Genetic control and evolution of sesquiterpene biosynthesis in Lycopersicon esculentum and L-hirsutum

Segregation analysis between Lysopersicon esculentum (cultivated tomato) an d L. hirsutum (wild form) in conjunction with positional verification by us ing near-isogenic lines demonstrated that biosynthesis of two structurally different classes of sesquiterpenes in these species is controlled by loci on two different chromosomes. A locus on chromosome 6, Sesquiterpene syntha se1 (Sst1), was identified for which the L. esculentum allele is associated with the biosynthesis of beta -caryophyllene and alpha -humulene. At this same locus, the L. hirsutum allele is associated with biosynthesis of germa crene B, germacrene D, and an unidentified sesquiterpene. Genomic mapping, cDNA isolation, and heterologous expression of putative sesquiterpene synth ases from both L. esculentum and L. hirsutum revealed that Sst1 is composed of two gene clusters 24 centimorgans apart, Sst1-A and Sst1-B, and that on ly the genes in the Sst1-A cluster are responsible for accumulation of chro mosome 6-associated sesquiterpenes. At a second locus, Sst2, on chromosome 8, the L. hirsutum allele specified accumulation of alpha -santalene, alpha -bergamotene, and beta -bergamotene. Surprisingly, the L. esculentum allel e for Sst2 is not associated with the expression of any sesquiterpenes, whi ch suggests that cultivated tomato may have a nonfunctional allele. Sesquit erpene synthase cDNA clones on chromosome 6 do not cross-hybridize on genom ic DNA gel blots with putative sesquiterpene synthases on chromosome 8, an indication that the genes in Sst1 and Sst2 are highly diverged, each being responsible for the biosynthesis of structurally different sets of sesquite rpenes.