Present and future of marker-assisted breeding in sweet and sour cherry

In the last decade, with the advent of next-generation sequencing technologies, numerous molecular markers have become available for genetic studies in sweet and sour cherries. Thanks to these tools, saturated linkage maps have been built in several countries allowing the detection of numerous quantitative trait loci (QTLs). Hence, marker-assisted selection (MAS) has now become a reality for these crops. Given that sweet cherry is a species with a long period of juvenility and that large areas are needed to evaluate thousands of new hybrids, MAS will allow breeders to make decisions efficiently based on DNA information, such as which parents to choose, what crosses to make, and which hybrids to plant so that only those hybrids with favorable allelic combinations for the most critical agronomic traits are advanced. Nevertheless, the majority of these traits are complex, multi-genic and for some of them highly influenced by the environmental conditions. In this study, a review of the main QTLs detected on sweet and sour cherry populations from INRA Bordeaux and Michigan State University is presented for the following traits: bloom time and its components chilling and heat requirements, maturity period, productivity, fruit weight and firmness, fruit color, fruit acidity and sugar content, tolerance to rain-induced fruit cracking and resistance to cherry leaf spot. MAS has already been implemented for self-fertility and fruit color in both sweet and sour cherry and fruit weight in sweet cherry. For the other traits, either MAS is still in a validation phase or stronger precision is needed in the QTL detection studies. In some cases, the co-localization of QTLs and promising candidate genes (CGs) has been demonstrated. Perspectives dealing with fine mapping approaches, study of QTL×E interactions and QTL and/or CG validation through association genetics will be summarized. The full sweet cherry genome sequence of cultivar 'Regina' will soon be available, and will represent an invaluable tool for the whole cherry community of geneticists and breeders.