Origin of the Texas Gulf coast island populations of Ambrosia psilostachya dc. (Compositae): a biochemical and numerical taxonomic investigation.




Potter, J.M.L.

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University of Texas at Austin


The populations of the ragweed A. psilostachya, which occur on the Texas gulf coast islands are believed to have been derived from a single, localized source as evidenced by the consistency in the morphology and chromosome numbers, and in the production of the structurally unusual sesquiterpene dilactones. The same sesquiterpene dilactones are produced by a few Texas mainland A. psilostachya populations and also by populations of a closely related species. A. cumanensis, in Vera Cruz, Mexico. The volatile oils and sesquiterpene lactones of A. psilostachya and A. cumanensis in Texas and Mexico were investigated to bring additional evidence to bear on the problem of the origin of the island populations of A. psilostachya. The volatile oils of each population were steam distilled from fresh plant material and analyzed by gas-liquid chromatography (GLC). The constituents of the volatile oils were isolated by preparative GLC and identified by comparison with known compounds, utilizing infra-red and NMR spectral data and GLC retention times. Nineteen mono- and sesquiterpenes were identified; the chemical structures of an additional twenty-five components were partially characterized. The volatile oil patterns were analyzed by two numerical techniques: 1/the Student-Newman-Keuls multiple range test for determining significant differences among the population means for 80 components; and 2) the calculation of similarity ratios (which were subsequently clustered by the single-linkage technique) based on the qualitative variations. The results indicate that the Texas Gulf coast island A. psilostachya populations are more similar to A. cumanensis (in Vera Cruz, Mexico) than they are to the populations of A. psilostachya on the Texas mainland. The sesquiterpene lactones were extracted from air-dried plant material and the resultant crude extracts were analyzed by NMR spectroscopy. Five south Texas populations were found to produce isabelin, which exhibits a germacronolide chemical structure. The other populations contained sesquiterpene lactones of the pseudoguaianolide structural type, which occurred either as monolactones (in all except three Texas mainland populations) or as dilactones (in the Texas island A. psilostachya and the Vera Cruz, Mexico, populations of A. cumanensis). The results of the analyses of the volatile oils and sesquiterpene lactones have indicated that 1) as a species A. cumanensis is characterized by the sesquiterpene dilactone chemistry; 2) a unique volatile oil pattern is produced by the same populations which contain the sesquiterpene lactone isabelin; 3) the sporadic occurrence of sesquiterpene dilactone-producing plants on the Texas mainland is probably the results of hybridization with the island populations by means of pollen carried landward by the prevailing winds; 4) the Texas Gulf coast island populations of A. psilostachya may have been derived from the same ancestral stock which also gave rise to the Vera Cruz, Mexico, populations of A. cumanensis.


144 p., Dissertation


botany, Ambrosia psilostachya, western ragweed, Ambrosia cumanensis, population dynamics, biological speciation