Hybridization in North American Asclepias. I. Morphological Evidence

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Abstract
Biometric study of natural populations and artificial hybrids revealed hybridization and possible introgression between Asclepias exaltata and A. syriaca. Herbarium specimens and plants from natural populations in Virginia, West Virginia, and Michigan [USA] were measured for 17 vegetative characters. The elliptic leaves of A. exaltata were glabrous to sparsely pubscent and tapered to an acute base and broadly acuminate apex. In contrast, the densely tomentose leaves of A. syriaca were ovate or broad throughout their length, typically with a rounded base and a cuspidate apex. Principal components analysis showed that the two parental species are highly distinct and suggested an intermediate placement of putative F1 hybrids. More complex patterns from certain natural populations suggested that backcrossing and introgression had occurred. A total of 20 reproductive characters also was measured for natural populations and for F1 hybrids from artificial pollinations. Again, parental populations were highly distinctive and putative hybrid populations were intermediate for most characters, including umbel size, pedical length, petal length, horn length, anther head length, number of hood teeth, median and lateral hood heights, hood width, the ratio of hood height to horn extension, and the width to length ratios for anther head and column. Bumblebees and butterflies were observed on both species and their hybrids, and considerable overlap in flowering time occurred. Experimental cross-pollinations were successful using either species as the maternal parent. Seeds from these interspecific crosses germinated and grew into viable, fertile, F1 hybrids, which were successfully backcrossed to A. syriaca. Vegetative and reproductive morphology of these plants was similar to that observed for naturally occurring hybrids and backcrosses. Contrary to prevailing dogma, It appears that hybridization does occur in milkweeds.