Sexual and Asexual modes of reproduction in various field and horticultural crops.

1. Prepared By: Prof. T.V.Rathod

2. MODE OF REPRODUCTION
• Mode of reproduction determines the genetic constitution of crop
plants, that is, whether the plants are normally homozygous or
heterozygous.
• A knowledge of the mode of reproduction of crop plants is also
important for making artificial hybrids.
• Production of hybrids between diverse and desirable parents is the
basis for almost all the modern breeding programmes.
• The modes of reproduction in crop plants may be broadly grouped
into two categories, asexual and sexual.

3. Asexual Reproduction
 Vegetative Reproduction
• In nature, a new plant
develops from a portion of
the plant body. This may
occur through following
modified forms,
1. Natural vegetative
• Underground stem
• Sub-aerial stems
• Bulbils
2. Artificial vegetative
• Stem cuttings
Asexual reproduction does not involve fusion of male and female gametes.
New plants may develop from vegetative parts of the plant (vegetative
reproduction) or may arise from embryos that develop without fertilization
(apomixis).
 Apomixis
• seeds are formed but the
embryos develop without
fertilization.
1. Parthenogenesis
2. Apospory
3. Apogamy
4. Adventive Embryony

6. Natural vegetative
• Underground Stems
• The underground modifications
of stem generally serve as
storage organs and contain many
buds. These buds develop into
shoots and produce plants after
rooting. Eg.,
 Tuber : Potato
 Bulb : Onion, Garlic
 Rhizome : Ginger, turmeric
 Corm : Bunda, arwi.
• Sub-aerial Stems
• These modifications include
runner, stolon, sucker etc.,.
• Sub-aerial stems are used for
the propagation of mint, date
plam etc.
• Bulbils
• Bulbils are modified flowers
that develop into plants directly
without formation of seeds.
• These are vegetative bodies;
their development does not
involve fertilization and seed
formation.
• The lower flowers in the
inflorescence of garlic naturally
develop into bulbils.
• Scientists are trying to induce
bulbil development in plantation
crops by culturing young
inflorescence on tissue culture
media ; it has been successfully
done in the case of cardamom.

7. Artificial vegetative
• It is commonly used for the propagation of many crop species,
although it may not occur naturally in those species.
• Stem cuttings are commercially used for the propagation of
sugarcane, grapes, roses, etc.
• Layering, budding, grafting and gootee are in common use for the
propagation of fruit trees and ornamental shrubs.
• Techniques are available for vegetative multiplication through tissue
culture in case of many plant species, and attempts are being made to
develop the techniques for many others.
• In many of these species sexual reproduction occurs naturally but for
certain reasons vegetative reproduction is more desirable.

8. Significance of Vegetative Reproduction
• Vegetatively reproducing species offer
unique possibilities in breeding.
• A desirable plant may be used as a variety
directly regardless of whether it is
homozygous or heterozygous.
• Further, mutant buds, branches or seedlings,
if desirable, can be multiplied and directly
used as varieties.

9. Apomixis
• In apomixis, seeds are formed but the embryos develop without
fertilization. (without the fusion of male & female gametes).
• Consequently, the plants resulting from them are identical in
genotype to the parent plant.
• In apomictic species, sexual reproduction is either suppressed or
absent.
• When sexual reproduction does occur, the apomixis is termed as
facultative. But when sexual reproduction is absent, it is
referred to as obligate.
• Many crop species show apomixis, but it is generally facultative.
• The details of apomictic reproduction vary so widely that a
confusing terminology has resulted.

12. • Parthenogenesis
• The embryo develops from embryo
sac without pollination. It is of two
types
 Gonial parthenogenesis – embryos
develop from egg cell,
 Somatic parthenogenesis –
embryos develop from any cell of
the embryo sac other than the egg
cell.
• Apospory
• The embryo may develop from egg
cell or some other cell of this
embryo sac.
• Apospory occurs in some species
of Hieraceum, Malus, Crepis,
Ranunculus, etc.
o Diplospory
• Embryo sac is produced from the
megaspore, which may be haploid
or, more generally, diploid.
• Apogamy
• In apogamy, synergids or
antipodal cells develop into an
embryo.
• Like parthenogenesis, apogamy
may be haploid or diploid
depending upon the haploid or
diploid state of the embryo sac.
• Diploid apogamy occurs in
Antennaria, Alchemilla, Allium
and many other plant species.
• Adventive Embryony
• In this case, embryos develop
directly from vegerative cells of
the ovule, such as nucellus,
integument, and chalaza.
• Development of embryo does not
involve production of embryo
sac.
• Adventive embryony occurs in
mango, citrus, etc.

15. Significance of Apomixis
• Apomixis is a nuisance when the breeder desires to obtain sexual
progeny, i.e., selfs or hybrids.
• It is of great help when the breeder desires to maintain varieties.
• The breeder has to avoid apomictic progeny when he is making crosses
or producing inbred lines.
• But once a desirable genotype has been selected, it can be multiplied and
maintained through apomictic progeny.
• Asexually reproducing crop species are highly heterozygous and show
severe inbreeding depression.
• Therefore, breeding methods in such species must avoid inbreeding.

16. SEXUAL REPRODUCTION
• Sexual reproduction involves fusion of male and female gametes
to form a zygote, which develops in to an embryo.
Significance of Sexual Reproduction
• Sexual reproduction makes it possible to combine genes from
two parents into a single hybrid plant.
• Recombination of these genes produces a large number of
genotypes.
• This is an essential step in creating variation thr ough
hybridization.
• Almost the entire plant breeding is based on sexual
reproduction.
• Even in asexually reproducing species, sexual reproduction, if it
occurs, is used to advantage, e.g., in sugarcane, potato, sweet
potato etc.

17. MODES OF POLLINATION
• Self-pollination
• Bisexuality
• Cleistogamy.
• Homogamy
• Chasmogamy
• Position of anthers in relation to
stigma.
• Genetic Consequences of Self-
Pollination
• Self-pollination leads to a very
rapid increase in homozygosity.
• Therefore, populations of self-
pollinated species are highly
homozygous.
• Self-pollinated species do not
show inbreeding depression, but
may exhibit considerable
heterosis.
• Therefore, the aim of breeding
methods generally is to develop
homozygous varieties.

19. • Cross-Pollination
• Unisexuality (Dicliny)
 monoecy
 dioecy
• Dichogamy
 protogyny
 Protandry
• Heterostyly
• Herkogamy
• Self incompatibility
• Male sterility
• Genetic Consequences of Cross-
Pollination.
• promotes heterozygosity in a
population.
• highly heterozygous and show
mild to severe inbreeding
depression and considerable
amount of heterosis.
• Usually, hybrid or synthetic
varieties are the aim of breeder
wherever the seed production of
such varieties is economically
feasible.
# Often Cross Pollination #

21. Heterosis
• The superiority of F1 hybrid in
one or more character over both
the parents.
• The term heterosis was first
coined by Shull in 1914.
• Heterosis is increased vigours,
growth, yield or function of a
hybrid over the parents,
resulting from crossing of
genetically unlike organisms.
• Hybrid vigour has been used as a
synonym of heterosis.
• GENETIC BASES OF
HETEROSIS
• There are three main theories to
explain heterosis
• (1) dominance,
• (2) over dominance, and
• (3) epistatis hypotheses.
• Heterosis results from the
masking of harmful effects of
recessive alleles by their
dominant alleles.
• Inbreeding depression, on the
other hand, is produced by the
harmful effects of recessive
alleles, which become homozygous
due to inbreeding.

22. Estimation of Heterosis
• 1) Mid parent heterosis (average/mean)
2) Better parent heterosis (heterobeltiosis)
3) Standard heterosis
(commercial/economic/useful)

23. Inbreeding and
Inbreeding Depression
BY PROF. T. V. RATHOD

24. INTRODUCTION
• 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.
• When the individuals are closely related E. g Full sib was mating,
half sib mating.
• The highest degree of inbreeding is achieved by selfing.
• The chief effect of inbreeding is to increase homozygosity in
the progeny, which is proportionate to the degree of inbreeding.
• Cross – pollinated and asexually reproducing species are highly
heterozygous in nature.

25. • These species show a severe reduction in fertility and vigour
due to inbreeding (inbreeding depression).
• It contrast to this hybridization between unrelated strains
leads to an increased vigour and fertility (hybrid vigour or
heterosis).
• These two aspects are of great significance in breeding of these
species.
• In fact heterosis and inbreeding depression may be considered
as the two opposite sides of the same coin.
INTRODUCTION CONT…

26. Inbreeding Depression:
• It refers to decrease in fitness and vigour due to
inbreeding or it may be defined as the reduction or
loss in vigour and fertility as a result of inbreeding.
• The most revealing impact of inbreeding is the loss
of vigour and the physiological efficiency of an
organism characterised by reduction in size and
fecundity.
• For example selfing reduces heterozygosity, by a
factor ½ in each generation.
• In fact the dwgree of inbreeding in any generation
is equal to the degree of homozygosity in that
generation.

27. • Inbreeding depression results due to fixation of unfavourable
recessive genes in F2, while in heterosis the unfavourable
recessive genes of one line (parent) are covered by favourable
dominant genes of other parent.
• Man has recognised inbreeding depression for a long time.
• In many species marriage between closely related ancestries
have been prohibited.
• In hindu society perhaps presents the extreme example, where
marriages between individual related by ancestry is prohibited.
Inbreeding Depression Cont…

28. Effects of Inbreeding
 Inbreeding is due to a reduction in vigour and reproductive capacity that is
fertility.
 There is a general reduction in the size of various plant parts and in yield.
 The effects of inbreeding may be summarised as under.
a) Appearance of Lethal and Sublethal Alleles:
 Inbreeding to the appearance of lethal, sublethal and subvital characteristics.
 Such characteristics include chlorophyll deficiencies E.g Albino, chlorine rootles
seedlings , defects in flower structure etc. generally, plants carrying such
characteristics cannot maintained and are lost from the population.
b) Reduction in Vigour:
 There is a general reduction in the vigour of the population.
 Plants become shorter and weaker because of general reduction in the size of
various plant parts.
c) Reduction in Reproductive Ability:
 The reproductive ability of the population decreases rapidly.
 Many lines (plant progenies) reproduction poorly that they cannot be maintained.

29. d) Separation of the Population into Distinct Lines:
 The population rapidly separates into phenotypically distinct lines.
 This is because of an increase in homozygosity due to which there is random
fixation of various alleles of different lines.
 Therefore, the lines differ in their genotype and consequently in phenotype.
e) Increase in Homozygosity:
 Each line becomes increasingly homozygous following inbreeding.
 Consequently, the variation within a line decreases rapidly.
 Ultimately, after 7 to 8 generations of selfing, the lines become almost uniform.
 Since they approach complete homozygosity (> 99 percent homozygosity).
 The lines, which are almost homozygous due to continued inbreeding and are
maintained through close inbreeding, are known as inbred lines or inbreds.
i) Reduction in Yield:
 Inbreeding generally leads to a loss in yield.
 The inbred lines that is able to survive and be maintained yield much less than
the open pollinated varieties from which they were derived.
 In maize, the best – inbred lines yield about half as much as the open pollinated
varieties from which they were produced.
 In alfalfa and carrot, the reduction in yields is much greater, while in onions and
many cucurbits the reduction in yield is very small.
Effects of Inbreeding cont…

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 be absent into the following four broad categories.
1) High inbreeding depression,
2) Moderate inbreeding depression,
3) Low inbreeding depression, and
4) Absence of inbreeding depression.

31. High Inbreeding Depression:
• Several plant species, Eg. alfalfa (M. sativa) carrot (D. carota) ,
hayfield, tarweed etc show very high inbreeding depression.
• A large proportion of plants produced by selfing shows lethal
characteristics and do not survive.
• The loss in vigour and fertility is so great that very few lines
can be maintained after 3 to 4 generation of inbreeding.
• The line shows greatly reduced yields, generally less than 25
percent of the yield of open – pollinated varieties.

32. Moderate Inbreeding Depression:
• Many crops species, such as maize, jowar, bajara etc. shows
moderate inbreeding depression.
• Many lethal and sublethal types appear in the selfed progeny,
but a substantial proportion of the population can be maintained
under self- pollination.
• There is appreciable reduction in fertility and many line
reproduce so poorly that they are lost.
• However, a large number of inbred lines can be obtained, which
yield upto 50 percent of the open- pollinated varieties.

33. Low Inbreeding Depression:
• Several crop plants, E. g onion (A. cepa), many cucurbits, rye (S.
cereale), sunflower (Hannus), hemp etc show only a small degree
of inbreeding depression.
• Only a small proportion of the plants show lethal or subvital
characteristics.
• The loss in vigour and fertility is small; rarely a line cannot be
maintained due to poor fertility.
• The reduction in yield due to inbreeding is small or absent.
• Some of the inbreds lines may yields as much as the open
pollinated varieties from which they were developed.

34. Lack of 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 of the cross- pollinated species exhibit heterozygous
balance.

35. Homozygous and Heterozygous Balance
• The concepts of homozygous and heterozygous balance were advanced
by Mather to explain the varied responses of different species to
inbreeding.
• The species that reproduce by cross- fertilization are highly
heterozygous.
• These species carry a large number of lethal, subvital and other
unfavourable recessive genes, which are of little value to the species.
• The sum total of these unfavourable genes constitutes genetic load of
these species.
• The harmful effects of such recessive alleles are masked by their
dominant allele as result of which they are retained in population.

36. • The population, therefore, develops a genetic organisation, which favours
heterozygosity as a result , homozygosity leads to detrimental effects.
• This type of genetic organisation in known as heterozygous balance, because it
promoted heterozygosity.
• The self fertilized species are naturally homozygous.
• They have no genetic load and are prompty eliminated (from the population).
• These species therefore develop a genetic organisation, which is adapted to
homozygosity i.e which does not produce undesirable effects in the homozygous
state.
• This type of genetic organisation is known as homozygous balance.
• The self – pollinated species are believed to have evolved from cross fertilized
species.
Homozygous and Heterozygous Balance

37. • It has been suggested that the self- fertilized species retain sufficient
heterozygous balance to show the beneficial effects of out crossing i.e
heterosis.
• The cross- fertilized species that is generally grown in very small
populations’ e. g Cucurbits would show some degree of homozygosity due to
inbreeding.
• This would leads to the development of homozygous balance in such cross
fertilized species.
• The homozygous and heterozygous balances are concepts of genetic
organisation of populations.
• These concepts are neither very clear nor very specific in terms of the
physical bases of this genetic organisation of the types of gene combination
involved.
Homozygous and Heterozygous Balance

38. Heritability
By Prof. T. V. Rathod

39. INTRODUCTION
• Components of Genetic Variance:
• In crop improvements programme, only the genetic components
of variation are important because only this component is
transmitted to the next generation.
• According to Fisher in 1918, components of genetic variance
divided into three components viz.
• 1) Additive,
• 2) Dominance
• 3) Epistatic

40. 1) Additive Components:
• It is the component arising from difference between the two
homozygotes for a gene, Eg. AA and aa.
2) Dominance Component:
• It is due to the deviation of heterozygote (Aa) phenotype from
the average of phenotypic value of the two homozygotes (AA
and aa).
• It is also referred as intra-allelic interaction.
3) Epistatic or Interaction Components:
• It results from an interaction between two or more genes.
• Later Hayman and Mather classified the epistatic components
into three types interaction viz.
1) Additive X additive,
2) Additive X dominance,
3) Dominance X dominance.

41. Heritability:
• The ratio of genetic variance to the total variance i.e phenotypic
variance is known as heritability.
• The extent of contribution of genotype to the phenotypic
variation for a trait in a population is ordinarily expressed as the
ratio of genetic variance to the total variance. i.e Phenotypic
variance.
• Thus heritability denotes the proportion of phenotypic variance
that is due to genotype. Heritability may be represented as
follows:
VG VG
Heritability H = --------- Or = -----------
VP VG + VE
• Where VG, VP and VE are the genotypic phenotypic and
environmental component of variance respectively.

42. Types of Heritability:
 There are two types of heritability viz
1) Broad sense heritability and
2) Narrow sense heritability.

43. 1) Broad Sense Heritability:
• It is the ratio of genotypic variance VG to the total phenotypic
variance (VP=VG+VE)
• h2 (bs) = VG/VP or VG/VG+VE
• Broad sense heritability estimates are valid for homozygous
lines, or populations.
• However, when we are dealing with segregating generation.
• The genetic variance consists of additive and dominance
component.
• Since in self pollinated crop we develop homozygous lines, the
dominance component will not contribute to the phenotype of
homozygous lines derived from a population.
• Consequently in such cases only the additive component of
variation is important.
• Therefore, for segregating generation broad sense heritability
is less important but narrow sense heritability is more important
because it cannot realize fully in the offspring.

44. 2) Narrow Sense Heritability:
• It is the ratio of additive genetic variance VA to the total
phenotypic variance VP (smith, 1952)
• h2 (ns) = VA/VP = VA/VG + VE
• Narrow sense heritability is reliable measures, as it is based on
breeding value.
• The magnitude of narrow sense heritability is always less than or
equal to broad sense heritability.

45. Methods of Estimation of Heritability:
• Heritability can be estimated by three different methods.
a) From analysis of variance table of a trial consisting of a large
number of genotypes.
b) Estimation of VG and VE from the variance of P1, P2, P3, P4
generation of a cross.
c) Parent – offspring regression upon doubling provides estimates
of heritability.
• Thus, H = 2b, where b is the regression of progeny means on
parent value.
• When heritability is estimated from the above three methods
is known as broad sense heritability.

46. Uses of Heritability:
 It is useful in predicting the effectiveness of selection.
 It is also helpful for deciding breeding methods to be followed
for effective selection.
 It gives us an idea about the response of various characters to
selection pressure.
 It is useful in predicting the performance under different
degree of intensity of selection.
 It helps for construction of selection index.
 Estimates of heritability serve as a useful guide to the breeder,
to appreciate the proportion of variation that is due to
genotypic or additive effects.

47. THANK YOU