1. Faculty of Science, School of Sciences, Natabua Campus Lautoka
BIO706 Embryology
Lecture 26: Self-incompatibility - II

2. Sporophytic Self-Incompatibility (SSI)
This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower.
In this system,
Rejection of self pollen is controlled by the diploid genotype of the sporophyte generation.

3. The control lies in the "S-locus", which is actually a cluster of three tightly-linked loci:
SLG (S-Locus Glycoprotein) which encodes part of a receptor present in the cell wall of the stigma;

4. SRK (S-Receptor Kinase), which encodes the other part of the receptor
SRK (S-Receptor Kinase), which encodes the other part of the receptor. Kinases attach phosphate groups to other proteins. SRK is transmembrane protein embedded in the plasma membrane of the stigma cell.
SCR (S-locus Cysteine-Rich protein), which encodes a soluble ligand for the same receptor which is secreted by the pollen

7. Because the plants cannot fertilize themselves, they tend to be heterozygous; that is, carry a pair of different S loci (here designated S1 and S2).
However, dozens of different S alleles may be present in the population of the species; that is; the S-locus in the species is extremely polymorphic.
The difference between the alleles is concentrated in certain "hyper variable regions" of the receptor.

8. The rules:
Pollen will not germinate on the stigma (diploid) of a flower that contains either of the two alleles in the sporophyte parent that produced the pollen.
This holds true even though each pollen grain — being haploid — contains only one of the alleles.
For example, the S2 pollen, which was produced by a S1S2 parent, cannot germinate on an S1S3 stigma.

10. The explanation:
The S1S2 pollen-producing sporophyte synthesizes both SCR1 and SCR2 for incorporation in (and later release from) both S1 and S2 pollen grains.
If either SCR molecule can bind to either receptor on the pistil, the kinase triggers a series of events that lead to failure of the stigma to support germination of the pollen grain. Among these events is the ubiquination of proteins targeting them for destruction in proteasomes.
If this path is not triggered (e.g., pollen from an S1S2 parent on an S3S4 stigma, the pollen germinates successfully.

12. 2) 2-loci gametophytic self-incompatibility
The grass subfamily Pooideae, and perhaps all of the family Poaceae, have a gametophytic self-incompatibility system that involves two unlinked loci referred to as S and Z. If the alleles expressed at these two loci in the pollen grain both match the corresponding alleles in the pistil, the pollen grain will be recognized as incompatible.

13. 3) Heteromorphic self-incompatibility
The genes responsible for self-incompatibility in heterostylous flowers are strongly linked to the genes responsible for flower polymorphism, so these traits are inherited together.
The associated concepts are distyly and tristyly.

14. What is Distyly ? Here, both stamens and styles are of two types.
Stamens may be low and high
styles short and long.
It is determined by a single gene, with two alleles.
The flower with short style and high stamen is called as thrum type and flower with long style and low stamen is called as pin type. Both thrum and pin flowers differ for six characters in addition to stamen and style length.

15. Distyly Stigma Anther Thrum Pin Cross Result Ss (thrum) X Ss (thrum)
Incompatible
ss (pin) X ss (pin)
Ss(thrum) X ss (pin)
1:1
ss (pin) X Ss(thrum)
Thrum Pin

16. What is Tristyly?
In tristyly, styles and stamens have three different positions.
It is determined by two genes S and M, each with two alleles.
S gives rise to short style,
S and M to medium style and
s and m to long style.
The number of possible genotypes is greater, but a 1:1 ratio exists between individuals of each SI type.

17. Style
Style
Short Style Medium Style Long Style

18. Cryptic self-incompatibility (CSI)
It exists in a limited number of taxa (for example, there is evidence for CSI in 
Bladder Campion-Silene vulgaris (Caryophyllaceae),
Viper's Bugloss or Blueweed -Echium vulgare(Boraginaceae),
Waterwillow or swamp loosestrife -Decodon verticillatus (Lythraceae),

19. (Decodon verticillatus)
Waterwillow or
 Swamp loosestrife
(Decodon verticillatus)
Bladder Campion
(Silene vulgaris)

20. Viper's Bugloss or Blueweed
(Echium vulgare)

21. In this mechanism, the simultaneous presence of cross and self pollen on the same stigma, results in higher seed set from cross pollen, relative to self pollen. However, as opposed to 'complete' or 'absolute' SI, in CSI, self-pollination without the presence of competing cross pollen, results in successive fertilization and seed set; in this way, reproduction is assured, even in the absence of cross-pollination.
Contd…

22. CSI acts, at least in some species, at the stage of pollen tube elongation, and leads to faster elongation of cross pollen tubes, relative to self pollen tubes. The cellular and molecular mechanisms of CSI have not been described.
The strength of a CSI response can be defined, as the ratio of crossed to selfed ovules, formed when equal amounts of cross and self pollen, are placed upon the stigma; in the taxa described up to this day, this ratio ranges between 3.2 and 11.5

23. Late-acting self-incompatibility (LSI)
It is also termed ovarian self-incompatibility (OSI).
In this mechanism, self pollen germinates and reaches the ovules, but no fruit is set.
 LSI can be pre-zygotic(e.g. deterioration of the embryo sac prior to pollen tube entry, as in Narcissus triandrus) or
post-zygotic (malformation of the zygote or embryo, as in certain species of Asclepias and in Spathodea campanulata).

24.  Narcissus triandrus

25. Spathodea campanulata

26. Late-acting self-incompatibility (LSI)
The existence of the LSI mechanism among different taxa and in general, is subject for scientific debate. Criticizers claim, that absence of fruit set is due to genetic defects (homozygosity for lethal recessive alleles), which are the direct result of self-fertilization (inbreeding depression).
 Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon.

27. Importance of Self-Incompatibility In
Plant Breeding
Self-incompatibility effectively prevents self-pollination; as a result, it has a profound effect on plant breeding approaches and objectives.
(1)  In self incompatible fruit trees, it is necessary to plant two cross-compatible varieties to ensure fruitfulness.
 (2) Self-incompatibility may be used in hybrid seed production. For that, two self-incompatible but cross-compatible lines are to be interpolated; seeds obtained from both the lines would be hybrid seed.

28. (3) Self incompatibility provides a way for hybrid seed production without emasculation and without resorting to genetic or cytoplasmic male sterility.
(4) Self incompatibility system permits combining of desirable genes in a single genotype from two or more different sources through natural cross pollination which is not possible in self compatible species .
(5) In case of pineapple, commercial clones are self-incompatible. As a result, their fruits develop parthenocarpically & are seedless.

29. Overcome the self incompatibility
It is very difficult to produce homozygous inbred lines in a self incompatible species.
Bud pollination has to be made to maintain the parental lines.
Self incompatibility is affected by environmental factors such as temperature and humidity..

30. 2. Incompatibility is reduced or. broken down at high
2. Incompatibility is reduced or broken down at high temperature and hu­midity.
3. There is a limited use of self- incompatibility due to problems associated with the maintenance of inbred lines through hand pollination as it is tedious and costly

31. Thank You
Questions are welcome