ABSTRACT
A key strategy to ensure sustained and sustainable apple production is breeding. New apple scion and rootstock cultivars can be genetically improved to better suit changing growing conditions, modern production systems and consumer preferences. Apple breeding is a long-term, resource-consuming, genetically complicated venture (Fig. 1). A juvenile phase of typically five years and up to ten years can be reduced by use of a precocious rootstock, but the generation time is greater than for many other crops. A plot of hundreds to thousands of apple seedlings takes up considerable land area, even on a dwarfing rootstock. Ongoing regular maintenance of plants over large areas for multiple years adds to the expense. Understanding both the genetic relationships among germplasm individuals and the genetic influences of important traits is fundamental to successful apple breeding. Apple is a self-incompatible, highly heterozygous, ancient autotetraploid of 17 chromosomes, features that have made genetic studies difficult. Many of our well-known apple cultivars have been around for decades, if not centuries, and a reasonably high proportion of them arose as chance seedlings rather than through controlled hybridization (Morgan and Richards, 2002) and hence parentage is unknown. As cultivars have been distributed via dormant buds around the world to either growers
for testing opportunities in commercial production or to germplasm collections, there have been, not surprisingly, labelling and propagation mistakes, which have generated confusion when cultivars have been compared among locations (Evans et al., 2011). The inability to readily determine identity, parentage, probability of shared alleles and general relatedness among breeding germplasm individuals has proved to be a long-term challenge for apple breeders. Dissection of the genetic control of apple traits, so that numerous target attributes, such as disease-and pest-hardy trees with high productivity for fruit that are crisp, flavourful, and with a long market life can be effectively assembled with available resources, is similarly challenged. The inherent difficulties described above have relegated it, and its supporting genetic research, almost entirely to the public sector in the United States. Yet apple is one of the most economically valuable fruit crops in the world and has significant contributions to human health (Boyer and Liu, 2004; Hyson, 2011). Therefore, solving breeding challenges will have large and positive consequences.
