Figure 1.

Schematic of diploid hybrid speciation hypotheses in Penstemon. Gray arrows represent directionality of hybrids as proposed by Straw (35–37). Length of the black arrows represent relative degree of gene flow between species supported by four molecular data sets (allozymes, nuclear rDNA and cpDNA restriction-site variation, and ISSR bands). Hypothesized hybrid species I: P. centranthifolius has red, tubular corollas and is primarily pollinated by hummingbirds, whereas P. grinnellii has pink flowers pollinated by large carpenter bees. The hypothesized hybrid derivative, P. spectabilis, purportedly was reproductively isolated from both progenitor species by adaptation to wasp pollination. The second putative hybrid species II, P. clevelandii, is primarily pollinated by solitary bees but is also visited by hummingbirds. P. clevelandii is adapted to different habitats than its purported progenitors, and ecological isolation is the proposed stabilizing factor in this hypothesis of hybrid speciation. Diploid hybrid speciation hypotheses I and II were supported by morphological similarities of the purported hybrid species with natural and artificial F₁s between the parental taxa and an examination of isolating mechanisms among species in the hybrid complex (35–37). The pattern of molecular markers in the hybrid complex supports the hypothesis of diploid hybrid speciation for P. clevelandii but not for P. spectabilis, and our results combined with previous studies (38–40) suggest pollen-mediated gene flow between P. centranthifolius and both P. spectabilis and P. grinnellii. We also observed low levels of introgression of ISSR molecular markers between each pair of taxa in section Spectabiles in the hybrid complex, except for P. grinnellii and P. clevelandii.