What is Sporophytic Self-incompatibility (SSI) – Definition?
It is a type of homomorphic self incompatibility. In this case, self-incompatibility is governed by genotype of pollen producing plant i.e. diploid genotype of the sporophyte generation.
This system inhibits pollen germination or pollen tube growth on the stigma of same flower. Progeny from cross between two genotypes is either fully fertile or complete sterile. It permits recovery of parent genotypes in some crosses.
It is controlled by single gene S with multiple alleles, where alleles may show individual action, dominance or interaction in pollen or style. S-locus is a cluster of three tightly-linked loci, SLG (S-Locus Glycoprotein), SRK (S-Receptor Kinase) and SCR (S-locus Cysteine-Rich protein). SLG codes for a part of a receptor present in the cell wall of the stigma; SRK codes for the other part of the receptor and SCR codes for a secreted ligand for the same receptor.
Pollen does not germinate on the stigma of a flower that contains either of the two alleles that are present in the sporophyte parent which produced the pollen. The same condition is applicable even though each pollen grain (haploid) contains only one of the alleles. So, pollen grains (S1 or S2) produced by S1S2 plant will germinate only on S3S4 plant not on S1S2 or S1S3 and so on. Order of dominance followed is S1 >S2> S3> S4
Examples of Sporophytic Self-incompatibility (SSI)
This type of incompatibility is observed in Brassicaceae, Polemoniaceae, Caryophyllaceae, Betulaceae, Asteraceae, Sterculiaceae and Convolvulaceae
Gametophytic self incompatibility is the second type of homomorphic incompatibility. Another must read link in the concept of compatibility is heteromorphic self incompatibility.