Genetic Relationships Between Basil (Ocimum spp.) Germplasm Provide Insights for Downy Mildew Resistance Breeding

Basil is a diverse genus of approximately 64 species that exhibit extensive variation in phenotype and essential oil chemistry. The most economically important species, Ocimum basilicum, includes sweet basil that is used primarily in fresh, culinary, and food flavoring industries. Sweet basil production is being threatened by the destructive disease downy mildew (Peronospora belbahrii), which was first introduced to the United States in 2007 and now affects all major basil growing regions. Resistant breeding lines are in development at Rutgers University through introduction of genetic resistance by inter- and intraspecific hybridization. A major obstacle to breeding efforts has been the prevalence of sexual incompatibility and hybrid sterility when attempting to introgress resistance genes from Ocimum spp. to commercial sweet basil. To better understand genetic relationships and characterize resistant and tolerant genotypes within the Ocimum genus, a genetic diversity study was performed using simple sequence repeat (SSR) markers developed from the NCBI basil expressed sequence tag (EST) database. In silico analysis provided 786 SSR primer pairs, from which 16 polymorphic EST-SSR markers were used to evaluate 95 basil genotypes including commercial varieties, USDA–NPGS accessions and Rutgers breeding lines, all of which were evaluated for response to downy mildew in greenhouse and/or field screening. Population structure analysis was performed using NTSYSpc, STRUCTURE 2.3.4 software and an AMOVA was used to determine whether resulting basil populations were statistically significant. Results indicate that sources of downy mildew resistance can be found throughout the Ocimum genus, but most commonly outside of the O. basilicum species.