The identification and analysis of meristematic mutations within the apple tree that developed the RubyMac sport mutation

Understanding the molecular basis of sport mutations in fruit trees can accelerate breeding of novel cultivars. For this, we analyzed the DNA of the apple tree that evolved the RubyMac phenotype through a sport mutation that introduced changes in fruit coloration in upper branches of the tree. Unexpectedly, we not only found 46 de novo mutations, but also 54 somatic gene conversions (i.e., loss-of-heterozygosity mutations) distinguishing the mutant and wild-type branches of the tree. Approximately 30% of the de novo mutations and 80% of the gene conversions were observed only in specific cells layers suggesting that they occurred in the corresponding meristematic layers. Interestingly, the de novo mutations were enriched for GC=>AT transitions, while the gene conversions showed the opposite bias for AT=>GC transitions suggesting that GC-biased gene conversions have the potential to counteract the AT-bias of de novo mutations. By comparing the gene expression patterns in fruit skins from mutant and wild-type branches, we found 56 differentially expressed genes including 18 that were involved in anthocyanin biosynthesis. While none of the differently expressed genes harbored a mutation, we found that some of the mutations affected the integrity of candidate genes in regions of the genome that were recently associated with natural variation in fruit coloration.