this presentation intends to familiarize students with the basic concept of male sterility. this is deemed essential to proceed with the cytoplasmic male sterility.
3. Origin of Male Sterility
• Male sterility may have two origins:
– (1) Nuclear, when the genetic locus is in the nuclear
genome. In this case this trait is inherited as a
classical Mendelian trait. The allele involved in this
phenotype is usually recessive. Nuclear sterile
mutants, spontaneous or induced, have been
described in nearly 200 plants.
– (2) Cytoplasmic, when the genetic locus is in the
genome present in the cytoplasm. It has maternal
inheritance . This type mutation is called cytoplasmic
male sterility (CMS). This type of mutation is linked to
mitochondrial genome. In CMS plants all molecules of
mtDNA are identical (homoplasmy)
4. Classification of male sterility in plants
(Kaul, 1988)
Male
Sterility
Inherited
(genetic)
Phenotypic
basis
Sporogenous
Structural or
positional
Functional
Genetic
basis
Genetic male sterility
(GMS):
Cytoplasmic male
sterility (CMS).
Cytoplasmic-genic male
sterility (CGMS).
Non-inherited (non-
genetic)
Chemical
Physiological
Ecological
5. Classification of male sterility in plants
(Kaul, 1988)
• (i) Inherited (genetic) male sterility
– Phenotypic basis
• (i) Sporogenous male sterility. Pollen formation is completely
disrupted. Stamens are formed but there is absence of pollen due
to abortion of microsporogenous cell which occurs before, during
or after meiosis.
• (ii) Structural sterility or positional sterility. Floral organs are
modified in such a way that selfing does not occur. Stamens are
either absent, malformed or modified into other floral parts which
leads to absence of microsporogenesis.
• (iii) Functional sterility. Viable pollens are produced but they are
unable to self fertilize due to some barriers (non-dehisent nature
of anthers). Seed setting occurs when artificially selfed by its own
pollen or crossed with other pollen.
6. Classification of male sterility in plants
(Kaul, 1988)
– Genetic basis
• (i) Genetic male sterility (GMS):
– Wide occurrence in plants.
– Caused by the gene(s) from nuclear compartment (nuclear gene).
– Mostly governed by a single recessive gene, ms.
– ms alleles arise spontaneously or may be artificially induced.
• (ii) Cytoplasmic male sterility (CMS).
– Determined by cytoplasm
– Result of mutation in the mitochondrial genome (mtDNA).
– Can be easily transferred to a given strain
• (iii) Cytoplasmic-genic male sterility (CGMS).
– Male sterility is caused by mitochonridal genes and restored by the nuclear
genes.
– Also known as nucleoplasmic male sterility.
– Fertility restorer gene R is required.
• Non-inherited (nongenetic) male sterility
– This kind of male sterility is temporarily induced by certain environmental
stresses e.g., temperature, etc. (i)Chemical, (ii) Physiological and (iii) Ecological
7. GMS/ Nuclear Male Sterility
• The male sterility, which is caused by nuclear genes, is
called GMS.
• As this type of male sterility is controlled by the gene(s)
from the nuclear compartment, it is also referred as
nuclear male sterility.
• GMS is the result of mutation in any genes involved in
stamen development process, male gamete
development process or in pollen development
process.
• Most of the transgenic male sterile plants are created
by transformation of male sterility related genes into
the nuclear genome.
8. CMS/ Mitochondrial Male Sterility
• The male sterility which is caused by cytoplasmic genes is
known as CMS.
• CMS is a maternally inherited trait, because mitochondrial
genome is responsible for the development of such type of
male sterility. It is also known as mitochondrial male
sterility.
• CMS may result from inter-specific or inter-generic crosses,
and also may be artificially induced through mutagenesis or
antibiotic effects on cytoplasmic genes (Kaul 1988).
• Protoplast fusion is another way to develop cytoplasmic
male sterile plants which has been successfully applied for
several vegetables crops (Pelletier et al. 1995).
• In near future, it will be more easy to develop cytoplasmic
male sterile plants through optimized transformation
protocol into mitochondrial genome.
9. Reason for Male Sterility
• Dysfunction of any genes which are involved in the
development of the male reproductive system are
associated with male sterility, such as genes involved in
sporogenous cells production pathway, tapetum and
MMCs development pathway, meiosis, haploid
microspores formation, degeneration of tapetum and
release of mature pollen grains.
• Any mutations in such genes negatively affect the
synthesis of amino acid and proteins. Consequently,
these are translated into failed production of functional
pollen and eventually causes male sterility.
10. Reason for Male Sterility: GMS
• GMS is an effect of mutation in the genes
associated with pollen development pathway.
• GMS is associated with abnormal tapetum
development as well as with other cytological,
biochemical and molecular changes.
• In many cases, the growing anthers of the
male sterile plants are involved with
quantitative and qualitative alterations in
different amino acids and enzymes.
11. Reason for Male Sterility: CMS
• Incompatibility between nuclear and mitochondrial
genome causes CMS, because many mitochondrial
enzymes are encoded jointly by the mitochondrial and
nuclear genes.
• This incompatibility leads to mitochondrial dysfunction,
particularly in the pollen producing organs like stamens,
anther wall or tapetum.
• And resulting in lower efficiency of mitochondria to give
energy to the tapetum cells which leads to the failure of
tapetum and pollen development and consequent result is
MS.
• Dysfunction of mitochondria is responsible for CMS
because pollen producing tissues require very high energy
compared to the other tissues, and dysfunctional
mitochondria is unable to supply this high energy .
12.
13. Restoration of fertility / Genetics of
restoration
• Restoration systems are classified as being either sporophytic or
gametophytic:
– Sporophytic restorers act prior to meiosis or in sporophytic tissues;
– Gametophytic restorers act after meiosis in microspores or pollen
grains.
• These differences lead to very different transmission patterns.
• A diploid plant that carries a male-sterile cytoplasm and is
heterozygous for a restorer will produce two classes of pollen
grains: those that carry the restorer and those that do not.
• In the case of a sporophytic restorer, both genotypic classes of
gametes will be functional.
• By contrast, in the case of a plant heterozygous for a gametophytic
restorer, only those gametes that carry the restorer will be
functional.
14. Mechanisms of restoration
• The mechanisms by which restoration occurs are probably
as diverse as the mechanisms by which mitochondrial
mutations cause CMS. The physical loss of a CMS-
associated gene from the mitochondrial genome is one
means by which this can occur.
• Nuclear restorers often alter the expression of CMS-
associated genes, and thereby presumably lessen or
eliminate the deleterious effects associated with their gene
products.
– For example, restoration of fertility is associated with processing
of CMS-associated transcripts in a number of systems, i.e., it
may be by transcriptional or post-transcriptional mechanisms.
• It is also possible that tissue-specific editing might allow a
CMS-associated sequence to become deleterious only at
microsporogenesis or microgametogenesis.
15. Need of Nuclear Restoration
• Nuclear restoration allows the commercial exploitation
of CMS systems in the production of hybrid seed. This
is because, in combination with CMS, it eliminates the
need for hand emasculation and yet ensures the
production of male-fertile, first-generation (F1)
progeny
• Breeders produce hybrid seed using a CMS system by
developing female lines that carry CMS cytoplasm but
lack restorer genes and by developing male lines that
carry the appropriate restorer genes. F1 hybrid seed
produced by the female lines carry the CMS cytoplasm
but yield fertile plants because of the action of the
paternally contributed nuclear restorers.
16. Strategies Used for the Production of
the Male-Sterile Phenotype
• The strategies used to obtain male-sterile plants, are
based either on molecule degradation or the
inactivation of cellular functions in plant organs
involved in pollen production.
• Genetic engineering of male sterility can selectively
and specifically destroy or interfere with the normal
development of anthers or pollen.
• All the transgenic male sterile lines developed till date
are GMS, since they have been developed through
transformation of male sterility causing gene
construct(s) inside the nuclear genome.
17. Strategies Used for the Production of the
Male-Sterile Phenotype: I
Barnase-Barstar system (Abolition restoration system)
• A system based on the specific expression of the
ribonuclease gene from Bacillus amyloliquefaciens
(barnase) in the anther tissues has been described
(Mariani et al. 1990).
• Barnase expression in tapetum cells leads to the
hydrolysis of cellular RNAs, thus avoiding pollen
production.
• Restoration to fertility was obtained by using a specific
inhibitor of RNase called "barstar".
• The co-expression of genes coding for barnase and
barstar proteins in tapetal cells results in the formation
of an inactive enzyme/inhibitor complex, thus restoring
male fertility of the offspring
18. Strategies Used for the Production of
the Male-Sterile Phenotype: II
• Callase (β-1,3-glucanase) is an important enzyme essential for
breakdown of the callose that encloses the pollen mother cells
(PMCs) and helps to release the pollen from tetrad after meiosis.
• Callase gene is naturally expressed in the tapetum at the end of
meiosis, and the enzyme is excreted in the anther loculi allowing
the release of microspores from their callose envelope.
• Another strategy to create CMS plants is based on the anther-
specific expression of the enzyme at an early stage of
microsporogenesis (Worral et al. 1992). Pre-mature pollen is
released if callase activity showed earlier.
• On the other hand, delayed callase activity led to delayed release of
pollens, both resulting in MS
• No fertility restoration approach is available for this strategy.
19. Strategies Used for the Production of
the Male-Sterile Phenotype: III
• Another approach to producing male-sterile
plants is based on the inhibition of the
flavonoid pigment synthesis at the step
catalyzed by chalcone synthase (CHS; Van
derMeer et al.1992).
• Chalcone biosynthesis was blocked by using
an antisense RNA complementary to CHS
mRNA.
20. Strategies Used for the Production of
the Male-Sterile Phenotype: IV
• Sporopollenin is a polymer which is essential
for pollen formation and it is believed that
esterase isozymes play important role in the
hydrolysis of sporopollenin.
• It is observed that male sterile plants showed
reduced activity of esterase.
