• Self-incompatibility refers to the failure of pollen to fertilize the same flower or other flower of the same plant, or It is the failure of pollen tube to penetrate the full length of style and effect fertilization.
  • Self-incompatibility is a widespread mechanism in flowering plants that prevents inbreeding and promotes outcrossing.
  • The self-incompatibility response is genetically controlled by one or more multi-allelic loci, and relies on a series of complex cellular interactions between the self-incompatible pollen and pistil.
  • Although SI functions ultimately to prevent self-fertilization, flowering plants have evolved several unique mechanisms for rejecting the self-incompatible pollen.
  • The self-incompatibility system in the Solanaceae makes use of a multi-allelic RNase in the pistil to block incompatible pollen tube growth.
  • In contrast, the Papaveraceae system appears to have complex cellular responses such as calcium fluxes, actin rearrangements, and programmed cell death occurring in the incompatible pollen tube. Finally, the Brassicaceae system has a receptor kinase signalling pathway activated in the pistil leading to pollen rejection.
  • It has reported in more than 300 species belonging to 70 families.
  • In self incompatibility non-fruitfulness or failure of fruit setting May occurs due to:
    I) Pollen grain fails to germinate on the stigma.
    II) Pollen grain germinates, but pollen tube fails to enter inside the stigma and style.
    III) Sometimes pollen tube enters inside the style but growth is very slow.
    IV) Pollen tube enters the ovule but there is no fertilization due to degeneration of eggs cells.
    V) Sometimes fertilization is effected but embryo degenerate at very early stage.
  • Lewis (1954) has suggested classifications of self-incompatibility
1. Heteromorphic system

2. Homomorphic system

1. Heteromorphic system

  • In this system, flowers of different incompatibility groups are different in morphology. eg. In Primula there are two types of flowers. Pin and thrum.
  • Pin flowers have long style and short stamens.
  • While thrum flowers have short styles and long stamens.
  • This situation is referred as distyly.
  • Tristyly is known in some plant species. E.g Lythrum in such case, the style of a flower may be either short long or of medium length.
  • In the case of distyly, the only compatible mating between pin and thrum flower.
  • This characteristic is governed by a single gene Ss produces thrum, while ‘ss’ produces 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 systems, therefore, is Heteromorphic Sporophytic.
  • The pollen grain produced by pin flowers, would all be‘s’ in genotypes as well as incompatibility reaction.
  • The pollen produced in thrum flowers would be of two type, genotypically ‘S’ and ‘s’ but all of them would be ‘S’ phenotypically.
  • The male between pin and thrum plants would produces Ss and ss progeny in each frequency.
  • This system of little importance in crop plants, it occurs in sweet potato and buckwheat.

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).
(A) Gametophytic system

  • Gametophytic incompatibility was first described by East and Mangelsdor in 1925 in Nicotiana sanderae.
  • The incompatibility reaction of pollen on which it is produced.
  • Generally, incompatibility reaction is determined by a single gene having multiple alleles. E .g Trifolium Nicotiana, Solanum, Petunia etc.
  • Sometimes , polyploidy may lead to a loss of incompatibility due to a competition between the two S alleles , present in diploid pollen, irradiation of pollen or buds with X rays or gamma-rays temporarily suppresses the incompatibility style.
  • Pollen tube grows very slowly in the style containing the same S allele as the pollen and fails to effect fertilization.
  • Therefore all the heterozygous at the S locus. In a single system there are three types of matings. 1) Fully Incompatible (S1 S2 X S1 S2)
    2) Fully Compatible (S1 S2 X S3 S4)
    3) Partially Compatible (S1 S2 X S2 S3)
(B) Sporophytic system

  • In the Sporophytic system also, the self – incompatibility is governed by a single gene. ‘S’ with multiple alleles, more than 30 alleles are known in Brassica oleracea.
  • In general, the number of S alleles is considerably larger in Gametophytic than in the Sporophytic system.
  • The incompatibility reaction of pollen is governed by the genotype of the plant on which the pollen is produced and not by the genotype of the pollen. Hughes and Babcock first reported it in 1950 in Crepis foetida and by Gerstel in Parthenium argentatum.
  • In the Sporophytic system, the alleles may exhibit dominance, condominance or competition.
  • Consequently, there may be much complex incompatibility relationship.
  • Lewis has summarised the following characteristics of this system. 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. As incompatibility is found in radish ( R.sativus), diploid Brassica crops. In many cases, different S alleles vary in their activity leading to varying degree of self-incompatibility. eg B. oleracea.
(Relevance of Self Incompatibility )

  • Self incompatibility effectively prevents self pollination. As a result, it has a profound effect on breeding approaches and objectives. 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 condition reducing fruit set. Therefore, it would be desirable to develop self- fertile forms in such cases. 2. Some breeding scheme. eg. Development of hybrids etc. Initially require some degree of inbreeding. Although summating 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. 3. Self –Incompatibility may be in hybrid seed production. For this purpose
    1) Two self incompatible, but cross – compatible, Lines are inter planted, seed obtained from both the lines would be hybrid seed,
    2) Alternatively, a self incompatible line may be interred planted with a self compatible line. From this scheme, seed from only the self – incompatible line would be hybrid.
    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.

About Author / Additional Info:
I am currently pursuing Ph.D. in Plant Breeding and Genetics from MPUAT-Udaipur (Raj.)