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Fine Mapping of the Fire Blight Resistance Locus in Malus × robusta 5 on Linkage Group 3
J. Fahrentrapp, G.A.L. Broggini and C. Gessler
Plant Pathology
Integrative Biology Zurich (IBZ) Swiss Federal Institute of Technology ETH Zurich
Universitaetstrasse 2, LFW 8092 Zurich
Switzerland A. Peil
Julius Kühn-Institut
Federal Research Centre for Cultivated Plants
Institute for Breeding Research on Horticulture and Fruit Crops Dresden
Germany
M. Kellerhals
Agroscope Changins-Wädenswil (ACW) PO Box, 8820 Wädenswil
Switzerland M. Malnoy
Fondazione Edmund Mach – FEM Istituto Agrario San Michele – IASMA
Via Mach 1, 38010 S. Michele all’Adige (TN) Italy
K. Richter
Julius Kühn-Institut
Federal Research Center for Cultivated Plants
Institute for Resistance Research and Stress Tolerance
Quedlinburg Germany
Keywords: apple, Erwinia amylovora, QTL-analysis, SSR Abstract
Fire blight is one of the most damaging diseases in the majority of apple (Malus × domestica) producing countries worldwide. The resistance to Erwinia amylovora of Malus × robusta 5 previously mapped by QTL mapping by Peil et al.
(2007) was remapped using the same data but as a single gene with the approach developed by Durel et al. (2007). The new region of the resistance (on LG3) was enriched with new SSR markers and fine mapped using additional recombinants identified in an enlarged segregating population of 2137 individuals. This allowed the positioning of the resistance locus into a short section at the top of linkage group 3 flanked by molecular markers.
INTRODUCTION
Fire blight is the most destructive disease of apple and pear orchards. In Europe its appearance and severity is highly variable between years and is difficult to predict. This situation renders its control uncertain and difficult. Currently the growers lack an outstanding and trustworthy strategy to control the disease. New and better biocontrol products, antibiotics and improved formulations may contribute to better control. Plant resistance should become an important element of disease management. The natural resistance, which is present in the wild apple Malus × robusta 5 (MR5), the ornamental cultivar of Malus × domestica ‘Evereste’ and the commercial cultivar ‘Fiesta’, have been mapped by QTL-mapping (Calenge et al., 2005; Khan et al., 2006; Peil et al., 2007; Durel et al., 2009). The major QTL for resistance against E. amylovora, explaining up to 80% of the phenotypic variation in MR5 at the top end of linkage group 3 (LG3), is flanked by two markers (CH03e03 and CH03g07). In an analysis of the same binarized phenotypic trait as a single gene, the position of the resistance locus was calculated to the very top of LG3. In this paper we report the fine mapping of the resistance locus. To do so, additional markers were developed on the top of LG3 and the mapping population was increased.
MATERIAL AND METHODS
Twelve SSR markers developed on the top of LG3 based on the ‘Golden
Proc. of the 13th Eucarpia Symposium on Fruit Breeding and Genetics Eds.: K.M. Evans et al.
Acta Hort. 976, ISHS 2013
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Delicious’ genomic sequence were applied to a subset (n=92) of the original mapping population. The size of the mapping population was then increased to reach 2137 individuals by performing crosses between genotypes carrying the MR5 resistance (either MR5 itself or one of its resistant progeny identified by Peil et al., 2007) and genotypes lacking this specific resistance (‘Idared’, ‘Mairac’, ACW 11303). Individuals of the whole population (n=2137) having a crossing-over-event between the two markers flanking the resistance locus at the top end of LG3 of MR5 were identified. These individuals were grafted and used for artificial shoot inoculation (3-12 replicates) by cutting two young leaves with scissors dipped into a suspension of E. amylovora strain EA222 (109 cfu/ml). The same individuals were analyzed with the 12 markers. The fine map of the top of LG3 was calculated using JoinMap3.0 (Van Ooijen and Voorrips, 2001).
RESULTS
Eight of the 12 new markers applied to the subpopulation (n=92) mapped as a cluster together with CH03e03 and four mapped between CH03g07 and CH03e03. Of the whole mapping population 1.7% carried a crossing over in the region of interest. The application of the 12 new markers to all recombination-carrying individuals as well as their phenotyping allowed the fine mapping of the markers to the top of LG3 which were clustered when mapped with the subpopulation. The resistance locus of MR5 was finally mapped between CH03e03 and CH03g07 with two new flanking markers with a distance of less than 1 cM.
DISCUSSION
The fine mapping of Malus × robusta 5 allowed the definitive determination of the position of the resistance locus on LG3. However, the results of Peil et al. (2007), who mapped the trait also as a single gene were not confirmed. The region is determined by a small window of less than 1 cM which can be used for positional cloning of the resistance gene. The presence or absence of the favorable allele (in marker assisted breeding experiments) can be followed by the two markers CH03g07 and CH03e03 (Liebhard et al., 2002).
ACKNOWLEDGMENTS
Thanks to the Genetic Diversity Center of the Swiss Federal Institute of Technology Zurich (GDC) who provided useful hardware and Swiss Federal Office of Agriculture (ZUEFOS Project) for financial support.
Literature Cited
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Durel, C.E., Denance, C. and Brisset, M.N. 2009. Two distinct major QTL for resistance to fire blight co-localize on linkage group 12 in apple genotypes ‘Evereste’ and Malus floribunda clone 821. Genome 52(2):139-147.
Khan, M.A., Duffy, B., Gessler, C. and Patocchi, A. 2006. QTL mapping of fire blight resistance in apple. Mol. Breed. 17:299-306.
Liebhard, R., Gianfranceschi, L., Koller, B., Ryder, C.D., Tarchini, R., Van de Weg, W.E.
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Van Ooijen, J.W. and Voorrips, R.E. 2001. JoinMap 3.0, Software for the Calculation of Genetic Linkage Maps. Plant Research International, Wageningen, the Netherlands.
