title: The disease resistance gene Dm3 is infrequent in natural populations of Lactuca serriola due to deletions and frequent gene conversions at the RGC2 locus.
abstract: Resistance genes can exhibit heterogeneous patterns of variation. However, there are few data on their frequency and variation in natural populations. We analysed the frequency and variation of the resistance gene Dm3, which confers resistance to Bremia lactucae (downy mildew) in 1033 accessions of Lactuca serriola (prickly lettuce) from 49 natural populations. Inoculations with an isolate of Bremia lactucae carrying avirulence gene Avr3 indicated that the frequency of Dm3 in natural populations of L. serriola was very low. Molecular analysis demonstrated that Dm3 was present in only one of the 1033 wild accessions analysed. The sequence of the 5' region of Dm3 was either highly conserved among accessions, or absent. In contrast, frequent chimeras were detected in the 3' leucine-rich repeat-encoding region. Therefore low frequency of the Dm3 specificity in natural populations was due to either the recent evolution of Dm3 specificity, or deletions of the whole gene as well as variation in 3' region caused by frequent gene conversions. This is the most extensive analysis of the prevalence of a known disease resistance gene to date, and indicates that the total number of resistance genes in a species may be very high. This has implications for the scales of germplasm conservation and exploitation of sources of resistance.
authors.affiliation: Department of Plant Science and Genome Center, University of California, Davis, CA 95616, USA.
authors.forename: Hanhui
authors.initials: H
authors.lastname: Kuang
authors.forename: Oswaldo E
authors.initials: OE
authors.lastname: Ochoa
authors.forename: Eviatar
authors.initials: E
authors.lastname: Nevo
authors.forename: Richard W
authors.initials: RW
authors.lastname: Michelmore
mesh_terms: D020121:5' Untranslated Regions; D001483:Base Sequence; D001665:Binding Sites; D017124:Conserved Sequence; D005785:Gene Conversion; D017353:Gene Deletion; D005787:Gene Frequency; D017343:Genes, Plant; D014644:Genetic Variation / Q000276:immunology*; D018545:Lactuca / Q000235:genetics* / Q000276:immunology / Q000382:microbiology; D000090263:Leucine-Rich Repeat Proteins; D008969:Molecular Sequence Data; D009868:Oomycetes / Q000502:physiology; D016133:Polymerase Chain Reaction; D011506:Proteins / Q000235:genetics
chemical_list: D020121:5' Untranslated Regions; D000090263:Leucine-Rich Repeat Proteins; D011506:Proteins