Evolution of Floral Color Patterning in Chilean <em>Mimulus</Em>
Evolution can be studied at many levels, from phenotypic to molecular, and from a variety of disciplines. An integrative approach can help provide a more complete understanding of the complexities of evolutionary change. This dissertation examines the ecology, genetics, and molecular mechanisms of the evolution of floral anthocyanin pigmentation in four species of Mimulus native to central Chile. Anthocyanins, which create red and purple colors in many plants, are a valuable model for studying evolutionary processes. They are ecologically important and highly variable both within and between species, and the underlying biosynthetic pathway is well characterized. The focus of this dissertation is dramatic diversification in anthocyanin coloration, in four taxa that are closely related to the genomic model system M. guttatus. I posed three primary questions: (1) Is floral diversification associated with pollinator divergence? (2) What is the genetic basis of the floral diversification? (3) What is the molecular mechanism of the increased production of anthocyanin pigment? The first question was addressed by evaluating patterns of pollinator visitation in natural populations of all four study taxa. The second question was explored using segregation analysis for a series of inter- and intraspecific crosses. One trait, increased petal anthocyanins in M. cupreus, was further dissected at the molecular level, using candidate gene testing and quantitative gene expression analysis. Pollinator studies showed little effect of flower color on pollinator behavior, implying that pollinator preference probably did not drive pigment evolution in this group. However, segregation analyses revealed that petal anthocyanin pigmentation has evolved three times independently in the study taxa, suggesting an adaptive origin. In addition to pollinator attraction, anthocyanins and their biochemical precursors protect against a variety of environmental stressors, and selection may have acted on these additional functions. Molecular analysis of petal anthocyanins in M. cupreus revealed that this single-locus trait maps to a transcription factor, McAn1, which is differentially expressed in high- versus low-pigmented flowers. Expression of the anthocyanin structural genes is tightly correlated with McAn1 expression. The results suggest that M. cupreus pigmentation evolved by a mutation cis to McAn1 that alters the intensity of anthocyanin biosynthesis.
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