‘Honeycrisp’ is a widely grown and acclaimed apple cultivar that is commonly used in breeding programs. It also has a well-documented tendency to develop the physiological disorder, zonal leaf chlorosis (ZLC). This disorder causes reduced photosynthetic capacity and is thought to be due to a problem with phloem loading, although the underlying genetics of the disorder have not previously been discerned. In order to understand the breeding implications of the disorder, six families with ‘Honeycrisp’ as a parent and one family with ‘Honeycrisp’ as both a maternal and paternal grandparent were evaluated for ZLC incidence over two years. One major quantitative trait locus (QTL) for ZLC incidence was identified on linkage group (LG) 9. A haplotype in ‘Honeycrisp’ that originated from grandparent ‘Duchess of Oldenburg’ was associated with increased ZLC incidence in offspring in both years and all families evaluated. The LG9 QTL was 5 to 10 cM from MdMYB1, which is a major gene regulating fruit skin anthocyanin production. ‘Honeycrisp’ is heterozygous for red fruit skin overcolor at MdMYB1. The ‘Honeycrisp’ haplotype at the LG9 QTL associated with increased ZLC is in linkage phase with the allele at MdMYB1 associated with red color. Selection for the red allele from ‘Honeycrisp’ at MdMYB1 will result in most offspring also inheriting the haplotype at the LG9 QTL associated with high ZLC. The occurrence of two copies of this haplotype was sub-lethal in seedlings of a family where both parents inherited both the red overcolor allele at MdMYB1 and the haplotype at the LG9 QTL associated with high ZLC. This is the first study to have identified a genetic component of ZLC with clear breeding implications.
Bibliographical noteFunding Information:
This work was partially supported by the USDA National Institute of Food and Agriculture—Specialty Crop Research Initiative projects, “RosBREED: Enabling marker-assisted breeding in Rosaceae” (2009-51181-05808) and “RosBREED: Combining disease resistance with horticultural quality in new rosaceous cultivars” (2014-51181-22378). Some genetic data was provided by the FruitBreedomics project: Integrated approach for increasing breeding efficiency in fruit tree crops (http://fruitbreedomics.com/), which was co-funded by the EU seventh Framework Programme, project N 265582.
© 2019 Howard et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.