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🌸 Male Sterility in Plants

Types of male sterility and their practical use in hybrid seed production.

This lesson covers core principles and exam-focused points from this topic in plant breeding.

Self-Incompatibility

Self-incompatibility and sterility are the two mechanisms, which encourage cross

pollination. More than 300 species belonging to 20 families of angiosperms show self

incompatibility.

Definition

In self incompatibility plants, the flowers will produce functional or viable pollen grains

which fail to fertilize the same flower or any other flower of the same plant.

  • Self incompatible pollen grain may fail to germinate on the stigmatic surface.

  • Some may germinate but fails to penetrate the stigmatic surface.

  • Some pollen grains may produce pollen tube, which enters through stigmatic surface, but

its growth will be too slow. By the time the pollen tube enters the ovule the flower will

drop.

  • Some time fertilziation is effected but embryo degenerates early.

Reason

Self-incompatibility is appeared to be due to biochemical reaction, but precise nature of

these is not clearly understood.

Classification of self incompatibility

According to Lewis (1954) the self incompatibility is classified as follows:

Self incompatibility

Col1 Col2








Distyly Tristylty Gametophytic system Sporophytic system

Heteromorphic system

In this case there will be difference in the morphology of the flowers. For example in

Primula sp there are two types of flowers namely PIN and THRUM. PIN flowers have long style

and short stamens while THRUM flowers have short style and long stamens. This type of

difference is known as Distyly. In case of distyly the only compatible mating is between PIN and

THRUM. The relative position of anthers is determined by single gene S/s. The recessive as

produces PIN and heterozygotes Ss produces THRUM.

Homozygous dominant SS is lethal and do not exist. The incompatibility reaction of

pollen is determined by the genotype of the plant producing them. Allele S is dominant over s.

This system is also known as heteromorphic – sporophytic system. Pollen grains produced by

PIN flowers will be all s in genotype as well as in incompatibility reaction: Whereas THRUM

flowers will produce two types of gametes S and s but all of them would be S phenotypically.

The mating between PIN and THRUM would produce Ss and ss progeny in equal frequencies.

This system is of little importance in crop plants. It occurs in sweet potato and bulk wheat.

PIN FLOWER Col2 THRUM FLOWER Col4
Mating Mating Progeny Progeny
Phenotype Genotype Genotype Phenotype
Pin x Pin ss x ss Incom. Mating -
Pin x Thrum SS x Ss 1 ss : Ss 1Thrum

1 Pin
Thrum x Pin Ss x ss 1 Ss : ss 1 Thrum

1 Pin
Thrum x Thrum Ss x Ss Incom. Mating -

TRISTYLY is known in some plants like Lythrum salicaria . In this case the style of the

flower may be short, long or medium length

Homomorphic System

Here the incompatibility is not associated with morphological difference among flower.

The incompatibility reaction of pollen may be controlled by the genotype of the plant on which it

is produced – (Sporophytic control) or by its own genotype – (Gametophytic control).

Gametophytic system

First discovered by East and Mangelsdorf in1925 in Nicotiana sanderae . Here the

incompatible reaction of pollen is determined by its own genotype and not by the genotype of the

plant on which pollen is produced. Genetically the incompatibility reaction is determined by a

single gene having multiple allele. Eg. Trifolium Nicotiana, Lycopersicon, Solanum,& Petunia .

Here codominance is assumed.

Genotype of plant S1S2 S3S4

(Sporophyte)

Genotype of gametes S1 S2 S3 S4

Incompatible

reaction of pollen S1 S2 S3 S4

Incompatible reaction

of style S1 S2 S3 S4

Mating S1S2 x S1S2 - Fully incompatible

S1S2 x S1S3 - Partially compatible

S1S2 x S3S4 - Fully compatible

Sporophytic system

Here also the self incompatibility is governed by a single gene S with multiple alleles.

More than 30 alleles are known in Brassica oleracea . Here dominance is assumed.

The incompatibility reaction isdetermined by the genotype of the plant on which pollen

grain is produced and not by the genotype of the pollen. This system is more complicated. The

allele may exhibit dominance, co-dominance or competition. This system was first reported by

Hugues and Babcock in1950 in Crepis foetida and by Gerstal in Parthenium argentatum . The

sporophytic system is found in radish, brassicas and spinach.

Lewis has summarized the characteristics of sporophytic system as follows :

  1. There are frequent reciprocal differences

  2. Incompatibility can occur with female parent

  3. A family can consist of three incompatibility groups

  4. Homozygotes are a normal part of the system

  5. An incompatibility group may contain two genotypes.

Mechanism of Self Incompatibility

This is quite complex and is poorly understood. The various phenomena observed in Self

incompatibility is grouped into three categories.

  1. Pollen – Stigma interaction

  2. Pollen tube – Style interaction

  3. Pollen tube – Ovule interaction

Pollen – Stigma interaction

This occurs just after the pollen grains reach the stigma and generally prevents pollen

from germination. Previously it was thought that binucleate condition of pollen in gametophytic

system and trinucleate condition in sporophyutic system was the reason for self incompatibility.

But later on it was observed that they are not the reason for SI. Under homomorphic system of

incompatibility there are differences in the stigmatic surface which prevents pollen germination.

In gametophytic system the stigma surface is plumose having elongated receptive cells which is

commonly known as wet stigma. The pollen grain germinates on reaching the stigma and

incompatibility reaction occurs at a later stage.

In the sporophytic system the stigma is papillate and dry and covered with hydrated layer

of protein known as pellicle. This pellicle is involved in incompatibility reaction. With in few

minutes of reaching the stigmatic surface the pollen releases an exine exudates which is either

protein or glycerol protein. This reacts with pellicle and induces callose formation, which further

prevents the growth of pollen tube.

Gametophytic system Sporophytic system
Stigma surface Plumose Commonly known
as wet Stigma		 Stigma surface Papillate and dry. Covered
with hydrated layers of protein known as
pellicle which involves in incompatibility
reaction
Pollen
grain
germinates
and
incompatibility reaction occurs at a later
stage		 Pollen grain releases exine exudates which
is protein or Glyco-protein
This protein reaction with pellicle and
induces callose formation and arrests
growth of pollen type.

Pollen Tube – Style interaction

Pollen grains germinate and pollen tube penetrates the stigmatic surface. But in

incompatible combinations the growth of pollen tube is retarded with in the style as in Petunia,

Lycopersicon, & Lilium . The protein and poly saccharine synthesis in the pollen tube stops

resulting in bursting up of pollen tube and leading to death of nuclei.

Pollen tube – Ovule interaction

In Theobroma cacao pollen tube reaches the ovule and fertilization occurs but the

embryo degenerates later due to some biochemical reaction.

Male Sterility

Male sterility is characterized by nonfunctional pollen grains, while female gametes

function normally. It occurs in nature sporadically.

Morphological features of male sterility

The male sterility may be due to mutation, chromosomal aberrations, cytoplasmic factors or

interaction of cytoplasmic and genetic factors. Because of any of the above reasons the following

morphological changes may occur in male sterile plants.

  • Viable pollen grains are not formed. The sterile pollen grains will be transparent and

rarely takes up stain faintly.

  • Non-dehiscence of anthers, even though viable pollens are enclosed within. This may be

due to hard outer layer, which restrict the release of pollen grains.

  • Androecium may abort before the pollen grains are formed.

  • Androecium may be malformed, thus there is no possibility of pollen grain formation.

Kinds of male sterility, maintenance and uses

Male sterility may be conditioned due to cytoplasmic or genetic factors or due to

interaction of both. Environment also induces male sterility. Depending on these factors male

sterility can be classified in to

  • Cytoplasmic male sterility (CMS)

  • Cytoplasmic-genetic male sterility (CGMS)

  • Genetic male sterility (GMS)

In this there are two categories.

  • Environment insensitive genic male sterility- commonly referred as Genetic male

sterility.

  • Environment sensitive genic male sterility or Environmental induced sterility which is

again sub divided in to

  • TGMS (Thermosensitive)

  • PGMS (Photo sensitive)

  • Photo thermo sensitive

Cytoplasmic Male Sterility (CMS)

It occurs due to the mutation of mitochondria or some other cytoplasmic factors outside

the nucleus. Nuclear genes are not involved here. There is considerable evidence that gene or

genes conditioning cytoplasmic male sterility. Particularly in maize DNA reside in mitochondria

and may be located in a plasmid like element.

Genetic structure

Sterile

Maintenance

A line sterile B line fertile

Sterile

Since mother contributes the cytoplasm to the offspring, the sterility is transferred to the F1.

Uses

Since there are no R lines available, this type of sterility is useful only in crops where seed is

not the end product. For example in onion and many ornamental plants the hybrids developed

exhibit maximum hybrid vigour with respect to longer vegetative duration and larger flower size

and larger bulb size. Cytoplasmic male sterility has successfully been exploited in maize for

producing double cross hybrids.

A B C D

RR

Sterile (A x B) x (C x D)

Genetic Male Sterility (Gms)

Genetic male sterility is normally governed by nuclear recessive genes ms ms. Exception to

this is safflower where male sterility is governed by dominant gene Ms Ms. This type of male

sterility is used in Redgram and Castor for production of hybrids.

Genetic structure

Maintenance

In genetic male sterility, the sterile line will be maintained from heterozygous condition.

sterile.

The pollen from the Fertile line will pollinate the sterile line and as a result seed set will

be there in the sterile line. These seeds are to be harvested and used for hybrid seed production.

For hybrid seed production, the seeds collected from sterile plants will be grown using

double the seed rate since it will segregate in the ratio of 1 fertile : 1 sterile line. At the time of

flowering, the fertile line will be identified by yellow plumpy anthers and removed from the

field. Only the sterile line will remain in field. These will be pollinated by the R line and the R1

obtained will be hybrid redgram.

Utilisation: Hybrid development. Eg: Redgram

Ms T21 x ICPL 87109

A line R line

Difficulties in use of Gms

  1. Maintenance of GMS requires skilled labour to identify fertile and sterile line. Labelling

is time consuming and costly.

  1. In hybrid seed production plot identification of fertile line and removing them is costly.

  2. Use of double the seed rate of GMS line is costly

  3. In crops like castor high temperature leads to break down of male sterility.

Transgenic Genetic Male Sterility

A gene introduced into the genome of an organism by recombinant DNA technology or

genetic engineering is called transgene. Many transgenes have been shown to produce genetic

male sterility, which is dominant to fertility. Consequently, it is essential to develop effective

fertility restoration system if these are to be utilized for hybrid seed production. An effective

restoration system is available in at least one case called Barnase or Barstar system.

The Barnase gene of Bacillus amyloliquefaciens encodes an RNAs. When Barnase gene is

driven by TA 29 promoter, it is expressed only in tapetum cells causing their degeneration.

Transgenic tobacco and Brassica napus plants expressing Barnase were completely male sterile.

Another gene, Barstar, from the same bacterium encodes a protein, which is a highly specific

inhibitor of Barnase RNAse. Therefore, transgenic plants expressing both Barstar and Barnase

are fully male fertile.

The Barnase gene has been tagged with bar gene, which specifies resistance to the herbaicde

phosphinothricin. This male sterile line is maintained by crossing with a male fertile line. The

progeny so obtained contain 1 male sterile : 1 male fertile plants; the latter are easily eliminated

at seedling stage by a phosphinothricin spray. The male sterile plants are crossed with the Barstar

line to obtain male fertile hybrid progeny. This system of male sterility is yet to be commercially

used.

Use of TGMS or PGMS eliminates this problem. These male sterile lines are maintained by

growing them in a locality where the temperatures and photoperiods during the sensitive

developmental stages are such that they exhibit complete male fertility (phenotypically).

The selfed seeds from such lines are then grown for hybrid seed production in those locations

where their ms genes produce complete male sterility due to the prevailing temperature and

photoperiod regimes. Clearly, all the plants in the male sterile line will be sterile and no rouging

will be required. TGMS and PGMS are being used for hybrid rice development in China.

Cytoplasmic Geneic Male Sterility

This is a case of cytoplasmic male sterility where dominant nuclear gene restorers fertility.

This system is utilized for the production of hybrids in bajra, jowar, maize, rice, wheat and many

other crops.

Genetic structure

A line

Male sterile

A line or ms line : This tem represents a male sterile line belonging to anyone of the above

categories. The A line is always used as a female parent in hybrid seed production.

B line or maintainer line : This line is used to maintain the sterility of A line. The B line is

isogenic line which is identical for all traits except for fertility status.

R line and restoration of fertility : It is other wise known as Restorer line which restores

fertility in the A line. The crossing between A x R lines results in F 1 fertile hybrid seeds which

is of commercial value.

Maintenance

A line B line

Sterile Fertile

Male sterile line

The A line which is male sterile is maintained by crossing it with isogenic B line which is

also known as maintainer line. The B line is similar to that of A line in all characters

(isogenic) except fertile cytoplasm.

Utilsiation

The male sterile. A line is crossed with R line (Restorer) which restorers fertility in F1.

A line R line

Sterile Fertile

Hybrid Fertile

Limitations of CGMS lines

  1. Fertility restoration is a problem. E.g. Rice.

  2. Seed set will be low in crops like rice where special techniques are to be adopted to

increase seed set.

  1. Break down of male sterility at higher temperature

  2. In crops like wheat having a polyploidy series it is difficult to develop effective R line.

  3. Undesirable effect of cytoplasm. Eg. Texas cytoplasm in maize became susceptible to

Helminthosporium. In bajra Tift 23 A cytoplasm became susceptible to downy mildew.

  1. Modifier genes may reduce effectiveness of cytoplasmic male sterility.

Summary Cheat Sheet

Quick Recall Points

  • This lesson focuses on key plant breeding concepts, terminology, and exam-relevant applications.
  • Review major definitions, classifications, and method-wise distinctions from the sections above.
  • Revise tables and examples from this lesson for fast pre-exam recall.

Exam Traps

  • Do not confuse similarly named breeding methods without checking their core selection logic.
  • Pay attention to crop-specific examples because the same principle can behave differently by species.

References

1 source • [1]

[1]

Standard Plant Breeding Class Notes (GPBR211)

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