FLAVONOID CHEMISTRY AND SYSTEMATICS OF TITHONIA (COMPOSITAE)

Twenty-four flavonoid compounds, including chalcones and aurones, are reported from 13 taxa of Tithonia. Pedunculin (4', 6, 7-trimethoxy, 5, 8-dihydroxyflavone), also is characterized. The distribution of these compounds is compared to the existing classification of the genus based primarily on morphological data. By comparison with the previously proposed phylogeny of the genus, the evolution of the flavonoid system may be elucidated. It is proposed that the ancestral stock of Tithonia had flavonols, diand tri-methoxyflavones, and 6-methoxyflavones. From this ancestral stock, a number of trends have occured in derivative taxa. These trends are: 1) a maintenance of a diversity of compounds in several evolutionary lines, 2) a reduction in the number of methoxyl substitutions, and 3) the elaboration of 5-deoxyflavones and flavonols. THE GENUS TITHONIA Desf. ex Jussieu (Compositae, Heliantheae) is composed of 11 species which are distributed primarily in Mexico and Central America (La Duke, 1980). The genera most closely related to Tithonia are Helianthus and Viguiera (Blake, 1918; La Duke, 1980), from which the former can be most easily distinguished by the presence of a large (to 15 cm diam) flowering head supported by a hollow, flared peduncle (Blake, 1921; La Duke, 1980). Recently, two sections have been recognized within Tithonia (La Duke, 1982): section Tithonia and section Mirasolia. Section Tithonia includes perennial and annual taxa with triangular or sharply fourangled achenes, a pappus of laterally fused squamellae, and pales with aristate tips. Section Mirasolia includes perennial taxa with achenes which are rounded-quadrangular, lacking a pappus, and have predominately acute-tipped pales. One purpose of taxonomy is to achieve a natural classification which reflects phylogenetic relationships (e.g., Mickevich, 1978). These classifications should be predictive and may be "tested" by the addition of new data 1 Received for publication 31 January 1981; revision accepted 26 September 1981. Fieldwork for this investigation was supported largely by NSF Grant DEB-7618763. This work represents part of a Ph.D. thesis in the Department of Botany, The Ohio State University. The author wishes to thank T. F. Stuessy for his guidance throughout the study, D. J. Crawford for his assistance with flavonoid analysis and for reading the manuscript critically, R. Doskotch and J. Fowble for NMR and MS analyses (The Ohio State University), and R. Bye (University of Colorado) for providing material of T. fruticosa. Special thanks go to my wife, Barbara, for her help in preparing the manuscript. such as: chromosome numbers, eletrophoretic markers, or in this case, flavonoids. If the new data do not support the existing classification, then the classification, as well as the data should be reevaluated and the inconsistancy explained. Characters which have been shown to be under genetic control, such as flavonoids (Wong and Francis, 1968a, b; van Brederode and van Nigtevecht, 1974; Grisebach, 1979; Saleh, 1979) rather than characters which are environmentally influenced, are the most helpful in formulating a more predictive classification. The distribution and types of flavonoid compounds detected in Tithonia, therefore, should provide data for reevaluating the present classification. Tithonia proved to be excellent for this type of analysis because the genus is small enough to allow sampling of all taxa, and it is well understood taxonomically (La Duke, 1980). More information than just the distribution of the compounds in the taxa can be derived from the flavonoids. The evolutionary changes as reflected by the flavonoids, hereafter called the flavonoid system, can be inferred to some extent by superimposing these data on an already existing phylogenetic tree of the taxa based on other types of information (usually morphology). This procedure reveals the loss or gain of compounds or a reversal of either of these trends during evolution of the group. The purposes of this paper are to: 1) present comparative flavonoid data for each of the taxa of Tithonia; 2) compare these data to the existing classification of the genus; 3) compare an existing phylogenetic tree with the flavonoid data to determine evolution of the flavonoid system; and 4) compare concepts of relative