Mode of Reproduction

1. Asexual Reproduction
This type of reproduction doesn't involve the fusion of male and female gametes.
a. Vegetative Propagation: New plants grow from vegetative parts.
Underground stems: Tubers (potato), bulbs (onion), and rhizomes (ginger).
Sub-aerial stems: Runners, stolons, and suckers, like those found in mint and date
palm.
 Bulbils: Found in garlic.
 Artificial Vegetative Propagation: Techniques like stem cuttings (sugarcane),
layering, budding, and grafting are used.
b. Apomixis: Seeds are formed from the embryo without fertilization. Orchids are an example.

2. Sexual Reproduction
This involves the fusion of male and female gametes to form a zygote, which then develops into an
embryo. Anthesis: In the process of flowering, the first opening of a flower.

Modes of Pollination
Pollination is the transfer of pollen grains from anthers to stigmas. The document outlines three
modes:
 Self-pollination (Autogamy): Pollen is transferred from an anther to a stigma within the
same flower. This is common in bisexual crops like rice and wheat.
 Cross-pollination (Allogamy): Pollen is transferred from the flowers of one plant to the
stigmas of the flowers of a different plant. Examples include maize, sunflower, and
sugarcane.
 Geitonogamy: Pollen is transferred from the flowers of one plant to the stigmas of another
flower on the same plant. Examples: maize.

Mechanisms Promoting Self-Pollination

The document lists mechanisms that ensure self-pollination occurs:
1. Cleistogamy: Flowers don't open at all, ensuring complete self-pollination since foreign
pollen cannot reach the stigma of a closed flower. Wheat, barley, and oats are examples.
2. Chasmogamy: The flowers open, but only after pollination has taken place. This is seen in
wheat, barley, and rice.
3. Position of Stamens and Stigmas: In some crops, the stigmas are closely surrounded by
the anthers, allowing for easy self-pollination (e.g., tomato, brinjal).
 4. Enclosed Stamens and Stigmas: In crops like pea and soybean, the stamens and stigma are
hidden by other floral organs, ensuring self-pollination.
5. Receptive Stigmas: In some plants, the stigma becomes receptive and elongates through
the staminal columns, promoting self-pollination.

Mechanisms Promoting Cross-Pollination

Cross-pollination (allogamy) is the transfer of pollen from the anther of one plant to the stigma of
another plant. This process is crucial for promoting genetic diversity and avoiding inbreeding
depression.
1. Unisexuality (Diclini): This is a condition where the flowers are either male or female,
preventing self-pollination.
 Monoecy: Male and female flowers are found on the same plant, either in same
inflorescences or or in different inflorescences (e.g., maize, cucurbits, mango,
coconut).
 Dioecy: Male and female flowers are found on completely different plants (e.g.,
papaya, palm, spinach).
2. Dichogamy: In a bisexual flower (containing both male and female reproductive parts), the
male and female parts mature at different times, which prevents self-pollination.
 Protogyny: The pistil matures before the stamens.
 Protandry: The stamen matures before the pistil.
3. Male Sterility: This is the absence of functional pollen grains in an otherwise
hermaphrodite flower. This makes self-pollination impossible and forces the plant to rely on
pollen from other plants.
4. Self-Incompatibility: The plant produces pollen and has functional male and female
organs, but the pollen is not compatible with the stigma of the same plant. This means the
pollen cannot fertilize the ovules of the same flower, effectively preventing self-
fertilization. This mechanism is found in around 300 species.

Genetic Consequences
1. Inbreeding Depression: When closely related individuals (same ancestors) repeatedly
self-pollinate or mate, their offspring can start lacking important traits like Vigour,
fertility and this is called inbreeding depression and it reduces the overall quality and
productivity of the plants.
2. Heterosis: The F1 generation can be better or worse than the parent generation, indicating
that heterosis can be either "positive" (better performance) or "negative" (worse
performance).
3. Heterozygosity: It preserves and promotes heterozygosity in a population.
Self-Incompatibility (SI)
The plant produces pollen and has functional male and female organs, but the pollen is not
compatible with the stigma of the same plant. This means the pollen cannot fertilize the ovules of
the same flower, effectively preventing self-fertilization. This mechanism is found in around 300
species. This ensures that cross-pollination occurs, which increases genetic diversity. Self-
incompatibility is further categorized into two main types based on the morphology and genetic
control.
1. Heteromorphic Self-Incompatibility
This type of SI is controlled by differences in the physical structure of the flowers, such as the
length of the style and stamens. The two types of flowers mentioned are:
 Pin Flower: Has a long style and short stamens.
 Thrum Flower: Has a short style and long stamens. In heteromorphic SI, both dominant
and recessive alleles are involved in the S-locus (the gene controlling SI), but the phenotype
is determined by the dominant allele.
2. Homomorphic Self-Incompatibility
Unlike heteromorphic SI, homomorphic SI is not based on flower morphology but is solely
controlled by a genetic mechanism. This is further divided into two types:
a. Gametophytic Self-Incompatibility (GSI): This type of SI is determined by the genotype
of the pollen grain itself, specifically by one or two genes.
 Monofactorial GSI: Controlled by a single gene, often designated as the S-gene, which
can have multiple alleles (e.g., S1, S2, S3, etc.). An example of a genus where this
occurs is Nicotiana and Solanum.
 Difactorial GSI: Controlled by two genes.
b. Sporophytic Self-Incompatibility (SSI): This type of SI is determined by the genotype of
the plant that produces the pollen, specifically the diploid sporophyte tissue.
 It's controlled by a single gene with more than 50 multiple alleles.
 Example: Reddish (radish) and Brassica (cabbage, broccoli, etc.).

Relevance of Self-Incompatibility in Plant Breeding.

1. Ensuring Fruitfulness in Fruit Trees: Self-incompatibility necessitates planting at least
two cross-compatible varieties to ensure successful pollination and fruit set. To overcome
poor cross-pollination in adverse weather, developing self-fertile forms is a desirable
breeding objective.
2. In case of pineapple, commercial clones are self-incompatible. As a result, their fruits
develop parthenocarpically and are seedless. Self-compatible clones would produce fruits
containing hundreds of very hard credible seeds; such fruits would not be acceptable to the
consumer. Therefore, at least in pineapple, commercial clones must be self-incompatible.
3. Some breeding schemes, e.g., development of hybrid varieties, etc., initially require some
degree of inbreeding. Although sib- mating leads to inbreeding, but for the same degree of
inbreeding it takes twice as much time as selfing. Further, for the maintenance of inbred
lines selfing would be necessary.
4. Hybrid Seed Production: Self-incompatibility can be a valuable tool for creating hybrid
seeds.
Method 1: By interplanting two self-incompatible but cross-compatible lines, all seeds
produced from both lines will be hybrids.
Method 2: A self-incompatible line can be interplanted with a self-compatible line. This
ensures that only the seed harvested from the self-incompatible line is a hybrid.
Advanced Schemes: The principles of self-incompatibility have been used to propose and
successfully demonstrate the production of double and triple cross hybrids, particularly in
the case of brassicas.
Male Sterility: This is the absence of functional pollen grains in an otherwise hermaphrodite
flower. This makes self-pollination impossible and forces the plant to rely on pollen from other
plants. Male sterility into four types:
1. Genetic Male Sterility (GMS)
2. Cytoplasmic Male Sterility (CMS)
3. Cytoplasmic Genetic Male Sterility (CGMS)
4. Chemical Induced Male Sterility

Genetic Male Sterility (GMS)

Mechanism: It is typically governed by a single recessive gene, often denoted as ms. A plant with
the genotype ms ms is male sterile. A plant with the genotype Ms Ms or Ms ms is male fertile. To
produce a male-sterile line for hybrid seed production, a male-sterile plant (ms ms) is crossed with a
heterozygous male-fertile plant (Ms ms). The progeny of this cross will segregate into 50% male-
sterile (ms ms) and 50% male-fertile (Ms ms) plants. The male-fertile plants can then be used to
maintain the male-sterile line.
Examples: GMS is found in many important crops, including maize, tomato, barley, pea, and rice.
Genetic Diversity: Different plants have a varying number of genes responsible for male sterility.
For instance, maize has over 70 different male sterility genes, tomato has 64, barley has 57, rice has
25, and pea has 54.

Q. How to maintain male sterile line?

Types of Genetic Male Sterility (GMS)

1. Environment Sensitive GMS: This is further divided into two types:

 TGMS (Temperature Sensitive GMS): In this system, the male sterility gene is activated
above a specific temperature, making the plant male sterile. Below that temperature, the
plant becomes male fertile again. For example, in rice, a temperature above 23.3°C can
induce male sterility, while a temperature below this threshold results in fertility.
 PGMS (Photoperiod Sensitive GMS): This is related to the duration of daylight. In rice,
for example, a temperature of 23-29°C combined with a day length of 13 hours and 45
minutes can cause male sterility. Outside of this specific temperature and photoperiod
range, the plant will become male fertile.

2. Environment Non-Sensitive GMS: In this type, factors like temperature and photoperiod do
not affect the expression of the male sterility gene.

Transgenic Genetic Male Sterility (TrGMS)

A gene introduced into the genome of an organism by recombinant DNA technology or genetic
engineering is called a transgene. Many transgenes have been shown to produce genetic male
sterility, which is dominant to fertility. In such cases, it would be essential to develop effective
fertility restoration systems for their use in hybrid seed production.
The Barnase/Barstar system is a good example of transgenic male sterility. The Barnase gene of
Bacillus amyloliquefaciens encodes an RNase. When the Barnase gene is driven by the TA29
promoter, it is expressed only in tapetum cells, causing their degeneration. Transgenic tobacco and
Brassica napus plants expressing Barnase in their tapetum cells were completely male sterile.
Another gene, Barstar, from the same bacterium encodes a protein that is a highly specific inhibitor
of Barnase RNase. Therefore, transgenic plants expressing both Barstar and Barnase are fully male
fertile.

Cytoplasmic Male Sterility

This type of male sterility is determined
by the cytoplasm. CMS is the result of
mutation in the mitochondrial genome
(mtDNA), which leads to an
unfavourable nuclear-mitochondrial
interaction or incompatibility, the
ultimate consequence of which
is male sterility.
Importance of CMS:
Hybrid Seed Production: CMS can be used to produce hybrid seeds for certain ornamental
species or crops where the vegetative part (like a leaf or root) is the economically valuable
product.
Limitation: CMS is not useful for breeding crop plants where the seed is the economic product.
This is because the hybrid offspring produced would be male sterile, and therefore, they wouldn't
be able to produce seeds themselves.

Q. How to transfer the Restorer (R) gene from a Restorer line to a new strain?
Q. Why male sterile line develops?
To Facilitate Hybrid Seed Production: Developing hybrid seeds requires crossing two specific
parent lines. Male sterility in one parent (the female line) eliminates the need for manual
emasculation (removing anthers), which is a labor-intensive and costly process. This makes large-
scale hybrid seed production more efficient and economical.