In this article we will discuss about the types and aspects of modes of pollination in plant breeding.
Types of Modes of Pollination:
Pollination is of two types, viz.:
(1) Autogamy or self-pollination and
(2) Allogamy or cross pollination.
These are briefly discussed below:
i. Autogamy:
Development of seed by self-pollination is known as autogamy. Autogamy is the closest form of inbreeding. Autogamy leads to homozygosity. Such species develop homozygous balance and do not exhibit significant inbreeding depression. There are several mechanisms which promote autogamy.
These are discussed below:
(i) Bisexuality:
Presence of male and female organs in the same flower is known as bisexuality. The presence of bisexual flowers is a must for self-pollination. All the self-pollinated plants have hermaphrodite flowers.
(ii) Homogamy:
Maturation of anthers and stigma of a flower at the same time is called homogamy. As a rule, homogamy is essential for self-pollination.
(iii) Cleistogamy:
When pollination and fertilization occur in unopened flower bud, it is known as cleistogamy. It ensures self-pollination and prevents cross pollination. Cleistogamy has been reported in some varieties of wheat, barley, oats and several other grass species.
(iv) Chasmogamy:
In plants, fertilization after opening of flower is known as chasmogamy. This also promotes self-pollination and is found in crops like wheat, barley, rice and oats.
(v) Position of Anthers:
In some species, stigmas are surrounded by anthers in such a way that self-pollination is ensured. Such situation is found in tomato and brinjal. In some legumes, the stamens and stigma are enclosed by the petals in such a way that self-pollination is ensured. Examples are green-gram, black-gram, soybean, chickpea and pea.
ii. Allogamy:
Development of seed by cross pollination is called allogamy. This is the common form of out-breeding. Allogamy leads to heterozygosity. Such species develop heterozygous balance and exhibit significant inbreeding depression on selfing. There are various mechanisms which promote allogamy.
These are briefly discussed below:
(i) Dicliny:
It refers to unisexual flowers.
This is of two types: viz.:
(a) Monoecy, and
(b) Dioecy.
When male and female flowers are separate but present in the same plants, it is known as monoecy. In some crops, the male and female flowers are present in the same inflorescence such as in mango, castor and banana. In some cases, they are on separate inflorescence as in maize. Other examples are cucurbits, grapes, strawberry, cassava and rubber. When staminate and pistillate flowers are present on different plants, it is called dioecy. In includes papaya, date palm, spinach hemp and asparagus.
(ii) Dichogamy:
It refers to maturation of anthers and stigma of the same flower at different times. Dichogamy promotes cross pollination even in the hermaphrodite species.
Dichogamy is of two types, viz.:
(a) Protogyny and
(b) Protandry.
When pistil matures before anthers, it is called protogyny such as in pearl-millet. When anthers mature before pistil, it is known as protandry. It is found in maize, sugar-beet and several other species.
(iii) Heterostyly:
When styles and filaments in a flower are of different length, it is called heterostyly. It promotes cross pollination, such as in Linseed.
(iv) Herkogamy:
Hindrance to self-pollination due to some physical barriers such as presence of hyline membrane around the anther is known as herkogamy. Such membrane does not allow the dehiscence of pollen and prevents self-pollination such as in alfalfa.
(v) Self-Incompatibility:
The inability of fertile pollens to fertilize the same flower is referred to as self-incompatibility. It prevents self-pollination and promotes cross pollination. Self-incompatibility is found in several crop species like Brassica, Radish, Nicotiana, and many grass species. It is of two types sporophytic and gametophytic.
(vi) Male Sterility:
In some species, the pollen grains are nonfunctional. Such condition is known as male sterility. It prevents self-pollination and promotes cross pollination. It is of three types: viz. genetic, cytoplasmic and cytoplasmic genetic. It is a useful tool in hybrid seed production.
Study of floral biology and aforesaid mechanisms is essential for determining the mode of pollination of various crop species. Moreover, if selfing has adverse effects on seed setting and general vigour, it indicates that the species is cross pollinated. If selfing does not have any adverse effect on these characters, it suggests that the species is self-pollinated.
The percentage of cross pollination can be determined by growing a seed mixture of two different varieties together. The two varieties should have marker characters say green and pigmented plants. The seeds are harvested from the recessive (green) variety and grown next year in separate field. The proportion of pigmented plants in green variety will indicate the percentage of outcrossing or cross pollination.
Aspects of Pollination in Plant Breeding:
The mode of pollination plays an important role in plant breeding.
It has impact on five important aspects, viz.:
(1) Gene action,
(2) Genetic constitution,
(3) Adaptability,
(4) Genetic purity and
(5) Transfer of genes.
These are briefly discussed below:
i. Gene Action:
Gene action refers to mode of expression of genes for various characters in a population.
Gene action is of two types:
(i) Additive and
(ii) Non-additive (dominance and epistasis).
Additive gene action is associated with homozygosity and is more in self-pollinated species. On the other hand, non-additive gene action is associated with heterozygosity and, therefore, is more in cross pollinated species.
Selfing of cross pollinated species gradually leads to conversion of non-additive gene action into additive gene action. Thus mode of pollination determines the type of gene action in a population which in turn helps in deciding a suitable breeding procedure for genetic improvement of crop plants.
ii. Genetic Constitution:
The breeding populations are of four types- viz. heterozygous, homozygous, heterogeneous and homogeneous. Heterozygous individuals have dissimilar alleles at the corresponding loci and segregate on selfing. Homozygous individuals have similar alleles at the corresponding loci and are pure types. They do not segregate on selfing.
The genotypically similar population is known as homogeneous. It may be homozygous or heterozygous. Similarly, genotypically dissimilar populations are known as heterogeneous. It may again be homozygous or heterozygous.
Self-pollination leads to homozygosity and cross pollination results in heterozygosity. Thus we have to exploit homozygosity in self-pollinated crops and heterozygosity in cross pollinated species, because in breeders have advantages of homozygosity and out breeders have advantage of heterozygosity.
iii. Adaptability:
Adaptability refers to stable performance of a variety over a wide range of environmental conditions. The cross pollinated crop species have better adaptability (buffering capacity) than self-pollinated species because of more heterozygosity and heterogeneity in the former case.
The heterogeneous populations have broad genetic base. Such populations have greater capacity to stabilize productivity over a wide range of changing environments. Heterogeneous populations give more stable yields over several environments than pure lines. Similarly, heterozygous individuals such as F1 hybrids are more stable to environmental variations than their homozygous parents. Thus mode of pollination plays a key role in the varietal adaptability.
iv. Genetic Purity:
Self-pollination maintains the genetic purity of a species and the purity of a variety of self- pollinated crop can be easily maintained for several years. Cross pollination creates heterozygosity and genetic composition of a cross pollinated variety change gradually. Hence, self-pollination is essential even in cross pollinated species to maintain the purity in parental lines.
v. Transfer of Genes:
Cross pollination permits transfer of desirable genes from one species to another. In other words, combining of desirable genes from different sources into a single genotype is possible only through cross pollination. Self-pollination does not permit combining of genes from different sources.
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