A mating system conundrum: hybridization in Apocynum (Apocynaceae)

Based upon an intermediate morphology, Apocynum x-floribundum Greene has long been considered a hybrid involving A. androsaemifolium and A. cannabinum. The floral morphology in this genus, however, appears to prohibit both import and export of pollen, and observations of numerous insect visitors reveal that pollen is not routinely carried from flower to flower. Furthermore, reproductive success as measured by fruit set is very low in most populations. Hybridization was thus called into question, with allozyme evidence used to test the hypothesis of a hybrid origin for A. x-floribundum. Six diagnostic loci, as well as two loci exhibiting highly disparate allele frequencies, were resolved for each parent. All examined populations of A. x-floribundum were heterozygous at these loci, thus supporting the hypothesis of hybridization. Evidence from additional loci indicated that all populations tend to be strongly clonal. Observed heterozygosity was very low in the parental species, suggesting a history of inbreeding or a severe bottleneck. There was no support for earlier assumptions that some intermediates are derived from backcrosses or "secondary hybrids." Statistical analyses of plant height, leaf shape, petal length, sepal length, follicle length, seed length, and seed number per follicle supported these conclusions. The persistence and vegetative spread of hybrid clones may contribute to the illusion that hybridization is common.