basilicum, different ploidies have been reported across other Ocimum species. While the x = 12 basic chromosome number is considered stable in tetraploid (2n = 4× = 48) O. In addition, cytological investigation suggests this genus has also undergone extensive genome augmentation resulting in ploidy variation. The flexibility of this reproductive system has been exploited by natural and artificial selection, likely serving as a catalyst for morphological and volatile diversification of Ocimum spp. demonstrate relatively high rates of outcrossing in the presence of pollinators, while readily self-pollinating in their absence. Plant breeding efforts have largely targeted sweet basil for improved disease resistance, chilling tolerance and novel volatile profiles (chemotypes). Ocimum basilicum L., sweet basil, is among the most economically important culinary herbs in the United States, Europe and Israel. A number of species are cultivated and processed throughout the world for application in medicinal, dietary supplement, essential oil, food flavoring and culinary industries. The primary center of diversity is described as being in Tropical Africa, while a secondary center exists in Tropical Asia and a tertiary center exists in the New World tropics. The genus Ocimum is estimated to contain approximately 64 herbaceous annual and perennial plant species. germplasm provide the foundation for improved DM resistance breeding strategies and more rapid response to future disease outbreaks. Elucidation of population structure and genetic relationships among Ocimum spp. ![]() ![]() EST-SSRs provided a robust evaluation of molecular diversity and can be used for additional studies to increase resolution of genetic relationships in the Ocimum genus. High levels of genetic diversity support the observed phenotypic diversity among Ocimum spp. The 90-accession k1 cluster was largely susceptible to downy mildew with accession ‘MRI’ representing the only source of DM resistance. The k2 cluster located between k1 and k3, represents a source of transferrable tolerance evidenced by fertile backcross progeny. Although the k3 cluster is a rich source of DM resistance introgression of resistance into the commercially important k1 accessions is impeded by reproductive barriers as demonstrated by multiple sterile F1 hybrids. UPGMA analysis provided best resolution for the 36-accession, DM resistant k3 cluster with consistently strong bootstrap support. basilicum cluster suggesting prevalence of gene flow. Evidence of cryptic population structure was demonstrated for the k1 O. Allelic frequency among clusters supported previously developed hypotheses of allopolyploid genome structure. Three (major) and seven (sub) population (cluster) models were identified and well-supported ( P < 0.001) by PhiPT (Φ PT) values of 0.433 and 0.344, respectively. Results obtained from nested Bayesian model-based clustering, analysis of molecular variance and unweighted pair group method using arithmetic average (UPGMA) analyses were synergized to provide an updated phylogeny of the Ocimum genus. ![]() ![]() accessions with varying response to downy mildew. In an effort to improve classification of relationships 20 EST-SSR markers with species-level transferability were developed and used to resolve relationships among a diverse panel of 180 Ocimum spp. Absence of such information has slowed breeding efforts and the development of sweet basil ( Ocimum basilicum L.) with resistance to the worldwide downy mildew epidemic, caused by the obligate oomycete Peronospora belbahrii. Characterization of population structure and genetic diversity among a representative sample of this genus is severely lacking. The basil ( Ocimum spp.) genus maintains a rich diversity of phenotypes and aromatic volatiles through natural and artificial outcrossing.
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