French Rose (Rosa L. sp) Germplasm from the 19th Century Shows a Continuous Shift throughout the Time from a European to an Asian Genetic Background

Innovation in ornamental plant breeding is commonly obtained by hybridization with newly introduced genetic resources. In France, the 19^th century was a golden age for rose breeding and this period corresponds to the introduction and subsequent use for breeding of Asian roses in Europe. Our objective here was to study and explain the evolution of roses genetic diversity during this period of time. A large sample of 1,228 garden roses sampled in ten French rose gardens was constituted. This sample summarized the conserved diversity of roses cultivated in Europe during the 18^th and 19^th centuries as it included 991 European garden roses from this period of time. As a comparison, Asian, botanical and modern roses were included to the study. Genotypes of various ploidy levels, ranging within 2 to 6 x, were found in the sample. The individuals were genotyped with 32 single sequence repeat (SSR) primer pairs and the genetic diversity and structure of the sample were assessed. A wide diversity, structured into sixteen genetic groups, was observed. A genetic differentiation was detected between ancient European and Asian accessions and a continuous temporal shift was observed in cultivated hybrids from a European to an Asian genetic background during the 19^th century. Frequent crosses with Asian roses along the 19^th century and/or selection for Asiatic traits may have induced this shift. Implications for rose breeding and germplasm conservation will be discussed.