1. Mid parent heterosis is more practical importance in comparing performance of F1 hybrids because it indicates the level of superiority of a hybrid over the average performance of its parents. 2. Yes, it is possible that a hybrid may register high MPH (%) due to wide difference between parents but its actual performance may be lower than other hybrids when compared to a standard/commercial check. In such cases, standard heterosis or economic heterosis would be a better indicator of hybrid's performance relative to commercially grown varieties.

1. Heterosis Breeding Crops

2. Definition and historical considerations of
heterosis
Heterosis is the genetic expression of the
superiority of a hybrid in relation to its parents
Heterosis or hybrid vigor was first coined by
shull in 1914
to denote the stimulation in size and vigor in
hybrid.

3. It referred to:-
 increased vigor,
size,
fruitfulness,
speed of development,
resistance to disease and to pest, or to
climatic rigors of any kind,
manifested by cross bred organisms as
compared with corresponding inbred
(Shull, 1952, Zirkel, 1952).

4. Self-pollination of hybrids over several
generations leads to a gradual reduction of
heterozygosity and vigour in these plants,
a phenomenon that is known as inbreeding
depression
Hence, heterosis and inbreeding
depression are different aspects of the
same phenomenon (Zirkle, 1952).

5. Plant hybridizers in the 18th century
were among the first to make significant
scientific contributions to the development
of a modern heterosis concept.
Much of their work was concerned more
with experimentation of the results of
divergent crosses rather than practical plant
improvement (Mayr, 1982).

6. In 1776, Kolreuter became the first of the
plant hybridizers to document in detail the
results of his crosses, in the genus
Nicotiana, and to describe significant
heterosis
Knight took these ideas one step further
and, in 1799, suggested that the widespread
existence of cross-pollination in nature
was proof that nature 'intended' this to be
the norm.

7. In 1828, Wiegmann described the results of
crosses leading to heterosis in the crucifers
(Mayr, 1982).
Gartner and Focke, in 1849 and 1881,
respectively, detailed the results of their crosses,
noting heterosis and encouraging other
scientists to think along the lines of enhanced
plant growth through such hybridization (Mayr,
1982).

8. Darwin detailed the results of 37 crosses
including maize in which he observed
increased height in 24 crosses.
He commented on the decrease in height
observed in the self-pollinated plants and
discussed many natural mechanisms by
which plants avoid inbreeding.

9. These observations became critical in
developing an understanding of the
significance of inbreeding and out
breeding in nature.
Interestingly, Darwin noticed that the
deleterious effects of inbreeding could be
reversed following the crossing of inbred
strains.

10.  In the cases he tested, the performance of the first cross-
pollinated generation was quite vigorous.
 As such, he suggested that the effects of inbreeding
were essentially reversible upon intermating.
 This hints quite closely at a modern concept of the
relationship between inbreeding and out breeding with
respect to heterosis;
 however Darwin's ideas did not draw a clear path
between the effects of inbreeding and those of out
breeding (Goldman, 1999).

11.  The phenomenon observed by Charles Darwin was
rediscovered independently by Shull and East in
1908.
 Since then, heterosis has been extensively exploited in
plant breeding, particularly in maize.
 Today, 95% of the maize acreage in the USA and 65%
of the maize acreage worldwide is planted to hybrids
(Swanson-Wagner, 2006).
 Heterosis is most evident for adult traits, but is
already manifested during embryo (Meyer, 2007) and
early seedling development (Hoecker, 2006).

12.  Heterosis has been under investigation for more than 100
years,
 but no consensus exists about the genetic basis
underlying this very important phenomenon
 The most prominent genetic hypotheses to explain
heterosis are the dominance hypothesis and the over
dominance hypothesis,
 which were proposed about one century ago

13. The dominance hypothesis attributes
heterosis to canceling of deleterious or
inferior recessive alleles contributed by
one parent,
by beneficial or superior dominant
alleles contributed by the other parent
in the heterozygous genotypes at
different loci

14.  The over dominance hypothesis attributes heterosis to
the superior fitness of heterozygous genotypes over
homozygous genotypes at a single locus
 Finally, the epistasis hypothesis, considers epistatic
interactions between non-allelic genes at two or more
loci as main factor for the superior phenotypic
expression of a trait in hybrids
 However, it has recently been demonstrated in tomato
introgression lines that heterosis is manifested even in
the absence of epistasis (Semel, 2006).

15. The dominance concept suggests that the hybrid
vigour is the result of action and interaction of
favorable/ beneficial dominant alleles.
Heterozygosity per se is not essential as log as
parents of the hybrid maximize the number of
dominant alleles that confer rapid growth
According to this theory, the inter crossing of
inbred lines lead to the formation of hybrids in
which deleterious recessives contributed by
one are hidden by dominant alleles contributed
by the other parent (Allard, 1960).

16. For example, in cross of two inbred lines A and B
having the genetic constitution as:-
 AAbbCCdd, and
 aaBBccDD
the cross of these in F1 would be AaBbCcDd.
This hybrid contains more number of (4) of
favourable dominant genes and
would be more vigorous than either of the parent
which is comparatively less number of dominant
genes (2)

17. The over dominance theory explains single loci
at which two alleles have the property that the
heterozygote is truly superior to either of
homozygote
This showed that the heterozygote is superior to
the two homozygotes for the same gene
i.e. AA<Aa>aa and vigor increases in the
proportion to the amount of heterozygosis (Mac
Key, 1976).

18.  According to this theory heterozygosity per se is
necessity for the full expression of heterosis
 Rhodes et al. (1992) and Crow (2000), based on critical
reviews of the dominance and over dominance
hypothesis concluded that:-
 even though the dominance hypothesis is largely
accepted as playing the most important role in causing
heterosis, there is still the possibility that a small
contribution may come from some over dominance
 over dominance loci may give a small, but important
boost to the best yield.

19. There were objections on these theories
 the two early objections to the dominance
hypothesis were:-
the absence of a skewed F2 distribution,
which would be expected from the
expansion of (3/4 + 1/4) n, and

20. second, the failure of selection to produce
inbreeds as good as hybrids
But Jones (1916) noted that with linkage
the two hypotheses became
indistinguishable and
Collins (1921) pointed out that with a
large “n” the F2 distribution is essentially
symmetrical.

21. The major objection to over dominance
theory stems from the fact that
most of the support and evidences obtained for
this have come from the cases of single locus
heterosis.
Doubts often expressed as to what is true for
qualitative gene may not hold good for the
expression of heterosis in the quantitative
characters like yield.

22. Early it was recognized that when a complex trait
is due to several underlying multiplicatively
acting traits
heterosis may occur even in the absence of
dominance
For example, if plant height is the product of
internode length and number of nodes
heterosis can result from a cross between a strain
with a large number nodes and a strain with large
internode length

23. The relative performance of an allele in one line
is not predictive of its performance in other lines
or crosses.
When this occurs heterosis at a particular locus is
a function of the genetic background and
an allele of mediocre performance in one line or
cross may lead to substantial heterosis when
moved to another genetic background.

24.  Conference conducted on genetics and exploitation of
heterosis in crops has reached to important conclusion
that dominance and epistasis are the principal genetic
factors in the explanation of heterosis (Coors and
Pondey, 1999).
 This conclusion is stated that heterosis results
 From combined action and interaction of allelic and
non-allelic factors usually closely and positively
correlated with heterozygosity.
 All the above explanation shows that genetic causes of
heterosis are not yet fully understood.

25. Hybrid is the 1st generation produce
Vegetable hybrids under optimum crop
management produce economically higher yield
than improved open pollinated varieties
About 75-90% in developed countries (Japan,
USA) and developing countries like India and
China up to 60% of total vegetable area is under
hybrids

26. Similarities
1. Inbreeding would produce inbreeding depression
2. Outcrossing would restore vigour and fertility.
3.The degree of heterosis would depend upon the
genotypes of the two parents.
 In general, the greater the genetic diversity between
the parents, the higher the magnitude of heterosis.

27.  Differences between the two hypotheses are:
1. Heterozygotes are superior to the two homozygotes
according to the overdominance hypothesis,
 while according to the dominance hypothesis they are
as good as the dominant homozygote
2. Inbreds as vigorous as the F1 hybrid can be isolated
according to the dominance hypothesis,
 but it will be impossible according to the
overdominance hypothesis.

28. 3. According to dominance hypothesis, inbreeding
depression is due to homozygosity of harmful
recessive alleles,
 while as per overdominance hypothesis; it is due to
homozygosity itself
4. According to the overdominance hypothesis, heterosis is
the consequence of heterozygosity per se
 But as per dominance hypothesis it is the result of
dominant alleles masking the deleterious effects of
their recessive alleles, and heterozygosity itself is not
the cause of heterosis.

29.  Manifestations of Heterosis
 Heterosis is the superiority of a hybrid over its
parents
 This superiority may be in yield, quality,
disease and insect resistance, adaptability,
general size or the size of specific parts, growth
rate, enzyme activity, etc.
 These various manifestations of heterosis may be
summarized as follows

30. 1. Increased yield: Heterosis is generally
expressed as an increase in the yield of hybrids.
 Commercially, this phenomenon is of the
greatest importance since higher yields are the
most important objective of plant breeding.
 The yield may be measured in terms of grain,
fruit, seed, leaf, tubers or the whole plant.

31. 2.Increased Reproductive Ability
 The hybrids exhibiting heterosis show
an increase in fertility or reproductive
ability
 This is often expressed as higher yield of
seeds or fruits or other propagules, e.g.,
tuber in potato (S. tuberosum), stem in
sugarcane (S. officinarum), etc.

32. 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
 The increase in size is usually a result of an increase in
the number and size of cells in various plant parts
 Some examples of increased size are increases in fruit
size in tomato, head size in cabbage, cob size in maize,
head size etc.

33. 4. Better Quality
 In many cases, hybrids show improved quality
 This may or may not be accompanied by
higher yields
 For example, many hybrids in onion show
better keeping quality, but not yield, than
open -pollinated varieties

34. 5. Earlier Flowering and Maturity
 In many cases, hybrids are earlier in
flowering and maturity than the
parents
 This may sometimes be associated
with a lower total plant weight

35. But earliness is highly desirable in many
situations, particularly in vegetables
Many tomato hybrids are earlier than
their parents
6.Greater Resistance to Diseases and Pests
Some hybrids are known to exhibit a greater
resistance to insects or diseases than their
parents.

36. 7. Greater Adaptability
 Hybrids are generally more adapted to
environmental changes than inbreds
 In general, the variance of hybrids is significantly
smaller than that of inbreds
 This shows that hybrids are more adapted to
environmental variations than are inbreds
 In fact, it is one of the physiological explanations
offered for heterosis

37. 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

38. 9.Increase in the Number of 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
Such hybrids are known in beans (P.
vulgaris) and some other crops.

39. These are some of the characteristics for
which heterosis is easily observed
Many other characters are also affected
by heterosis,
e.g. enzyme activities,
cell division,
vitamin content (vit. C content in tomato),
other biochemical characteristics, etc., but
they are not so readily observable.

40. However, the hybrids also possess a
number of disadvantages and they are:-
Hybrid seeds costs higher than open
pollinated seeds and it may put off
subsistence farmers
Difficult in obtaining parental inbred lines
Difficult in maintaining inbred parental lines

41. Lastly, the superior characters of F1 hybrids
plants, unlike that of open pollinated varieties,
cannot be maintained by saving their seeds for
growing the next crop.
The uniformity, vigor, and overall performance of
the hybrid is lost during seed multiplication.
Therefore, growers need to buy seeds from the
seed producer every time they want to plant

42. Estimates of heterosis
(1) Mid-parent heterosis = (F– mp)/mp*100
(2) Heterobetiosis ( better parent heterosis)=
(F – bp)/bp*100
Where F = Mean of F1
mp = mean of the two parents
bp = better parent mean value
3) Economic heterosis= (F– cv)/cv*100
cv= commercial cultivar/variety mean value

43. P1= 20qt/ha P2=15qt/ha P3= 24qt/ha P4=18qt/ha, Standard check
25qt/ha?
P1 x P2=25qt/ha
P1 x P3= 28qt/ha
P1 x P4= 23 qt/ha
P2 x P3= 20qt/ha
P2 x P4= 21qt/ha
P3 x P4=26qt/ha
1. Calculate MPH (%), BPH (%) and SH(%)
2. Which estimate of heterosis is practical importance in
comparing performance of F1hybrids? Why?
3. Have you find that F1 registered high MPH (%) while the actual
performance is lower than other hybrids when compared to
standard check?

44. Cross MPH (%) BPH (%) SH (%)
P1 x P2
P1 x P3
P1 x P4
P2 x P3
P2 x P4
P3 x P4