Location: Plant Genetic Resources ResearchTitle: Dw2 a new dwarfing locus in apple rootstocks and relationship to induction of early bearing in apple scions Author
Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2013
Publication Date: 3/15/2014
Citation: Fazio, G., Wan, Y., Kviklys, D., Romero, L., Adams, R., Strickland, D., Robinson, T. 2014. Dw2 a new dwarfing locus in apple rootstocks and relationship to induction of early bearing in apple scions. Journal of the American Society for Horticultural Science. 139(2):87–98. Interpretive Summary: Grafting desirable fruit bearing scions onto different tree root systems is a very ancient technique and perhaps one of the first forms of clonal propagation and selection. Most temperate fruit trees planted in commercial orchards are the result of the union of two genotypes: the scion (which yields the fruit) and the rootstock (which serves as an interface between the scion and the soil). This dual system has allowed for remarkable efficiencies in the development of new fruit cultivars because it divides selection and breeding for the whole plant into its component environmental interactions and functions that require very different selection traits for improvement: the root system interactions with the soil (anchorage, soil profile exploration, water and nutrient absorption, and interaction with soil biota) and the scion (fruit production, quality, storability, photosynthesis, tree architecture, disease resistance and climate stress tolerance). In this research we identify two major genetic factors that increase productivity of apple trees by dwarfing and inducing early bearing of grafted scions. Knowledge and implementation of these genetic factors will allow increased productivity in apple orchards, especially in modern high density systems.
Technical Abstract: The ability of certain apple rootstocks to dwarf their scions has been known for centuries and spurred a revolution in apple production. In this investigation, several breeding populations, in multiple replicated field and pot experiments were used to ascertain the degree of dwarfing of segregating rootstocks onto multiple scions. A previously produced genetic map of a breeding population derived from parents ‘Ottawa 3’ and ‘Robusta 5’ was used for QTL analysis of traits related to scion vigor suppression, induction of early bearing and other tree size measurements on own-rooted and grafted trees. The QTL analysis confirmed a previously reported QTL that imparts vigor control (Dw1, LOD=7.2) on linkage group 5 (LG5) of the apple genome and a new QTL named Dw2 (LOD=6.4) on LG11 that has a similar effect on vigor control. The data from this breeding population were used to study the interaction of these two loci. To validate these findings, a new genetic map comprised of 1,841 SNP loci was constructed from a cross between dwarfing, precocious rootstocks G.935 and B.9, resulting in the confirmation and modeling of the effect of Dw1 and Dw2 on vigor control of apple scions. Flower density and fruit yield data from these experiments allowed the identification of genetic factors Eb1 (LOD=7.1) and Eb2 (LOD=7.6) that cause early bearing of scions, roughly co-located with the dwarfing factors. The major QTL for mean number of fruit produced per tree co-located with Dw2 (LOD=7.0) and a minor QTL was located on LG16 (LOD=3.5). These findings will aid the development of a marker-assisted breeding strategy, and the discovery of additional sources for dwarfing and predictive modeling of new apple rootstocks in the Geneva® apple rootstock breeding program.