FLAVONOID DIVERSITY INRELATION TO SYSTEMATICSAND EVOLUTION OFTHE TARWEEDS1'2William J. Crins3 andBruce A. Bohm4ABSTRACT The tarweeds produce a diverse array of flavonoids, including glycosylated compounds and methylated aglycones.Extra hydroxylation at positions 6 and/or 8, with frequent O-methylation at these and other positions, is prevalent.This structural diversity coupled with the variety of classes of flavonoids accumulated (flavanones, dihydroflavonols,flavones, flavonols) provides a rich source of characters for study of relationships within and among genera. Thedistribution of flavonoid aglycones within Dubautia is consistent with morphological, cytogenetic, electrophoretic, andphytogeographic evidence bearing on the evolution of its taxa. Flavonoids have also provided a means of assessingthe dynamics of hybridization in Dubautia. In Hemizonia, multivariate analyses of flavonoid arrays have corroboratedthe view, based on cytogenetic and morphological criteria, that each perennial taxon is independently derived. Ongoinginvestigations suggest that Argyroxiphium, Calycadenia, Holocarpha, Layia, Wilkesia, and other genera eachexhibit distinctive substitution and accumulation tendencies. Flavonoids may be helpful in developing and testingphylogenetic hypotheses in the Madiinae. The North American Madiinae (Asteraceae: He-liantheae) consist of about a dozen genera distrib-uted from Baja California to British Columbia, witha major concentration of taxa in California. LayiaHook. & Arn. and Madia Molina, two of the largergenera, contribute conspicuously to spring floraldisplays in California. The Hawaiian Madiinae con-sist of three genera: Argyroxiphium DC. with fivespecies, Dubautia Gaudich. with 21 species (33taxa in ail), and Wilkesia A. Gray with only twospecies. Arguably the most spectacular membersof the island taxa are the subspecies of A. sand-wicense: subsp. sandwicense grows on Mauna Keaon the island of Hawaii; subsp. macrocephalum(A. Gray) Meyrat, grows on Haleakala on Maui.Both of these have suffered serious predation andare maintained only with special effort. Species ofDubautia occur on ail major islands of the archi-pelago. They exhibit a very wide range of mor-phological forms and are ecologically the most di-verse within the Hawaiian tarweeds. Wilkesiaconsists of two species, W. gymnoxiphium A. Grayand W. hobdyi H. St. John, both of which arerestricted to western Kauai. The former is muchmore common, occurring in comparatively largepopulations along the western edge of WaimeaCanyon. Wilkesia hobdyi is known from only twosites on the far western ends of two ridges andconsists of perhaps no more than 200 individuals;it is under severe predation from ferai goats andmay be on the verge of extirpation. The Madiinae provide ideal opportunities for thestudy of evolutionary divergence. In a few genera,hypotheses of species formation have been pro-posed, and recently tests of these ideas have beenconducted using electrophoretic data (Warwick &Gottlieb, 1985; Witter & Carr, 1988). However,it is surprising that in a group as well studied asthe tarweeds, so few explicit hypotheses of phy-logenetic relationships have been proposed. Therenow exists a large enough body of data that suchhypotheses could be constructed, and these couldserve as frameworks for assessment of congruenceamong different types of data and for testing modelsof speciation. Flavonoid data could serve in bothroles. Since the genetic bases of their biosynthesisare well known (Heller, 1986), these compoundsultimately can be used to construct phylogeniesand to assess phylogenies based on other data.SURVEY AND BIOSYNTHETIC TRENDS Before dealing with the flavonoids from a phy-logenetic viewpoint, we will discuss their structural This research was supported by operating and equipment grants from the Natural Sciences and EngineeringResearch Council of Canada to B.A.B. Chemosystematic studies of the tarweeds (Asteraceae: Heliantheae: Madiinae) Number 6. SNew York State Museum, Biological Survey, Albany, New York 12230, U.S.A. 'Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B1. ANN. MISSOURI BOT. GARD. 77: 73-83. 1990.
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