The identification of the Rosa S-locus provides new insights into the breeding and wild origins of continuous-flowering roses

This study aims to: (i) identify the Rosa S-locus controlling self-incompatibility (SI); (ii) test the genetic linkage of the S-locus with other loci controlling important ornamental traits, such as the continuous-flowering (CF) characteristic; (iii) identify the S-alleles (S(C)) of old Chinese CF cultivars (e.g, Old Blush, Slater’s Crimson China) and examine the changes in the frequency of cultivars with Sc through the history of breeding; (iv) identify wild species carrying the Sc-alleles to infer wild origins of CF cultivars. We identified a new S-RNase (S(C2)) of Rosa chinensis in a contig from a genome database that has not been integrated into one of the seven chromosomes yet. Genetic mapping indicated that S(C2) is allelic to the previously-identified S-RNase (S(C1)) in chromosome 3. Pollination experiments with half-compatible pairs of roses confirmed that they are the pistil-determinant of SI. The segregation analysis of an F(1)-population indicated genetic linkage between the S-locus and the floral repressor gene KSN. The non-functional allele ksn is responsible for the CF characteristic. A total of five S-alleles (S(C1–5)) were identified from old CF cultivars. The frequency of cultivars with S(C) dramatically increased after the introgression of ksn from Chinese to European cultivars and remains high (80%) in modern cultivars, suggesting that S-genotyping is helpful for effective breeding. Wild individuals carrying S(C) were found in Rosa multiflora (S(C1)), Rosa chinensis var. spontanea (S(C3)), and Rosa gigantea (S(C2), S(C4)), supporting the hypothesis of hybrid origins of CF cultivars and providing a new evidence for the involvement of Rosa multiflora.