Diploid hybrid speciation in Penstemon (Scrophulariaceae) revisited

Diploid hybrid speciation remains a central issue in plant evolutionary biology. Here we test hypotheses of diploid hybrid speciation and introgressive hybridization in Penstemon section Peltanthera where P. spectabilis is a purported hybrid derivative species of the progenitors P. centranthifolius and P. grinnellii, and P. clevelandii is regarded as a hybrid derivative of P. centranthifolius and P. spectabilis. Forty-five populations representing P. centranthifolius (subsection Centranthifolii); P. spectabilis, P. clevelandii, P. grinnellii, P. palmeri in subsection Spectabiles; and P. × parishii (a purported hybrid between P. centranthifolius and P. spectabilis) were examined for allozyme differentiation. Penstemon centranthifolius has high-frequency allozymes at seven isozyme loci that differentiate it from species in subsection Spectabiles, whereas no genetic markers are detected that reliably differentiate species within Spectabiles. Because our data reveal that P. centranthifolius is genetically distinct compared to species in Spectabiles, we conclude that a recent hybrid origin of P. spectabilis and P. clevelandii with P. centranthifolius as a progenitor is not supported. Bidirectional gene flow and introgression between P. centranthifolius and species of subsection Spectabiles is indicated, however, based on the low-frequency occurrence of P. centranthifolius marker alleles in species of Spectabiles and of Spectabiles marker alleles in P. centranthifolius in areas of sympatry.