Identical Dormancy Gene Mutations Reveal Unanticipated Relatedness Among Low-Chill Apples

Apples (Malus x domestica) are popular fruits grown in temperate regions of the world. The various genotypes must meet a specific threshold amount of cold exposure before they are competent to break dormancy, a quantity approximated as "chill hours". Several varieties have been identified that exhibit an ultra-low-chill requirement, or more precisely shallow dormancy, breaking vegetative and floral buds early in spring in response to minimal cold exposure. These ultra-low-chill genotypes originated from the Bahamas ( Dorsett Golden,1960s), Israel ( Anna, 1950s) and Alabama, USA ( Shell of Alabama, 1880s). The separation in time and space implies that each would feature distinct genetic lesions that govern dormancy control, providing discrete mechanisms to incorporate a low-chill trait in variety improvement. However, analysis of microsatellites and ultimately genome sequence indicates that Dorsett Golden and Anna share strong concordance with the Shell of Alabama genotype, as well as other ultra-low-chill varieties. Kinship analysis confirms that all are closely related, despite differences in year and place of origin. All three low-chill genotypes share common mutations in the DORMANCY ASSOCIATED MADS-BOX1(DAM1) gene, a known repressor of vegetative growth during dormancy. Genomic sequence diversity is observed among Shell of Alabama individuals, including differences in DAM1 that match differences in flowering time. The results of this study call into question the pedigrees of the ultra-low-chill apple germplasm and indicate variation in an otherwise narrow genetic base for use in future breeding efforts.