Hybrids
The most prevalent type of hybrid that
was grown in the United States in the 1930’s
and 1940’s is known as a double-cross
hybrid. As the name implies, producing a
double-cross hybrid requires two stages of
crossing involving two pairs of inbreeds (see
diagram below). In Step 1, two pairs of
inbreeds, A and B and Y and Z, are crossed
to produce single-cross hybrids, AB and YZ.
In Step 2, the two single-cross hybrids
produced in Step 1 are crossed to produce
the double-cross. Typically, A and B are
closely related and Y and Z are also closely
related, but neither A nor B is closely related
to Y or Z. Unlike a single-cross hybrid, plants
of a double-cross hybrid are not genetically

14.  Compared to single-cross hybrid production, production of
double-cross requires an extra step. During the early history
of the hybrid seed industry in the United States, this extra
step was necessary because the inbreeds available at that
time produced so little grain that making commercial
quantities of seed of single-cross hybrids was difficult. Even
though the inbreeds of each pair of a double-cross hybrid
were related, the resulting single-cross hybrids exhibited
sufficient vigour to allow those single crosses to be used
successfully as parents in mass production of commercial
seed. In most environments, the best single-cross hybrid will
have superior performance to the best double-cross hybrid.
As breeders gradually improved the performance of inbreeds
through selection, it became possible to commercially
produce the more desirable single-cross hybrids.
 Double-cross hybrids may become important in the organic
corn market. In production of organic hybrid seed corn,
herbicides cannot be used. Therefore, seed producers desire
parents that have good vigour and can compare successfully
against weeds. The single-cross parents of double-cross

15. Single-cross Hybrids
A hybrid plant results from a cross of two
genetically different plants. The two parents
of a single-cross hybrid, which is also known
as a F1 hybrid, are inbreeds. Each seed
produced from crossing two inbreeds has an
array (collection) of alleles from each parent.
Those two arrays will be different if the
inbreeds are genetically different, but each
seed contains the same female array and the
same male array. Thus, all plants of the same
single-cross hybrid are genetically identical.
At every locus where the two inbred parents
possess different alleles, the single-cross
hybrid is heterozygous.

16. Figure 2a
Corn Plants: Inbred B73
(left), Inbred
Mo17(middle),
Single cross B73 x Mo17
(right) (UNL, 2004)
Figure. 2b:
Corn Ears: Inbred B73
(left), Inbred Mo17(right),
Single cross B73 x Mo17
(middle) (UNL, 2004)

17. For example, in Figure 2a the height
of the single-cross hybrid is 3.0 m (this
equals Hyb), the average height of the
inbreeds is 2.0 m (this equals MP),
and the value of hybrid vigour is 50%.
Hybrid vigour calculated in this way is
called mid-parent hybrid vigour.
Another type is high-parent hybrid
vigour. This is the superiority,
expressed as a percentage, of the
hybrid over the parent with the better
or higher value of the trait. Corn
breeders will be successful in
increasing hybrid performance if the
hybrid vigour of a new hybrid

18. Hybrid
A hybrid is the offspring of genetically
dissimilar parents or stock, especially the
offspring produced by breeding plant or
animals of different varieties, species or
races, something of mixed origin or
composition, such as a word whose
elements are derived from different
languages. A hybrid is something having
two kinds of components that produce
the same or similar results. For instance,
the offspring of two plants of different bre
eds, varieties, or species, especially as p
roduced through human manipulation for
specific genetic characteristics.

19. Examples of hybrid plants include :
 Sweet corn: The vast majority of U.S. corn grown
are hybrid varieties. The characteristics of these
varieties have made it easier for home gardeners
to grow and they are sweeter than past crops.
 Meyers lemons: Meyer lemons, originating in
China, are a cross between a true lemon tree
and mandarin orange tree. The fruit is much
sweeter than traditional lemons, which makes
this variety a favourite of gardeners and chefs
alike.
 Better Boy tomatoes: Better Boys have been
bred to be resistant to verticillium wilt, fusarium
wilt and nematodes, which are all common
tomato plant problems. Gardeners and tomato
lovers favour the large, bright red fruit, which can
weigh up to 1 lb. and mature within 75 days of
seedlings being transplanted into the ground.

20. Intergeneric Hybrid
 An intergeneric hybrid is a cross between plants
in two different genera in the same family. They
are closely related enough that pollination will
produce a hybrid, though the seeds of this hybrid
are usually sterile. The more distant
the relation between the two genera, the greater
the difficulty of intergeneric hybridization. Genera
that generate intergeneric hybrid are
always genetically related members of the
same taxonomic Tribe . Intergeneric hybridization
represents an opportunity to combine genomes
from distinctly different plants and to introgress
traits not found in the main genus of
interest. Many intergeneric hybrids are infertile.
 This type of hybridization is more for scientific
interest than for any other use.

21. Cypress
 For example, the Leyland cypress (×
Cupressocyparis leylandii) is a result
of crossing a Monterey cypress
(Cupressus macrocarpa) with a
Nootka cypress (Chamaecyparis
nootkatensis).
 Propagation of the Leyland cypress is
usually carried out through cuttings,
though it is possible for this tree to
produce seeds that will germinate.
Asexual propagation, however, is
preferable since it will ensure that the

22.  Cactus
A viable example is x Ferobergia, a
hybrid between two cacti of the
genus Ferocactus and of the
genus Leuchtembergia.
 Orchids
Vascostylis Prapawan 'Tanzanite' is a
combination of three orchid genera:
Ascocentrum x Rhynchostylis x Vanda