Catch a match: new insights into the genetics of self-(in)compatibility in fruit tree species

Most economically important fruit tree species exhibit gametophytic self-incompatibility (GSI) and hence require the presence of pollenizer trees in orchards to achieve fruit set. This trait is controlled by a multiallelic locus (S-locus), and fertilization is only possible between individuals carrying different S-alleles. An increasing number of S-alleles in a population or a set of cultivars will directly increase the chance of mating (sexual compatibility) between two individuals. This study will show the most recent progress in the identification of S-alleles and S-genotype assignment for Malus, Pyrus and Prunus cultivars, with special focus to the polyploid species where such analyses are not so advanced. In Rosaceae, the pistil and pollen-specificities are provided by an S-ribonuclease (S-RNase) enzyme and an F-box domain containing protein (S-haplotype-specific F-box, SFB), respectively. The mechanism of recognition is different in two subfamilies of Rosaceae with Prunoideae (stone fruit species) having self-recognition and Malinae (apples and pears) having a collaborative non-self-recognition process with multiple SFB genes, the S-haplotype-specific F-box brothers, SFBBs. Recently new fruit bearing taxa (e.g., Rhamnaceae, Rutaceae, Cactaceae and Rosoideae) have been also described to have S-RNase based GSI, pointing to its significance in Angiosperm evolution. Several proteins putatively taking part in the underlying molecular mechanisms were identified, some of those are encoded outside the S-locus. Transcriptomic analysis and metabolic fingerprinting were also exploited to find components of GSI and an explanation for the rapid growth arrest of incompatible pollen tubes. The mutations resulting in self-compatibility (SC) are not only important for orchard planning but may also help understand the involved molecular interactions. The consequences of most frequent mutations (SNPs, transposons, epigenetic alterations, etc.) will be also considered from the aspects of molecular biology and crop evolution as well as their potential application in breeding and cultivation.