| Diploid hybrid speciation remains a central issue in plant evolutionary biology. In this research, I have tested 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. Three distinct sources of data were used to evaluate these hypotheses of speciation and introgression: allozymes, nuclear ribosomal DNA (rDNA) and chloroplast DNA (cpDNA) restriction site variation. Effective evaluation of diploid hybrid speciation using allozyme and molecular data requires that there be unequivocal markers to differentiate parental species from one another. I found nuclear genetic markers at seven isozyme loci that differentiated P. centranthifolius (subsection Centranthifolii) from the other species examined, but no allelic differences that could be used to differentiate P. grinnellii. P. spectabilis, P. clevelandii and P. palmeri (subsection Spectabiles) from one another. However, unambiguous molecular markers were found using rDNA and cpDNA restriction site variation that clearly differentiated all of the purported parental taxa. The results from investigations employing nuclear genes did not support the hybrid speciation hypotheses, but provided evidence suggesting that introgression between P. centranthifolius and the species in subsection Spectabiles occurs. However, when the data sets derived from nuclear genes were added to the data gathered from the chloroplast genome, there was sufficient evidence to conclude that P. spectabilis ssp. subviscosus is a hybrid derivative species with P. grinnellii, and P. spectabilis ssp. spectabilis as the parental taxa. From my results, I formulated criteria to differentiate among the evolutionary processes of recent and ancient diploid hybridization, and introgression. I also inferred from the my results that the patterns of introgression observed for section Peltanthera are predominantly from pollen-mediated gene flow. Additionally, a comparison of the phylogenetic reconstructions based on rDNA and cpDNA data suggest several chloroplast-capture events having occurred in Penstemon section Peltanthera. |
