The role of pollinator-mediated reproductive isolation in plant speciation: Phylogeny, ecology and floral evolution in Antirrhinum (Veronicaceae)

The fundamental pursuit of biology is the understanding of a complex system: the diversity of life. One of the more enduring conundrums has been the study of the process of speciation, or how this diversity is generated and maintained. In plants, the involvement of animal pollinators presents an extra dimension of complexity to their reproduction. This led to a paradigm of the flower as an adaptive structure and, given its centrality in angiosperm reproduction, a conviction that animal pollination holds a key role in the evolution and speciation of plants. The correlation between increasing insect diversity and increasing plant diversity in the paleontological record has often been cited as causational evidence.; The genus Antirrhinum (Veronicaceae) provides an excellent system with which to take an integrative approach to the study of the role of pollinators in the speciation of plants. The existence of A. majus , a model organism for the study of the genetics of development, within the genus and the diversity of floral morphologies, flower colors and geographic distributions found in the group provide many research opportunities. Two species of snapdragon native to northern California (A. subcordatum and A. vexillo-calyculatum) appear to be in the process of speciation. These two species are sympatric and differ mainly in their floral color with A. subcordatum having white flowers and A. vexillo-calyculatum having purple flowers.; These two species are investigated from a variety of approaches to obtain a comprehensive picture of their natural history. Recognizing the need for a good phylogenetic hypothesis of relationships, the Internal Transcribed Spacer region (ITS) is used to reconstruct a phylogeny for the genus Antirrhinum and some close relatives. The resulting tree provides a more quantitative way to assess the relatedness of the two species being studied than what is available for most plant groups being studied for pollinator mediated speciation. Along with ITS, the third intron of the gene Nitrate Reductase is employed to investigate the relationships among the very closely related populations of A. subcordatum and A. vexillo-calyculatum. Pollinator observations and array experiments are used to explore the effect of the flower color difference on the behavior of the animal pollinators. Finally, some preliminary work on the potential genetic basis for the interspecific difference is presented. Integrating these multiple perspectives allows us to see that many elements of recently proposed and realistic models of sympatric speciation are present in this system.