Identification a moderate effect QTL for fire blight resistance from Malus sieversii, the primary progenitor species of apples

Authors: TEGTMEIER, RICHARD - Cornell University; COBB-SMITH, DELLA - Cornell University; Zhong, Gan-Yuan; KHAN, AWAIS - Cornell University

Submitted to: Tree Genetics and Genomes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2023
Publication Date: 10/23/2023
Citation: Tegtmeier, R., Cobb-Smith, D., Zhong, G., Khan, A. 0203. Identification a moderate effect QTL for fire blight resistance from Malus sieversii, the primary progenitor species of apples. Tree Genetics and Genomes. https://doi.org/10.1007/s11295-023-01626-6.
DOI: https://doi.org/10.1007/s11295-023-01626-6

Interpretive Summary: Fire blight is a bacterial disease and a major threat to apple production. Most commercial apple cultivars are susceptible to fire blight and there is a pressing need to develop highly resistant cultivars. One way to accelerate development of fire blight resistant apple cultivars is to develop and identify molecular markers which are highly associated with genetic factors (such as QTL) controlling resistance to the disease. Such molecular markers can be used to select individuals carrying the resistance QTL in breeding. There are several major fire blight resistance QTLs identified from wild apple species, but their uses in breeding are much limited due to many reasons. In this study we identified a major QTL from M. sieversii, the primary progenitor of domesticated apples. The study was conducted by crossing a M. sieversii apple with the widely cultivated apple cultivar ‘Royal Gala’, inoculating the cross progeny with fire blight bacteria in a greenhouse, observing the reactions of the progeny to the inoculation and developing and identifying molecular markers associated with resistance to the diseases (or QTL). The major QTL identified from M. sieversii was on its chromosome 7 and responsible for about 50% of the resistance variation observed in the progeny. Additionally, a minor resistance QTL explaining 18% of the variation was identified on the chromosome 10 from ‘Royal Gala’. These findings were based on greenhouse experiments over two years and could help accelerate the development of fire blight-resistant cultivars in future.

Technical Abstract: Fire blight, a bacterial disease caused by Erwinia amylovora, is a major threat to apple production. Most commercial apple cultivars are susceptible to fire blight driving the need to develop fire blight resistant cultivars. Although several major fire blight resistance QTLs have been identified from wild species of Malus, the challenges of breeding apples due to long juvenile phase and heterozygosity greatly limit their use. M. sieversii, the primary progenitor of domesticated apples, is one of the wild Malus species that is sexually compatible with M. domestica and has some favorable fruit quality traits. In this study, we performed QTL analysis on two F1 apple populations of M. domestica cv. ‘Royal Gala’ × M. sieversii (GMAL4591 and GMAL4592) to identify fire blight resistance QTL. Parental linkage maps were constructed for each family using marker sets of approximately 20K GBS-SNPs. Phenotype data was collected from parents and progeny through controlled fire blight inoculations in the greenhouse for two subsequent years. A significant (P < 0.0001) moderate-effect fire blight resistance QTL on linkage group 7 of GMAL4591 was identified from the paternal parent M. sieversii ‘KAZ 95 17-14’ (Msv_FB7). Msv_FB7 explains about 48-53% of the phenotyping variance. Additionally, a significant (P < 0.001) minor effect QTL explaining 18% of the phenotypic variance was identified in population GMAL4592 on LG10 from ‘Royal Gala’. We developed diagnostic SSR markers flanking the Msv_FB7 QTL to use in apple breeding. These findings have the potential to accelerate the development of fire blight-resistant cultivars.