APPLE SELF-INCOMPATIBILITY GENOTYPES: AN OVERVIEW

Apple (Malus x domestica) flowers express a self-incompatibility system that reduces self-fertilization by arresting the growth of self-pollen tubes inside the pistil.
Many alleles of the S-gene controlling the female “receptivity” to self-pollen have been identified in previous years through either pollination studies, S-RNase protein identification or S-allele cDNA cloning and sequencing.
The independent nature of many of these studies has created confusion on the relationship between incompatibility phenotypes, gene products and nucleotide sequences.
We therefore attempted to clarify the incompatibility puzzle in apple and compiled data from the literature and from our own research on S-allele identification in apple.
The allele-specific PCR technique was optimised for the detection of the 15 sequenced S-alleles, corresponding to 14 incompatibility phenotypes.
We were able to relate all sequenced S-alleles to their corresponding S-RNases, except for the second ‘Granny Smith’ allele, called S₂₃, the crabapple-derived S₂₆ and S₂₈. Nine other S-alleles that are known from their S-RNase bands only, have been discovered in a single cultivar each up to now.
As a major result of our investigation, the S-genotypes of 130 North-American, European and Asian apple cultivars are presented, arranged into incompatibility groups.