Incompatibility

Self-incompatible pollen grains fail to germinate on the stigma. If some pollen grains do germinate, pollen tubes fail to enter the stigma. In many species, the pollen tubes enter the style, but they grow too slowly to effect fertilization before the flower drops.

Classification of self-incompatibility

1. Heteromorphic system

In this system, flowers of different incompatibility groups are different in morphology. For example, in Primula there are two types of flowers, pin and thrum. Pin flowers have long styles and long stamens. This situation is referred to as distyly. Tristyly is known in some plant species e.g., Lythrum; in such cases, the style of a flower may be either short, long or of medium length. In the case of distyly, the only compatible mating is between pin and thrum flowers. This characteristic is governed by a single gene s, Sx producing thrum and ss producing pin flowers. The incompatibility reaction of pollen is determined by the genotype of the plant producing them. Allele S is dominant over s the incompatibility system, therefore, is heteromorphic-sporophytic. This system is of little importance in crop plants; it occurs in sweet potato and buck-wheat.

2. Homomorphic system

In the homomorphic system, incompatibility is not associated with morphological differences among flowers. The incompatibility reaction of pollen may be controlled by the genotype of the plant on which it is produced (sporophytic control) or by its own genotype (gametophytic control).

• Gametophytic system

Gametophytic incompatibility was first described by East and Mangelsdorf in 1925 in Nicotiana sanderae. The incompatibility reaction of pollen is determined by its own genotype, and not by the genotype of the plant on which it is produced. Generally, the incompatibility reaction is determined by a single gene having multiple alleles, e.g., Trifolium, Nicotiana, Lycoperscion, Solanum, Petunia etc. sometime, polyploidy may lead to a loss of incompatibility due to competition between the two S alleles in diploid pollen. Irradiation of pollen or buds with X-rays or gamma rays temporarily suppresses the incompatibility reaction, and thus allows the pollen tube to grow through incompatible style.

In a single gene system, there are three types of mating

(i) Fully incompatible, e.g., S₁S₂xS₁S₂

2. Fully compatible, e.g., S₁S₂ x S₃S₄
3. Partially (i.e., 50% of the pollen) compatible e.g., S₁S₂ xS₂S₃

In some cases, an allele for self-fertility, S_f, is found. Pollen carrying the S_f allele does not show incompatibility reaction. Thus in a plant with the genotype StS1, selfing produce S_fS_f and S_fS₁ progeny. Mutations for S_f allele may be induced by irradiating the pollen used for self-pollination. There is another allele, Sf, which retards the growth of S_f pollen tubes, thus enforcing self-incompatibility.

• Sporophytic System

The incompatibility reaction of pollen is governed by the genotype of the plant on which the pollen is produced, and not by genotype of the pollen. There may be many complex incompatibility relationships.

1. There are frequent reciprocal differences.
2. Incompatibility can occur with the female parent.
3. A family can consist of three incompatibility groups.
4. Homozygotes are a normal part of the system.
5. An incompatibility group may contain two genotypes.

The sporophytic incompatibility is found in radish (R.sativus), diploid Brassica crops and Sinapis.

Mechanism of Self-Incompatibility

The various phenomena observed in self-incompatible matings are grouped into three broad categories:

1. pollen-stigma interaction,
2. pollen tube-style interaction, and
3. pollen tube-ovule interaction.

• Pollen-Stigma interaction

These interactions occur just after the pollen grains reach the stigma and generally prevent pollen germination. At the time they reach stigma, pollen grains generally have two nuclei in the gametophytic system, while they have three nuclei in the sporophytic system. This was once considered to be the basis for the two incompatibility systems, but the available evidence indicates otherwise.

In the gametophytic system, the stigma surface is plumose having elongated receptive cells and is commonly known as ‘wet’ stigma. The pollen grains generally germinate on reaching the stigma, and the incompatibility reaction occurs at a later stage. There are clear cut serological differences among the pollen grains with different S genotypes; such differences have not been observed in the sporophytic system.

In the sporophytic system, the stigma is papillate and dry, and is covered with a hydrated layer of proteins known as ‘pellicle’. Within few minutes of reaching the stigmatic surface, the pollen releases an exine exudate which is either protein or glycoprotein in nature. This exudate induces immediate callose formation in the papilae of incompatible stigma. Thus in the sporophytic system, stigma is the site of incompatibility reaction; once the pollen tube crosses the stigmatic barrier, there is not further inhibition of the pollen tube growth. In the hormomorphic sporophytic system, the incompatibility reaction of pollen is probably due to the deposition of some compounds from anther tapetum on to the pollen exine.

• Pollen tube-style interaction

In most cases of the gametophytic system, pollen grains germinate and pollen tubes penetrate the stigmatic surface. But in incompatible combinations, the growth of pollen tubes is retarded within the stigma, e.g., in Oenothera, or a little later in the style, e.g., in Petunia, Lycoperscion, Lilium etc.

• Pollen Tube-Ovule Interaction

Pollen tubes reach the ovule and effect fertilization. However, in incompatible combinations, embryos degenerate at an early stage of development.

Relevance of Self-Incompatibility in Plant Breeding

1. In self-incompatible fruit trees, it is necessary to plant two cross-compatible varieties to ensure fruitfulness. Further, cross-pollination may be poor in adverse weather conditions reducing fruit set. Therefore, it would be desirable to develop self-fertile forms in such cases.
2. 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 degreeof inbreeding it takes twice as much time as selfing. Further, for the maintenance of ibred lines selfing would be necessary.
3. Self-incompatibility may be used in hybrid seed production. For this purpose, (1) Alternatively, a self-incompatible, but cross compatible, lines are interplanted; seed obtained from both the lines would be hybrid seed. (2) Alternatively, a self-incompatible line may be interplanted with a self-compatible line. (3) Schemes for the production of double cross and triple cross hybrids have also been proposed and their feasibility has been demonstrated in the case of brassicas.

Overcoming Self-Incompatibility

• High temperature

In some species e.g., Trifolium, Lycopersicon, Brassica, Oenothera etc. exposure of pistils to temperature upto 600C induces pesudo-fertility.

• Irradiation

In the single-locus gametophytic system, e.g., in Solanaceae, acute irradiation with X-rays or gamma rays induces a temporary loss of self-incompatibility.

• Grafting

Grafting of a branch onto another branch of the same plant or of another plant is reported to reduce the degree of self-incompatibility in Trifolium pratense. There is only one report on this phenomenon, and the mechanism of this reduction is not known.

• Double Pollination

In some species, self-incompatible mating become possible when incompatible pollen is applied as a mixture with compatible pollen, or it is applied after pollination with compatible pollen.

• Other Techniques

A number of other techniques have been tried with varying degrees of success, but they are not commonly used. These techniques are: treatment of flowers with carbon monoxide, injecting styles with immunosuppressants, application of electrical potential difference of about 100 V between the stigma and pollen grains, treatment of pistil with phytohormones and with protein synthesis inhibitors, and steel brush pollination.

• Bud Pollination

Bud pollination means applications of mature pollen to immature nonreceptive stigma, generally 1-2 days prior to the anthesis of flowers. This is the most practicable and successful method both in the gametophytic and sporophytic systems

• Surgical Techniques

Removal of the stigmatic surface, the whole of stigma or a part or whole of the style may permit an otherwise incompatible mating. Removal of the stigma is very useful in the sporophytic system.