Genetics, Cytogenetics, and Genetic Diversity in the Genus Ocimum

Abstract

The genus Ocimum of the family Lamiaceae has been known since centuries primarily for its essential oil, which is used extensively in pharmaceutical, culinary, and perfumery industries. In addition to the wild Ocimum spp., commercially cultivated varieties, popularly known as basils, are found in different geographical regions of the world. The genus is well known for its morphological and chemical diversity. This widespread variability is attributed to inter- and intraspecific hybridization, polyploidy, aneuploidy, synonymous names, various varieties and cultivars, and huge plethora of chemotypes. These have resulted in ambiguities in classification of the genus and consequently a complex taxonomic scenario. Initially, basils were identified according to their geographic origin that had distinct chemotypes in the world market. Over the years, morphological, chemical, karyological, and molecular means are being applied to get a better understanding of the genetic relationship among basils. The base chromosome numbers suggested are x = 12 (O. basilicum clade as tetraploid; O. americanum clade as hexaploids), x = 10 (O. gratissimum), and x = 9 (O. tenuiflorum). Variations in genome size and chromosome number among Ocimum spp. imply that sequence deletion/amplification, chromosome rearrangements, and polyploidization have played a role in the course of evolution. Recent revision by nuclear DNA content has divided the section Ocimum into two clades—the first comprising O. basilicum and O. minimum, whereas the second comprising O. americanum, O. africanum, and two O. basilicum var. purpurascens accessions. O. tenuiflorum was found to be the most divergent species. It was also seen that O. basilicum clade species are tetraploids, while species belonging to O. americanum clade are hexaploids. For the first time, DNA linkage map for sweet basil has been constructed, which is anchored by SSRs (42 EST-SSR) and saturated by SNPs (1847) spanning 3030.9 cM and QTLs identified for basil downy mildew response. The plant genetic resources existing in Ocimum have been taken up for diversity studies and constitute the much required raw materials for future breeding programs for desired chemotypes, disease resistance, and better agronomic traits.