Hybridization in North American Asclepias. III. Isozyme evidence
1992
Wyatt, R. | Broyles, S.B.
We sampled 100 adult milkweeds along a 3 km transect through the Dolly Sods Wilderness Area in northeastern West Virginia. These plants were measured for 13 flower and 17 leaf characters. We also used horizontal starch-gel electrophoresis to score variation in 11 enzyme loci. A composite character index was constructed that yielded negative scores for species-specific alleles of A. syriaca and positive scores for species-specific alleles of A. exaltata. The 30 plants from the middle portion of the transect were highly variable, with many plants achieving scores near zero. These plants presumably represent a complex hybrid swarm, consisting of F1 interspecific hybrids and backcrosses. Plants with intermediate character-index scores were significantly more likely not to flower than were plants with higher (A. exattata-like) or, especially, lower (A. syriaca-like) scores. On the basis of a study of reproductive characters alone, one might conclude that there is no evidence of hybridization, as there appear to be no intermediates among the 54 flowering plants. Vegetative characters, however, reveal a more complex pattern of variation that agrees better with the isozyme analysis of all 100 plants. It does appear that hybridization and localized introgression are occuring between A. exaltata and A. syriaca in the Dolly Sods region. Several unique features of milkweed reproduction appear to promote interspecific hybridization and to magnify its effects.
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