Hybridization in North American Asclepias. II. Flavonoid Evidence

Flavonoids were isolated from leaf extracts of three putative hybrid milkweeds and the four presumed parental species of Asclepias. A total of 16 flavonoids, all of which are glycosides of the flavonols quercetin and kaempferol, was identified. The hybrid A. exaltata x A. quadrifolia contained both of the marker compounds characteristic of A. quadrifolia and three of the four compounds usually produced by A. exaltata. In addition, two novel flavonoids were detected. Naturally occurring A. exaltata x A. syriaca produced three of four compounds diagnostic for A. exaltata and all but one of the common constituents of A. exaltata, yielding a profile similar to that of known hybrids produced by experimental crosses. Four novel compounds occurred in field-collected hybrids but were present only in trace amounts. Detailed analyses of a larger population of milkweeds in West Virginia suggested that F1 interspecific hybrids of A. exaltata and A. syriaca had backcrossed to the parents, yielding a complex hybrid swarm. The putative hybrid A. purpurascens x A. syriaca contained two of the four compounds characteristic of A. syriaca and four of the six variably present in A. purpurascens. Two novel flavonoids were observed. Analyses of flower extracts indicated that the novel compounds detected in leaves of hybrid milkweeds usually result from a breakdown in tissue-specific regulation. Chemical data therefore lend strong support to morphological evidence of hybridization between these species of Asclepias. Recent studies suggest that hybridization is common and widespread in many groups of angiosperms (Grant 1971; Stebbins 1950). A notable exception is the genus Asclepias, which includes 108 species in North America (Woodson 1954). Stevens (1945a, 1945b) produced artificial hybrids of A. speciosa Torrey x A. syriaca L. , and several botanists have collected from nature plants that closely resemble these hybrids (Adams et al. 1987; Woodson 1954). Aside from these, Woodson (1954) reported that he had not "encountered many more than a dozen plants that [he] would construe as interspecific hybrids amongst the thousands of specimens" he had examined either in the herbarium or in the field. The pollinating apparatus of milkweeds is so elegantly contrived that it has been only natural to assume that it plays a major role in preventing hybridization (Woodson 1954). Kephart and Heiser (1980), however, reviewed the literature on mechanical reproductive isolation in Asclepias and presented evidence that its importance has been exaggerated. They found high levels of interspecific pollination between sympatric species, which were never observed to produce hybrids in nature. Their conclusions therefore paralleled those of Moore (1946), Woodson (1954), and Wyatt (1976), who argued that postpollination physiological barriers are of primary importance in reproductive isolation of milkweeds. Nevertheless, recent morphological studies of A. exaltata L. and A. syriaca and certain intermediate populations in Virginia, West Virginia, and Michigan have demonstrated that hybridization does occur between species of milkweeds (Kephart et al. 1988). The use of flavonoid evidence to document hybridization has gained wide acceptance since its first application by Alston and Turfter (1963). The rationale is simple: hybrids should produce the summation of the compounds present in the two parental species. In many studies, such chemical complementation has been observed (e.g., Smith and Levin 1963). In other cases, however, a more complex pattern involving the production of "novel" compounds (i.e., compounds not accumulated at detectable levels by either parental species) by hybrids has been reported (Alston et al. 1965; King 1977; Levy and Levin 1971, 1974, 1975; and references therein). In most cases these novel compounds have been shown to result either from deletion or addition of steps along an existing biosynthetic pathway or from deregulation of tissue-specific flavonoid production. The purpose of this paper is to use chemical evidence to test the hypothesis that milkweed plants from Virginia and West Virginia that are morphologically intermediate between A. ex-