Mosaic Eucalypt Trees Suggest Genetic Control at a Point That Influences Several Metabolic Pathways

Mosaic trees contain more than one phenotype. The two Eucalyptus mosaic trees studied here (E. melliodora and E. sideroxylon) are predominantly susceptible to insect herbivory, with the leaves on a single large branch on each tree resisting herbivory. We used gas chromatography–mass spectrometry and high-pressure liquid chromatography to analyze the chemical profile of leaves of the mosaic trees, as well as leaves of adjacent non-mosaic con-specifics. We show that the leaves of the two phenotypes are distinctly different. Compared to the susceptible (S) leaves on the same tree, the resistant (R) leaves on the mosaic trees had low concentrations of sesquiterpenes (E. melliodora: 2 vs. 24 mg·g−1 dry matter; E. sideroxylon: 8 vs. 22 mg·g−1 dry matter), high concentrations of FPCs (E. melliodora: 5.4 vs. 0.3 mg·g−1 dry matter; E. sideroxylon: 9.8 vs. 0.2 mg·g−1 dry matter) but similar concentrations of nitrogen (E. melliodora: 15.4 vs. 16.8 mg·g−1 dry matter; E. sideroxylon: 13.1 vs. 14.0 mg·g−1 dry matter). The only difference between the two mosaic trees was in the levels of monoterpenes. The R leaves from the mosaic E. melliodora contained higher concentrations of monoterpenes compared to the S leaves (12 vs. 6 mg·g−1 dry matter). In contrast, the leaves from the E. sideroxylon mosaic contained much higher concentrations of monoterpenes with a reversed pattern (R: 26 vs. S: 45 mg·g−1 dry matter). There were qualitative differences too on the mosaic trees. The R leaves of both species contained much higher concentrations of the monoterpene, 1,8-cineole, whereas the S chemotype of E. sideroxylon only had high concentrations of phellandrenes. Furthermore, the chemical differences between leaves on the R and S branches of the mosaic trees resembled those between the leaves of R and S con-specific trees in the same population. We use these data and knowledge of secondary metabolite biosynthesis to propose that high-level transcriptional differences may control the profile of specialized metabolites in eucalypts.