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Self-incompatibility system can be divided into two basic groups: 1. Heteromorphic Systems 2. Homomorphic Systems.
Group # 1. Heteromorphic Incompatibility:
This incompatibility refers to those plants where the same species produces more than one morphological types of flower, where the relative length of the style (distyly and tristyly) and stamens differ. In distyly the plants produce two forms of flowers, called long styled or pin morph and short styled or thrum morph. Pollination is successful in intermorph but incompatible in intramorph plants (Fig. 6.9).
Besides the differences in length of style and stamens, they also differ in the size of pollen, ornamentation of exine and stigmatic surface, and structure of the stigmatic papillae. For instance, in several members of Plumbaginaceae the long-styled flowers produce non-papillate type of stigma and reticulate ornamented exine. On the other hand short-styled flowers produce papillate stigma and spinulose exine (Fig. 6.10 A & B)
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Distyly is controlled by a single gene complex, S, with two alleles S arid s. The allele for short-style (S) is dominant over the long-style (s) allele. The long-styled plants are homozygous recessive (ss), and short-styled are heterozygous (Ss).
Thus compatible pollination (Ss x ss) results in progeny of nearly equal number of long-styled and short-styled form. Lewis (1954) believes that the S gene (super gene) has six linked genes, viz., G S I’ I” P A, where, G= length of style, S = surface of stigma, I’ = pollen incompatibility, I” = stylar incompatibility, P = pollen size and /or shape, and A = height of stamen.
The tristyly plants produce three floral morphs, namely, long-styled, mid-styled and short-styled. Each of these morphotypes produces anthers at different levels that correspond to the level of stigma in the other two forms.
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In a successful pollination pollen from long stamens of short-styled morph is compatible on stigma of long-styled morph and not on mid-styled morph, while those from mid- stamens are compatible on stigma of mid-styled morph and not on long-styled morph (Fig 6.9b).
Tristyly is controlled by two genes, M and S where S is epistatic to M, and both the two alleles. The long-styled morph is homozygous recessive for S (ss) and homozygous dominant or heterozygous for M (ss MM/ ss Mm). The short-styled morph is heterozygous for S, and M (Ss mm/Ss Mm/Ss MM).
The zone of inhibition is an important factor in incompatibility and is co-related with the pollen cytology or crosses among different floral morph. For instance the 3-celled pollen of Linum and Limonium show stigmatic inhibition, whereas the 2-celled pollen of Primula shows stylar inhibition. In Fagopyrum, inhibition in thrum x thrum pollination is in the stigma, while in pin x pin pollination it is in the style.
It has been seen that in Primula vulgaris incompatible pollen is inhibited at different sites, viz., on stigmatic surface due to failure of germination or pollen tube penetration through the stigmatic head or transmitting tract of the style. In Linum thrum or short-styled stigma, support adhesion and germination of both thrum and pin pollen, however, only thrum pollen is effectively inhibited in the stigma.
While working with the mechanism of inhibition in Linum grandiflorum, Lewis (1943) recorded that incompatibility depends on the differences in osmotic pressure of pollen and style of the two morphs. An osmotic ratio of 4:1 between pollen and style is optimum for normal pollen hydration, and growth of the pollen tube.
In Forsythia intermedia the long-style pollen and the short-style pollen contain the inhibitors quercitrin and rutin respectively. Rutin in thrum pollen is degraded by a specific enzyme present only in the pin style, while quercitrin present in pin pollen is destroyed by another specific enzyme present only in the thrum style. Incompatible pollinations however, cannot break this inhibitor.
The involvement of a stylar protein dialysates in the intercellular secretions of the transmitting tissue in Primula vulgaris for incompatibility responses was recorded by Shivanna (1978a) in both the morph types. The protein inhibited tube growth of pollen of the same morph more strongly than that of the other.
Group # 2. Homomorphic Incompatibility:
In this incompatibility the flowers produced by different plants do not show morphological variations. The physiological barriers act in such a way that pollens do not germinate on a stigma of similar genetic constitution or their growth is so slow that by the time the pollen tube reaches the embryo sac, the latter withers out.
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Almost all the molecular work on self-incompatibility has come from homomorphic systems which have been reported in over 250 genera.
East and Mangelsdorf (1925) have given a satisfactory genetic interpretations of homomorphic incompatibility in Nicotiana. They found that the rate of pollen tube growth on stigma and style of Nicotiana is governed by a gene S with multiple alleles (S1, S2, S3, S4,……. Sn ) at a single locus. Each plant has only two of these alleles.
Cells of styles and stigmas are diploid and carry both the alleles of the plant. Pollen grains are haploid and, therefore, carry only one of the two alleles. A pollen grain carrying a self sterility allele does not grow well on a female style carrying the same allele, but grows and fertilizes a plant that carries the other allele. For instance, a plant with the genotype S1, S2 will produce two types of pollen grains, S1 and S2, none of which will grow on a style with these alleles (Fig 6.11).
Thus self-fertilization is ruled out. But if a plant with the genotype S2S3 is pollinated by S1 and S2 pollen grains, only the S, pollen grains will germinate and fertilize, whereas S2 pollen grains will fail to grow (Table 6.2). On the other hand all the pollen grains of a S1S2 plant will be viable and active on a S3S4 tissue.
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Homomorphic incompatibility is divided into gametophytic and sporophytic control. In the gametophytic self-incompatibility phenotype is determined by the genotype of the pollen grain, whereas in the sporophytic type, pollen rejection is imposed by the genotype of the pollen parent plant and not by the pollen genotype (Table 6.3 & 6.4).
In the gametophytic type, pollen rejection mechanism operate in the style, leading to the inhibition of pollen tube growth, whereas the stigma is the site for pollen rejection in the sporophytic type due to failure of pollen germination or pollen tube penetration. Gametophytic type is common in Solanaceae, Fabaceae, Papaveraceae Rosaceae, Rubiaceae, Liliaceae, Poaceae, and Commelinaceae.
The sporophytic type is seen in the Brassicaceae, Asteraceae, and Convolvulaceae. According to Lewis (1954) and Pandey (1970), there exists a difference in the time of S-gene action in the two types of incompatibility. In the sporophytic type the S-gene is activated in meiocytes before the completion of meiosis, as a result the product of both the genes is distributed in all the four microspores.
In the gametophyte type S-gene activation is delayed until the completion of meiosis, as a result two of the four microspores receive the products of one S-allele and the other two the products of the other S-allele. Pandey (1979) explained the action of S-gene in two distinct phases, viz., i. the transcription of the S-gene produces S-specific precursors, and ii. translation of the precursors into specific proteins (Fig 6.12).
In some sporophytic species, S-allele specific products are synthesized in the tapetum and are incorporated into the pollen exine following the breakdown of the tapetum. (Fig 6.13).
The zone of inhibition for the incompatible pollen or pollen tube is the stigmatic surface or the stylar tissue. Pollen grains that are shed at 2-celled stage show gametophytic incompatibility and the zone of inhibition is in the style.
Pollen grains shed at 3-celled stage show sporophytic incompatibility, and the zone of inhibition is in the stigma. In the gametophyte system S-gene activation takes place after meiosis and, therefore, there is a delay in the synthesis of incompatibility substances, and the completion of mitotic division. Since the incompatibility substances are synthesized only in the pollen tubes, inhibition is also delayed.
In the sporophytic system the incompatibility substances are already present in the pollen cytoplasm, the activation of incompatibility reactions and inhibition of incompatible pollen tubes takes place in the stigma.
A phenotypic expression of incompatibility is seen in the form of a lenticular plug of callose between the plasma membrane and pectocellulosic layer of the stigmatic papillae, just below the point of contact with the pollen (Fig 6.14).
This deposition is very rapid and often within minutes after pollination. In gametophytic self- incompatibility systems, like grasses, where rejection reaction occurs on the stigma rather than in the style, there is no callose deposition in the stigmatic papillae following incompatible pollination, however, callose deposition is very distinct in the pollen tube. Further the earliest morphological manifestation in incompatible pollen tube is the apposition of microfibrillar pectic material.
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In gametophytic self-incompatibility systems, the incompatibility reaction occurs in the style after the pollen tube has grown to about two-third the length of the style.
Pollen tube growth in the incompatible style is arrested in various ways, for instance in Petunia hybrida the pollen tube walls become thickened and the cytoplasm undergoes degeneration; the generative cell does not divide or the vegetative nucleus disappears within the first few hours; pollen tip becomes branched; increased number of callose plug or a high density of cellulose microfibrils in the thickened walls, etc.
