Selective sweep at the Rpv3 locus during grapevine breeding for downy mildew resistance.

The Rpv3 locus is a major determinant of downy mildew resistance in grapevine (Vitis spp.). A selective sweep at this locus was revealed by the DNA genotyping of 580 grapevines, which include a highly diverse set of 265 European varieties that predated the spread of North American mildews, 82 accessions of wild species, and 233 registered breeding lines with North American ancestry produced in the past 150 years. Artificial hybridisation and subsequent phenotypic selection favoured a few Rpv3 haplotypes that were introgressed from wild vines and retained in released varieties. Seven conserved haplotypes in five descent groups of resistant varieties were traced back to their founders: (1) 'Munson', a cross between two of Hermann Jaeger's selections of V. rupestris and V. lincecumii made in the early 1880s in Missouri, (2) V. rupestris 'Ganzin', first utilised for breeding in 1879 by Victor Ganzin in France, (3) 'Noah', selected in 1869 from intermingled accessions of V. riparia and V. labrusca by Otto Wasserzieher in Illinois, (4) 'Bayard', a V. rupestris × V. labrusca offspring generated in 1882 by George Couderc in France, and (5) a wild form closely related to V. rupestris accessions in the Midwestern United States and introgressed into 'Seibel 4614' in the 1880s by Albert Seibel in France. Persistence of these Rpv3 haplotypes across many of the varieties generated by human intervention indicates that a handful of vines with prominent resistance have laid the foundation for modern grape breeding. A rampant hot spot of NB-LRR genes at the Rpv3 locus has provided a distinctive advantage for the adaptation of native North American grapevines to withstand downy mildew. The coexistence of multiple resistance alleles or paralogues in the same chromosomal region but in different haplotypes counteracts efforts to pyramidise them in a diploid individual via conventional breeding.