This document discusses cytoplasmic male sterility (CMS), a maternally inherited trait in plants where the plant is unable to produce functional pollen. CMS is caused by mitochondrial mutations or rearrangements that interfere with pollen development. Nuclear restorer genes can suppress CMS by interacting with the mitochondrial genes. CMS is used in hybrid seed production systems in many crops.

1. DR. VIBHA KHANNA
ASSO. PROF. (BOTANY)
S.P.C. GOVERNMENT COLLEGE
AJMER (RAJASTHAN)

2. CYTOGENETICS
• BLOCK 1
• PRESENTATION 5:
CYTOPLASMIC MALE STERILITY
https://www.slideshare.net/vibhakhanna1/male-sterility

3. Cytoplasmic Male Sterility (CMS)
• Cytoplasmic male sterility (CMS) is a commonly observed
phenotype in higher plants.
• It is often cytoplasmically based and maternally inherited.
• Viable pollen is not produced by plants exhibiting the
cytoplasmic male sterility trait.
• Although all CMS plants are alike in that they are unable to
produce viable pollen, the specific mechanism differs
among plant species.
• The extensive variation among CMS indicates that distinct
and diverse factors are responsible for the male sterility
trait.
• Moreover, the effect is apparently limited to pollen
development because female fertility is generally
unaffected by CMS.

4. Cytoplasmic Male Sterility (CMS):
Mitochondria or Chloroplast
• Plant mitochondria and chloroplasts have their own
genome which is maternally inherited.
• Cytoplasmic male sterility or CMS phenotype has been
linked to morphological and functional damage of the
mitochondrial compartment.
• Protoplast fusion experiments showed that
chloroplasts were not linked to the CMS trait. On the
contrary, the comparison of mtDNA from fertile and
CMS plants of the same species indicated significant
differences. The mutation analysis of the traits involved
in the reversion of the CMS phenotype constituted
strong support for a link between CMS and the
mitochondrial genetic elements.

5. Plant mtDNA
• Plant mtDNA are a group of molecules having different
sizes and forms, composed of a large circular DNA
molecule, containing direct or inverted repeated
sequences. These sequences are involved in homologous
recombination events which generate either subgenomic
DNA ("loop-out" mechanism), or an inversion of the
sequence involved in the recombination process ("flip-flop“
mechanism).
• Plant mtDNA codes for some of the polypeptides of the
respiratory chain, the ATP synthase and ribosomes.
• Plant mitochondrial genomes also have some genes not
found in the mtDNA from other organisms including
a number of ‘orfs’ of unknown function.
• Some orfs, found in plant mtDNA introns, code for RNA
maturases. The latter enzymes play a crucial role in the
splicing and mobility of introns as shown in the yeast.

6. Origin Of CMS
• Cytoplasmic male sterility (CMS) is a maternally
inherited condition in which a plant is unable to
produce functional pollen.
• CMS is characterized by some deleterious effects, like
abnormal development of the male organs (anthers), a
diminished function of these organs, etc. Cytological
studies of the anther tissue showed a degeneration of
the microspore feeder layer, i.e., tapetum.
• It is often associated with chimeric mitochondrial open
reading frames (ORFs) found in mitochondrial genomes
• In a number of cases, transcripts originating from these
altered open reading frames are translated into unique
proteins that appear to interfere with mitochondrial
function and pollen development.

7. Origin Of CMS
• Thus, CMS is a phenotype produced by the
incompatibility between nuclear and mitochondrial
genomes of a given plant.
• In the plant mitochondrial genome, the rate of
gene recombination is high. Mitochondrial gene
recombination may lead to the formation of
chimeric genes.
• Chimeric proteins having an altered mitochondrial
function, may lead to the emergence of the CMS
phenotype.

8. Origin Of CMS
• CMS can arise spontaneously in breeding lines, as
a result of wide crosses or the interspecific
exchange of nuclear and cytoplasmic genomes, or
following mutagenesis
• {CMS Vs C-GMS: In Cytoplasmic male
sterility, male sterility is maternally transmitted
and all progeny will be male sterile, however,
in cytoplasmic–genetic male sterility restoration
of fertility is done using restorer lines carrying
nuclear genes.}

9. Origin Of CGMS
• In many cases, it has been found that male fertility can
be restored by dominant restorer allele (located in
nuclear genome) Rf gene(s).
• Nuclear restorer (Rf or Fr) genes function to suppress
the deleterious effects of CMS-associated
mitochondrial abnormalities by diverse mechanisms.
• CMS/Rf systems are examples of interactions between
nuclear and mitochondrial genomes.
• On the one hand, sterility results from mitochondrial
genes causing cytoplasmic dysfunction, and on the
other, fertility restoration relies on nuclear genes that
suppress cytoplasmic dysfunction.

10. Cytoplasmic- Genic Male Sterility
(CGMS)
• Hence male sterility in CMS expressed under the
presence of sterile mt-genome located in
cytoplasm (S-cytoplasm) and recessive allele of
restorer (maintainer allele; r) located in the
nuclear genome.
• This male sterility is conditioned by the
interaction of nuclear gene and sterile cytoplasm
but neither the genetic factor nor the cytoplasmic
factor alone can regulate sterility.
• Fertility restorer gene should be present in
homozygous Rf/Rf state i.e. either S Rf/Rf or N
Rf/Rf where seed production is important.

11. Inheritance & Transfer of CMS
• Cytoplasmic male sterility is a maternally inherited trait, because
mt-genome is responsible for the expression of male sterility and
the mitochondria are usually excluded from the pollen during
fertilization. Since the cytoplasm of a zygote comes primarily from
egg cell, the progeny of such male sterile plants would always be
male sterile.
• CMS can be transferred easily to a given strain by using that strain
as a pollinator (recurrent parent) in the successive generations of
backcross programme. After 6-7 backcrosses, the nuclear genotype
of the male sterile line would be almost identical to the recurrent
pollinator strain.
• Cytoplasmic male sterility can be maintained by crossing a male
sterile line (A line) with the pollinator strain (maintainer line) used
as a recurrent parent in the back cross programme since the
nuclear genotype of the pollinator is identical with that of the new
male sterile line.

12. Significance of CMS
• CMS systems represent a valuable tool in the
production of hybrid seed in self-pollinating
crop species, including maize, rice, cotton, and
a number of vegetable crops. Hybrids often
exhibit heterosis, more commonly known as
hybrid vigor, whereby hybrid progeny exhibit
superior growth characteristics relative to
either of the parental lines. CMS systems
facilitates efficient hybrid seed production.