The following points highlight the top two ways to promote cross pollination. The ways are: 1. Incompatibility 2. Male Sterility.
Way # 1. Incompatibility:
Self-incompatibility refers to the situation when in a bisexual flower fertile pollen and ovule are produced but the pollen is unable to fertilize the ovule due to some reasons which may be classified as follows:
(a) Heteromorphic System:
When the species has two or three-different kinds of arrangement of floral part each type is self-incompatible but compatible with others. Here the genes or alleles associated with incompatibility are also linked with length of style and filament.
(i) Distyly: In Primula, there are two types of flowers:
‘Pin type’ – long style, short filament, large stigmatic cell, small pollen;
‘Thrum type’ – short style, long filament, small stigmatic cell, large pollen;
Both are self-incompatible but cross compatible. The ‘thrum’ is governed by Ss and ‘pin’ by ss. As the reaction of pollen is controlled by the gene of the sporophyte, the pollen of the ‘thrum’ type will behave like S (Fig. 2.4). The s gametes are incompatible with s type but compatible with S type and the vice-versa.
In Lythrum, three types of flowers with different stylar length exist. Here the stylar length is governed by two independent loci M and S. Plants with S have short style irrespective of the nature of other allele. The three different morphological types are self-incompatible but cross compatible.
(b) Homomorphic System:
In this case there is no morphological distinction between the self-incompatible flowers and the incompatibility is governed by multiple alleles.
This system can be of two types:
1. Pollen tube growth is controlled by genotype of sporophyte; and
2. Pollen tube growth is governed by genotype of the pollen.
(i) Sporophytic Incompatibility:
Here the incompatibility is governed by multiple alleles but has the dominant-recessive reaction. Here the pollen tube growth is not controlled by the genotype of pollen but by the genotype of the plant on which it is being produced. Actually the behaviour of gametophyte is controlled by dominant allele of the genotype of sporophyte.
For example, multiple alleles for incompatibility are S1, S2, S3, S4; S1 is dominant over others, S2 is dominant over S3 and S4, S3 is dominant over S4, etc. Irrespective of the gametophytic genotype, the reaction will be of dominant type of the sporophyte.
S1S2 will produce all S, type gametophyte;
S3S4 will produce all S3 type gametophyte;
S2S3 will produce all S2 type gametophyte;
and all these are incompatible with the same dominant type (Fig. 2.5).
(ii) Gametophytic Incompatibility:
In this system the incompatibility criteria is determined by multiple alleles of S gene and the reaction of the pollen is determined by its own genotype, not by the dominant allele of genotype of the plant on which it is produced. Therefore the pollen tube growth depends on the difference in the genotype of pollen and female parent; here all the plants are heterozygous in relation to this particular ‘S’ locus.
Way # 2. Male Sterility:
Male sterility is characterised by the deficiency in pollen formation or the nonfunctional pollen production, i.e., there may be some deficiency in the process of micros-poro-genesis. But in some cases the viable pollen are formed but anthers fail to dehisce. The process of male sterility is totally under genie control which may be nuclear gene or cytoplasmic-gene.
Depending on this factor male sterility can be grouped as follows:
(a) Genetic male sterility;
(b) Cytoplasmic male sterility;
(c) Cytoplasmic-genetic male sterility.
(a) Genetic Male Sterility:
When the male sterility is governed by a single recessive nuclear gene, ms and there is no influence of cytoplasm then it is called as genetic male sterility.
In this case the genotypes will be:
Ms Ms — Fertile (R line)
Ms ms — Fertile (B line)
ms ms — Sterile (A line)
A-Line or Male Sterile Line:
This term is used to represent a male sterile line which can be used as a female parent.
B-Line or Maintainer Line:
This line is crossed with the male sterile line to produce normal hybrid seeds, F1 plants fail to produce normal pollen, i.e., remain male sterile. This line should be isogenic line with A line so that by crossing A x B always the A-line can be maintained.
R-Line or Restorer Line:
Restoration means when this line is crossed with the male sterile line then F1 hybrid seeds will give the plants which will produce the normal viable pollen. But in the F1 progenies the male sterile plants will reappear due to segregation.
Maintenance of Genetic Male Sterility:
Restoration of Fertility in Genetic Male Sterility:
Restoration of fertility can be done by crossing this A line with restorer line or R line.
Uses/Importance in Plant Breeding:
Genetic male sterility though can be used in plant breeding but the seed from 50% of the plants (heterozygous) cannot be harvested, so this type of behaviour has less value in hybrid seed production. This criteria is utilised for genetical studies and preservation of variability. Genetic male sterility is found in nature in crops like tomato, bean, barley, sorghum, corn, etc.
(b) Cytoplasmic Male Sterility (CMS):
This type of male sterility occurs due to the mutation of mitochondrial gene or some other cytoplasmic factors outside the nuclear genome which make the plant male sterile.
Here the genotypes can be designated as:
Maintenance of Cytoplasmic Male Sterility:
As there are only two types of genotypes and cytoplasm is always inherited from the male sterile (female) plant so the maintenance of sterility is straight-forward here.
Transfer of Male Sterility to New Genotype:
This cytoplasmic male sterility may be transferred easily to a given strain by using that strain as pollinator in successive generations of a backcross programme. After 6-7 generations the genotype of male sterile line would be identical to pollinator line bearing the male sterile cytoplasm (Fig. 2.7). Through more repeated backcrossing total nuclear gene (variety B) will be transferred with the male sterile cytoplasm of variety A.
Uses/Importance in Plant Breeding:
As in cytoplasmic male sterility there is no restorer gene, so restoration of fertility is not possible, therefore this type of male sterility is useful for crops where seeds are not desired product, e.g., the vegetables like onion, broccoli, cauliflower, cabbage or other ornamental plants.
The hybrids developed in this process exhibit maximum advantage of hybrid vigour with respect to longer vegetative duration, larger flower size in ornamental plants and larger bulb size as in onion.
(c) Cytoplasmic-Genetic Male Sterility:
Here the male sterility character arises from the interaction of nuclear genes and cytoplasmic factor, i.e., when in the cytoplasmic male sterile line fertility can be restored with the help of nuclear gene (restorer gene, R) then this behaviour is called as cytoplasmic- genetic male sterility. In this kind of male sterility the combination of both nuclear genes and cytoplasmic factors determines the fertility or sterility in such plants.
Different combinations of these show only one male sterile line:
Maintenance of Sterility in Cytoplasmic-genetic Male Sterility:
For this purpose, A line and B line should be isogenic. If the nuclear gene is heterozygous in nature then the outcome will segregate into 1: 1 ratio.
Restoration of Fertility in Cytoplasmic-genetic Male Sterility:
Fertility can be restored when a homozygous restorer line, i.e., RR is crossed with male sterile A line, the progenies will become fertile. New male sterile lines can be produced in this kind of male sterility behaviour, but the pollinator should always be recessive homozygous.
Uses/Importance in Plant Breeding:
Cytoplasmic-genetic male sterility has much importance in the exploitation of hybrid vigour in crops where seed is desired. By using this cytoplasmic-genetic male sterility hybrid seed in maize, bajra and jowar has been produced. But owing to restriction on cross pollination, it has limited use in these crops.
Limitations of Using Male Sterility in Plant Breeding:
1. Undesirable Effects of the Cytoplasm:
In many cases it has been found that the male sterile cytoplasm has some undesirable effects, e.g., Cms-T cytoplasm of maize slightly retards growth and more susceptible to Helminthosporium.
2. Unsatisfactory Fertility Restoration:
In many cases restoration of fertility is not always possible, then those sources cannot be used in production of hybrid seed.
3. Unsatisfactory Pollination:
In case of many self-pollinating crops natural cross pollination is a problem. For hybrid seed production if pollination is not 100% then seed production cost increases due to less harvest.
4. Nuclear Gene Effect:
Sometimes some of the nuclear genes (modifier gene) may effect the cytoplasmic male sterility, as a result during back-crossing for transfer of male sterile cytoplasm to a new genotype, the male sterility character may get disturbed and fertile pollen grains may be produced.
5. Environmental Factors:
Under certain environmental conditions the male sterility mechanism may breakdown resulting in production of fertile pollen by the male sterile line.
6. Contribution of Male Cytoplasm:
During maintenance of male sterility sometimes the small amount of cytoplasm contributed by the pollen may lead to breakdown of male sterility mechanism.