4.

Androgensis and gynogenesis; Breeding system; Pollination biology; structural and functional
aspects of pollen and pistil; Male sterility; Self and inter-specific incompatibility;
Fertilization; Embryo and seed development.

POLLINATION BIOLOGY

The mode of reproduction determines the genetic constitution of crop plants i.e whether the plants are
normally homozygous of heterozygous. This is turn determines the goal of the breeding programme.
If the crop plants were naturally homozygous e.g. as in self-pollinated crops like wheat, a
homozygous line would be desirable as a variety. But, if the plants are heterozygous naturally, as in
cross-pollinated crops like maize, a heterozygous population has to be developed as a variety.

Reproduction in crop plants maybe broadly grouped in to two categories.

1. Asexual - Vegetative reproduction and apomixis
2. Sexual

MODES OF POLLINATION

Pollination refers to the transfer of pollen grains from anthers to stigmas. Pollen from an anther may
fall on the same flowers leading to “Self pollination " or " Outo gamy".

When pollen from flowers of one plant are transmitted to the stigmas of flowers of another plant, it is
known as “Cross pollination" or allogamy.

A third situation "Geitonogamy" results when pollen from a flower of one plant galls on the stigmas
of other flower of the same plant e.g. maize. The genetic consequences of geitonogamy are the same
as those of quitogamy.

SELF POLLINATION

Many cultivated plant species reproduce by self-pollination. These species as a rule must have
hermaphrodite flowers. But in most of this sp., self-pollination is not complete and cross-pollination
may occur upto 5%. Several factors like variety, environmental conditions like temperature humidity
and location affect the degree of cross-pollination. There are various mechanisms that promote self-
pollination.

1. CLEISTOGAMY - In this case, flowers do not open at all. This ensures complete self-
pollination. Since, foreign pollen cannot reach the stigma of closed flowers. It occurs in some
varieties of wheat (Triticm sp. ) Oats (Avena sp.), Barley (Hordaum vulgare) and in a number of other
grasses.
2. CHASMOGAMY - In some sp. the flower open, only after pollination has taken place. This occur
in many cereals, such as wheat, barley, rice and oats.
3. In crops like tomato (Lycopersicum esculentum) and brinjal ( Solanum melongena), the stigma are
closely surrounded by anthers. Pollination generally occurs after the flowers open. But the position of
anthers in relation to stigma ensures self-pollination.
4. In some species, flowers open but the stamens and the stigma are hidden by other floral organs. In
several legumes e.g. pea, mung bean, urd bean, soybean, bengalgram. Two petals forming a keel
enclose the stamens and the stigma.
5. In a few sp. stigmas become receptive and elongate through stamina columns. This ensures pre-
dominant self-pollination.
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 in breeding
depression, but may exhibit considerable heterosis. Therefore the aim of breeding methods generally
is to develop homozygous varieties.

CROSS POLLINATION

In cross-pollination species, the transfer of pollen from a flower the stigmas of the other may be
brought about the wind (anemophilic) water (hydrophilic) or insects (entomophilic). Many of the crop
plants are naturally cross-pollinated. In many sp. a small amount (upto 5-10 per cent) of selfing may
occur. There are several mechanisms that facilitate cross-pollination.

1. DICLINY - or unisexuality is a condition in which flowers are either staminate (mala) or

pistillate (female)
a. Monecy - Staminate and pistillate flowers occur in the same plant, either in the same
inflorescence e.g. castor, mango, banana, coconut or in separate inflorescence e.g maize. Other
monoecious species cucurbits, walnut, chestnut, straw berry, rubber, grapes and cossava.
b. Dioecy- The male and female flowers are present on different plants. E.g. Papaya, date, hemp,
asparagus and spinach.

2. DICHOGAMY - Stamens and pistils of hermaphrodite flowers may mature at different times
facilitating cross pollination.
a. Protogzymy - In crop sp. like cumbu, gynoecium matures fruits.
b. Protandry - In maize and sugar beets androecium mature first.

3. In lucerene, stigma are covered with a waxy film. The stigma does not become receptive until the
waxy film is broken. The waxy membrane is broken by the visit of honey bees which also effect cross
pollination.
1. A combination of two or more of the above mechanisms may occur in some species. This
improves the efficiency of the system in promoting cross pollination. E.g. Maize exhibits
bothmonecy and protandry.
2. Self incompatibility - It refers to the failure of pollen from a flower to fertilize the some flowers
or other flowers on the same plant. It is highly effective in preventing self pollination. e.g. self
icompatibility in common in Brassica, Nicotiana, Radish, Rye and Many grasses.
3. Male sterility - It refers to the absence of functional pollen grains. But it is of great value for the
production of hybrid seeds.

Genetic consequences of cross-pollination

Cross-pollination preserves and promotes heterozygosity in a population. Cross-pollinated sp. are

highly heterozygous and show mild to severe inbreeding depression and a considerable amount of
heterosis. The breeding methods in such species aim at improving the crop species with out reducing
heterozygosity to an appreciable degree.

OFTEN CROSS-POLLINATED SPECIES

In many crop plants, cross pollination often exceeds 5 per cent and may reach 30 per cent. Such sp.
are generally known as "often cross pollinated crops" e.g. Sorghu, cotton, redgram and safflower etc.,

The genetic architecture of such crops is intermediate between self-pollinated and cross-pollinated sp.
Consequently in such species breeding methods suitable for both of them may be profitably applied.
But often hybrid varieties are superior to others.
INCOMPATIBILITY

It is the inability of a plant producing functional female and male gametes to self-seed when self-
pollinated. It is met with both in heteromorhphic species with differences in morphology of flowers of
different plants and in homomorphic species with no differences in floral morphology. The term " Self
incompatibility" was originally coined by Stout in 1917.

Incompatibility is due to some physiological hindrance to fertilization caused by the failure of pollen
to germinate on the stigma or slow growth of the pollen tube along the style sometimes fertilization is
effected, but the embryo degenerates at a very early stage.

The main features of self-incompatibility are;

i. It is an important out breeding mechanism, which prevents autogamy and promotes
allogamy.
ii. Self-incompatible species do not produce seed on self-pollination but lead to normal seed
set on cross-pollination.
iii. It maintains high degree of heterozygosity in a species due to out breeding and reduces
homozygosity due to elimination of in breeding of selfing.
iv. Self-incompatibility results due to morphological genetic, physiological and biochemical
causes. It is not under single genetic control.
v. Self-incompatibility reaction can operate at any stage between pollination and
fertilization.
vi. Self-incompatibility has been reported in about 70 families of angiosperms including
several crop species.

Self-incompatibility can be classified on the basis of

i. flower morphology
ii. genes involved
iii. site of expression of self-incompatibility reaction and
iv. pollen cytology

Basis of Types of Brief description

classification incompatibility
Flower a. Heteromorhphic Self incompatibility is associated with differences in
morphology flower morphology
i. Distyly Styles and stamens are of two types i.e. short and
long.
ii. Tristyly Styles and stamens have three positions i.e. short
medium and long.
b. Homomorhphic Flowers do not differ in morphology
i. Saprophytic Self-incompatibility is governed by genotype of
pollen producing plant.
ii. Gametophytic Self-incompatibility is governed by genetic
constitution of gametes.
Genes involved a. Monoalble Self-incompatibility is governed by single gene
b. Diallelic Self-incompatibility is governed by two genes
c. Polyallelic Self-incompatibility by several genes
Site of expression a. Stigmatic Self-incompatibility genes express on the stigma
b. Stylar Self-incompatibility express in the style
c. Ovarian Self-incompatibility express in the ovary
Pollen cytology a. Binucleate Pollen grains have two nuclei
b. Trinuleate Pollen grains have three nuclei.
SELF INCOMPATIBILITY
Based on flower morphology, self-incompatibility system is of two types.
i. Heteromorphic system.
ii. Homomorphic system a. Gametophytic control b. Sporophytic control

HETEROMORPHIC SYSTEM

When self-incompatibility is associated with differences in floral morphology. It is known

heteromorphic system. In this system, self-incompatibility results due to differences in the length of
style and stamen. This system is divided into two types viz., a. Distly and b. Tristly.
a. Distly: It refers to two types of styles (short and long) and stamens (low and high). This system
operates in the family Primulaceae. In Primula, there are two types of flowers. i.. Thrum type
which has short style and high anthers. ii. Pin type with long style and high anthers. The crosses
are compatible only between the style and stamen of matching length. In otherwords, crosses are
compatible between Pin x Thrum or Thrum x pin but not between Pin x Pin and Thrum x Thrum
flowers.
Lateron, it was discovered that, incompatibility barrier between Pin and Pin and Thrum x Thrum
are govered by a single gene S which behaves in a heterozygous (Ss) and Pin is homzygous
recessive (ss). Thus thrum is dominant over Pin. Cross between thrum and Pin produce thrum and
Pin in 1:1 ratio in F1.
Distyly
MATING PROGENY
Phenotype Genotype Genotype Phenotype
Pin x Pin Ss x Ss Incompatibility mating -
Pin x Thrum Ss x Ss 1 Ss x 1 Ss 1 Thrum : 1 Pin
Thrum x Pin Ss x ss 1Ss x 1 ss 1 Thrum x 1 Pin
Thrum x Thrum Ss x Ss Incompatible -

The incompatibility reaction of pollen is determined by the genotype of the plant producing them. The
incompatibility system therefore in heteromorphic saprophytic.
Several variations were observed in other plants. For example in Linum grandiflorum, flowers have
long and short styles, but pollengrains are of same size. The limonium vulgare stamens and styles are
monomorphic, but stigmatic surface and pollen size are dimorhphic.

b. Tristly
When style has three positions (short, medium and long) and filaments are of three lengths
corresponding to the length of style. Any one plant has one style length and four stamens with
two different lengths of filaments pollination are compatible only between stigmas and anthers at
the same level. Thus each type of plant can effectively fertilize the other two types.

HOMOMORPHIC SYSTEM
In this system, the plants do not have differences in the length of style stamens or other floral parts.
This system is very much important in crop plants. It can operate in various ways as given below:
i. The pollen grains do not germinate on the stigma of the same flower. If they germinate the
pollen tube fails to penetrate the stigma as in Rye, Cabbage and Radish.
ii. The pollen grains may germinate but there is retard action of pollen tube growth.
iii. In some cases, there is slow rate of pollen tube growth and it rarely reaches the ovary.
iv. In some cases, pollen tubes growth may be normal but it does not release the male gamete.

Homomorphic system is of two types viz.,

a. Gametophytic system and b. Sporophytic system.
MAIN FEATURES OF GAMETOPHYTIC SYSTEM
 Self-incompatibility in majority of species is governed by a single gene S, which has large
number of multiple alleles.[In Rye self-incompatibility reaction is governed by two loci.
 In this system alleles have individual action in the style without interaction.
 Pollen grains are unable to germinate or function on a pistil having similar allele as that of pollen.
The pollen tube growth is usually inhibited in the style or ovary.
 This system gives rise to three types of pollination's v.z, (1) Fully incompatible (S 1S2 x S1S2) in
which both alleles are common in the pollen and ovule (2) Half the pollen is compatible (S 1S2 x
S1S3), in which one allele is different, and 3) Fully fertile (S 1S2 x S3S4) when both alleles differ in
pollen and ovule.
 Gametophytic system permits recovery of male parent only in the partially fertile crosses which
are obtained when one allele differs in the cross, viz., S 1S2 x S1S3. This cross would give rise to
S1S3 and S2S3 progeny.
 Plant sps. belonging to gametophytic self-incompatibility system have binucleate pollen.

MAIN FEATURES OF SPOROPHYTIC SYSTEM

 Here the self-incompatibility is controlled by a single gene S that has multiple alleles.
 The alleles may show dominance, individual action or interaction in either pollen or style as per
allelic combinations involved.
 This system exhibits inhibition of pollen germination or pollen tube growth on the stigma of same
flower.
 The Sporophytic systems contains a form of dominance in which S 1 is dominant over all others, S2
is dominant over all except S1, and so on (S1 > S2 > S3 > S4 ). In this system, crosses between
different genotypes are either fully fertile or completely sterile.
 Pollen grains from both heterozygous or homozygous plants react in a similar fashion due to
dominance effect of male parent. For example, pollen grains from S 1S1 or S1S2 plants would have
S1 phenotype and from S2S2 or S2S4 as S2 phenotype.
 This system permits recovery of parental genotypes in some crosses. For example, a cross
between S1S3 female and S2S3 male will produce S1S2 , S1 S3 , S2S3 and S3S3 genotypes, which
represent parental genotypes also.
 Plant species belonging to this system of self-incompatibility generally have trinucleate pollens.
Comparison of gametophytic and Sporophytic systems of self incompatibility
S. N. Gametophytic system Sporophytic system
1 Self incompatibility is controlled by the Self incompatibility is controlled by the
genetic constitution of pollen genotype of pollen producing plant
(sporophyte)
2 Incompatibility is governed by a single S Self-incompatibility is also governed by a
gene with multiple alleles single S gene with in either alleles.
3 Alleles have individual action in the style Alleles may show dominance, individual
without interaction action or interaction in either pollen or style
4 The pollen tube growth is usually inhibited Pollen germination or pollen tube growth is
in the style or ovary inhibited on the stigma
5 Plant species belonging to this system have Plant species belonging to this system have
binucleate pollen grains trinucleate pollen grains
6 Recovery of only male parent is possible Recovery of both male and female parents is
from crosses possible from crosses
7 Does not permit production of Permits production of some homozygotes
homozygotes
8 Crosses may be sterile, partially fertile or Crosses would be either fully sterile or full
fully fertile fertile
9 Such incompatibility can be overcome by This cannot be overcome by polyploidy
polyploidy
10 Examples of this system include red clover, Examples of this system include radish,
white clover, rye, potato, tomato etc., cabbage, cauliflower etc.,
 HOMOMORPHIC SPROPHYTIC SYSTEM OF INCOMPATIBILITY

Genotype of plant (Sporophyte)

S1 S2 S1 S3 S1 S4 S2 S3 S2 S4 S3 S4

S S S S S S S S S S S S
1 2 1 3 1 4 2 3 2 4 3 4
1 1

Genotype of gametes

Incompatibility reaction
of pollen grains. All S1 All S1 All S1 All S2 All S2 All S3

Incompatibility
Reaction of style S1 S1 S1 S2 S2 S3

S1 S2 x S1 x S2 Complete incompatibility

S1 S2 x S1 x S3 Complete incompatibility

S1 S2 x S1 x S4 Complete incompatibility

S1 S2 x S2 x S3 Complete incompatibility

Significance of self incompatibility

Self-incompatibility is of great significance to plant breeders. It is an important pollination control

device, which prevents autogamy and promotes allogamy. In plant breeding it is useful in two main
ways.
i. Production of hybrids: Self-incompatibility provides a way for hybrid seed production without
emasculation and without resorting to genetic or cytoplasmic male sterility. IT has been utilized
for production of commercial hybrids in Brassica and sunflower. Two self incompatible lines are
planted in the alternate row for hybrid seed production. Harvest from both the lines as hybrid
seed. In Japan it used for cruciferous crops.

ii. Combining desirable genes: Self-incompatibility system permits from 2 or more different sources
through natural cross pollination that is not possible in self-compatible species. Moreover
knowledge of self-incompatibility specially in fruit crops, helps fruit growers to increase the yield
of fruits by providing suitable pollination.

Limitations
i. It is very difficult to produce homozygous inbred lines in a self-incompatible species. Bud
pollination has to be made to maintain the parental lines.
ii. Self-incompatibility is affected by environmental factors like temperature and humidity self-
incompatibility is reduced or broken down at high temperature humidity.
iii. Sometimes bees visit only one parental line in the seed production resulting in sibmating.
STERILITY
It is due to failure of any of the process concerned with normal alteration of generation viz.,
development of pollen, embryo sac, and embryo endosperm causing non- functional gametes. So
failure to set seeds, may be due to sterility, caused by non - functional gametes. Sterility may be
caused by chromosomal aberrations, gene action or cytoplasmic influences.

Chromosomal sterility: Sterility in auto polyplids, interspecific hybrids eneuploids and individuals
carrying chromosomal aberrations in very often due to chromosomes.

Sterility in autopolyploids.

Autopolyploids are usually highly sterile because the behaviour of chromosomes during meiosis in
autopolyploids in peculiar due to the fact that they posses more than two homologous chromosomes.

We shall consider meiosis in an auto triploid as an example of autopolyploids. During zygonema

stage, genetically homologous regions of the chromosomes pair in such a very that at any one place,
chromosome pairing in between two chromosomes only. Two of the three homologous chromosomes
may completely pair as bivalent, leaving one chromosome unpaired as an univalent. There are seven
possible configurations at diakinesis of an auto triploid as follows:
1. Three univalents
2. A ring bivalent and an univalent
3. A rod bivalent and an univalent
4. A trivalent having the form of ring of two, with third chromosome attached to the ring at the one
end.
5. A trivalent having the form of a ring of two, with the third chromosomes attached to the ring at
each end.
6. A trivalent having the form of a 'Y'
7. A trivalent having the form of chain

Behaviour of univalent in Anaphase-I.

An univalent that reaches the spindle either remains without division on the equatorial plate and so is
not included in the daughter nuclei or divides into two chromotids, one moving to each pole so
slowly that it may not be included in the daughter nuclei. An univalent that does not reach the spindle
in either lost in the cytoplasm or occasionally caught up by chance into one of the two daughter
nuclei.

Behaviour of Trivalents
Two chromosomes may move to opposite poles leaving one on the equatorial plate (false movement).
Triploids do not have a balanced complement of chromosome, only a few or viable.

Chromosome pairing in autotetraploid

One quadrivalent or two bivalents may be formed at the zygonema stage, if the pairing between the
four homologous chromosomes is complete. If the pairing is incomplete, they may form one bivalent
and two univalents or one trivalent and one univalent or very rarely four univalents.

In auto tetraploid, the sterility is not however usually as high as in triploids, because tetraploids have
even number of homolgous chromosomes and regular segregation is more likely than in triploids.

Sterility in interspecific hybrids

The most characteristic feature of interspecific hybrids in sterility in a greater or lesser degree. The
fact that amphidiploids derived from several hybrids become fertile once each chromosome has a
fully homologous partner with which to pair shows that the sterility of the F1 is sometimes due
sterility to chromosomes.
Sterility in Aneuploids
Aneuploids tend to be irregular at meiosis and as a result they are highly sterile. Monosomics and
trisomics posses an uneven numbers of some particular chromosome and therefore form a high
percentage of unbalanced gametes. In the meiosis of a monosomic, the old chromosome passes at
random to either pole resulting in formation of two kinds of gametes (n and n-1). Frequently, the
included in either daughter nucleus.

At meiosis, in a trisomic, all the three chromosomes may form a trivalent or bivalent and univalent. It
gives rise to gametes with n +1 or n chromosomes. Gametes with n+1 chromosome, especially on the
male side, do not offer function in fertilization.

Sterility due to chromosomal aberrations

Chromosomal aberrations very often cause sterility because they give rise to gametes that carry
deficiencies or duplications for some genes. In most plants, embryosac with deficiencies or
duplications may be functional but pollen grains with deficiencies or duplications are mostly inviable.
Consequently plants carrying chromosomal aberrations may produce fewer seeds than normal plants.

MALE STERILITY
It is characterized by non-functional pollen grains while female gametes function normally. It occurs
in nature sporadically perhaps due to mutation.
It is classified into three groups;
1. Genetic male sterility
2. Cytoplasmic male sterility
3. Cytoplasmic genetic.

Genetic male sterility

Genetic male sterility is governed by single recessive gene ms, but dominant genes governing male
sterility are also known. (e.g. safflower)

A male sterile line may be maintained by crossing it with heterozygous male fertile plants such a
mating produces 1:1 male sterile and male fertile plants.

Inheritance of genetic male sterility

ms ms x Ms Ms
(male stetile) (male fertile)

F1 Ms ms - Male fertile

F2 1 MsMs : 2 Ms ms : 1 ms ms
3:1
Male fertile Male sterile

Maintenance of male sterile lines

Male sterile " A" Male fertile " B"
Ms ms x Ms ms

1 Ms ms : 1 ms ms
( male sterile) (Male sterile)
(seed harvested)
Maintained by sib-mating seeds from male sterile plants one harvested
Genetic male sterility utilization in plant breeding: Genetic male sterility may be used in hybrid
seed production. The male sterile lines are inter planted with homozygous male fertile pollination.
The genotypes of the ms ms and Ms ms lines are identical except for the ms locus i.e. they are
isogenic and are known as male sterile (A0 and maintain (B) lines respectively. The female line would
therefore, contain both male and sterile and male fertile plants. The male fertile plants must be
identified and removed before pollen shedding. This is done by identifying the male fertile plants in
seedling stage, either due to the pleiotropic effect of the ms gene or due to the phenotypic effect of a
closely linked gene. Roguing of a male fertile plants from the female line is costly as a result of
which the cost of hybrid seed is higher. Due to these difficulties, genetic male sterility has been
exploited commercially only in a few countries. In USA it has been successfully used in castor. In
India, it has been used in redgram by some private seed companies.

CYTOPLASMIC MALE STERILITY

Plants carrying particular types of cytoplasm are male sterile but will produce seed if pollinated by
pollen from male fertile plants. These seeds produce only male sterile plants since their cytoplasm is
derived entirely from the female gametes. This type of sterility has been found in maize, onion etc.,

X rr
♀ rr
rr
♂
F S
S

Male sterile Male fertile Male sterile

The male sterile line is called " A" line and male fertile line is known as the maintainer line ♂ "B"
line, as it is used to maintain the male sterile line. The genes conditioning cytoplasmic male sterility,
particularly in maize residue in mitochondria, and may be located is a plasmid like element.

CMS may be transferred easily to a given strain by using that strain as a pollinator (Recurrent parent)
in the successive generation of a backcross programme. After 6 to 7 backcrosses, the nuclear genotype
of the male sterile line will be almost idendtical to that of the recurrent pollinator strain.

Utilization in plant Breeding

CMS may be utilized for producing hybrid send in certain ornamental species, or in species where, a
vegetative part is economic value. But in these crop plants where seed is the economic part, it is of no
use because the hybrid progeny would be male sterile.

CYTOPLASMIC GENETIC MALE STERILITY

In this system, a nuclear gene for restoring fertility in the male sterility line is known. The fertility
restorer gene 'R' is dominant and found in certain strains of the species or maybe transferred from a
related species. e.g. wheat. This gene restores male fertility in the male sterile line, and hence it is
known as restorer gene. This system is known in maize, cholam, pearl millet, sunflower, rice and
wheat.

The plants would be male sterile in the presence of male sterile cytoplasm, if the nuclear genotype
were rr, but would be male fertile, if the nucleus were Rr or RR. New male sterile lines can be
developed by repeated back crossings as in the case of cytoplasmic system. But the nuclear genotype
of the pollinator strain used in the transfer must be "rr" otherwise the fertility would be restored.
CYTOPLASMIC GENETIC MALE STERILITY - Various genotypes & phenotypes

rr rr

S F
Male sterile Male fertile
Cytoplasm sterile Cytoplasm fertile
Nuclear gene- non-restorer Nuclear genes non-restorer
Recessive allele

RR Rr
S
S
Male fertile
Cytoplasm sterile Effect of sterile cytoplasm
Nuclear gene- non-restorer engaged by restorer gene.
in homozygous (RR) or heterozygous (Rr)

Results of various matings:

X
rr
rr Rr rr
♂
♀
S F/s S S

Male sterile Male fertile Male fertile Male sterile

Utilization in plant breeding

The cytoplasmic genetic male sterility is used commercially to produce hybrid seeds in maize, pearl
millet, cholam, Rice and wheat.

Origin of male sterile cytoplasm

Male sterile cytoplasm arises spontaneously in nature or maybe produced by the breeder. The
various sources of the male sterile cytoplasm are as follows
i. Spontaneous mutation: arise in low frequencies isolated in maize, pearl millet and sunflower.
ii. Interspecific hybridization: Transfer of the full somatic chromosome complement of a crop
species, through repeated back crossing, into the cytoplasm of a related wild species often
leads to cytoplasmic male sterility. In cross-pollinated sp. the male sterile cytoplasms have
generally originated through mutation, while in self-pollinated crops, they have been
transferred from related species.
iii. Induction through Ethidium Bromoide: Male sterile cytoplasm may be induced by seed
treatment with Ethidium bromide. E.g. Petunia.
Chemically induced male sterility: The chemical at a particular concentration is sprayed on the
foliage prior to flowering and this inhibits production of viable pollen without injuring the
gynoecium. The flower set seed on cross-pollination. The chemicals used for inducing male sterility
are called male gametocides, pollen sterility are called male gametocides, pollen suppressants and
chemical hybridizing agents.

Environmental influence on male sterility: Environment influences the expression of male sterility
in certain crops. Temperature sensitive (TGMS) and photoperiod sensitive (PGMS) genic male
sterility has been identified in certain crops like rice, sorghum, tomato, maize castor and sugar beet.

Use of environmental genetic male sterility (EGMS), either TGMS or PGMS is involving hybrids is
known as "Two line breeding". In China two line hybrids based on TGMS system are popular in rice.

LIMITATIONS OF CYTOPLASMIC GENETIC MALE STERILITY FOR USE IN PLANT

BREEDING

i. Undesirable effects of cytoplasm: Male sterile cytoplasm generally has undesirable side
effects. For example - Texas cytoplasm in maize is the most successful cytoplasm
commercially, but slightly retards growth, reduces yield, plant height and leaf number. It also
makes the plants susceptible to helminthosproium leaf blight. The male sterile cytoplasm in
tobacco could not be used due to its severe undesirable side effects.
ii. Unsatisfactory fertility restoration: In many cases, restoration of fertility is not satisfactory.
As a result, these sources cannot be used in the production of hybrid seed.
iii. Unsatisfactory pollination: Natural pollination is often not satisfactory, except in wind
pollinated crops line maize. This reduces the production of hybrid seed and thereby increases
its cost. In some sp. the capsicum, this has prevented the use of male sterility in hybrid seed
production.
iv. Modifier genes: This may reduces the effectiveness of cytoplasmic male sterility and lead to
some pollen production by the male sterile lines.
v. Some times, cytoplasm may also be contributed by the sperm, which is the long run, may lead
to a breakdown of the male sterility mechanism.
vi. Male sterility Mechanism may breakdown partially under certain environmental condition,
resulting in some pollen production by the male sterile liens. This problem is encountered in
maize, barja and sorghum.
vii. In crops like wheat, polyploid nature of the crop and undesirable linkage with the restorer
gene make it very difficult to develop a suitable restorer (R) line.

APOMIXIS

Seeds are formed but the embryos develop without fertilization. The plants resulting from them are
identical in genotype to the parent plant. In apomixis, sexual reproduction in 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.

CLASSIFICATION

Adventive Embryony: Embryos develop directly from vegetative cells of the ovule, such as
nucellus, integument and chalaza. Adventive embryony occurs in Mango (Mangifera indica) citrus
etc.,

Apospory: Some vegetative cells of the ovule develop into unreduced embryosacs after meiosis. The
embryo may develop from egg cell or some other cell of this embryo sac. It occurs in some species of
Hieraceum, Malus, crepis, Ranum culus etc.,
Diplospory: Embryo sac is produced from the megaspore, which may be haploid or more generally
diploid. Generally the meiosis is no modified that the megaspore remains diploid. Diplospory leads to
parthenogenesis or apogamy.

Parthenogenesis: The embryo develops from egg cell. Depending upon whether the embryosac is
haploid or diploid, parthenogenesis is termed as haploid or diploid parthenogenesis. Haploid
parthenogenesis occurs accidentally and has been reported in Solanum nigera, Nicotiana, Crepis and
Maize. Diploid parthenogenesis occurs in many grasses e.g. Taraxacum.

Apogamy: Synnergids or antipodal cells develop into an embryo. Like parthenogenesis, apogamy
may be haploid or diploid depending upon the haploid or diploid state of the embryosac. Diploid
apogamy occurs in Anternna, Alchemilla, Allium and many other plant species.

Significance of Apomixis

Apomixis is a nuisance when the breeder desires to obtain sexual progeny. But it is of great help when
the breeder desires to maintain varieties. Thus in breeding of apomicitc species, 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. This
would keep the genotype of a variety intact.

Classification based on stability

Based on the stability of apomixis in subsequent generations, they are classified as,
1. Recurrent (stable ) apomixis
2. Non-recurrent (unstable) apomixis

Diploid apospory, diploid parthenogenesis and diploid apogamy are recurrent apomixis. Haploid
apospory, haploid parthenogenesis and haploid apogamy are non-recurrent apomixis.

Androgamy: Refers to the development of embryo from one of the male gametes, inside or outside
the embryo sac and it is haploid in nature.
Apomicts conserve the genetic constitution of the parent. Heliozygosity and consequent
hybrid vigour can be permanently fixed through apomixis. Genetically uniform individuals can be
rapidly multiplied, as it does not involve segregation. Breeding for apomicts is called "Single line
breeding".