A central goal in evolutionary biology is to understand how organisms adapt to their environments. Progress toward this goal requires an integrative set of experiments that connects the ecological significance of putatively adaptive traits with a functional characterization of their molecular basis. My research addresses these issues by investigating the evolutionary consequences of transitions in flower color in several species, with a focus on the phenotypically variable species Mimulus aurantiacus. Because M.aurantiacus is closely related to the emerging model system, Mimulus guttatus, I am able to take advantage of valuable sources of new genomic tools to address these issues.
An additional area of interest involves the molecular evolution of transcriptional regulation, especially as it relates to how functional constraint and pleiotropy determine the types of mutations fixed during evolutionary change.
Ecological genetics of phenotypic differentiation
While it generally has been assumed that the tremendous diversity in flower color among angiosperms largely is a result of adaptive change associated with attracting animal pollinators, surprisingly few studies provide direct evidence that flower color divergence is adaptive, and in most cases non-adaptive change cannot be ruled out. By contrast, my previous studies in M. aurantiacus provide several lines of evidence that point directly toward an adaptive change in flower color between populations.
Ongoing lab and field studies are seeking to further clarify the role of selection on divergence. In particular, we are using: i) molecular population genetics to identify signatures of selection in genomic regions implicated in causing these flower color differences, and ii) we are using our knowledge of the molecular basis of flower color differences to more definitively test the direct and indirect effects of selection on these individual alleles in the field.
Molecular characterization of adaptive traits
The goal of these projects is to functionally characterize the genetic changes responsible for adaptive differentiation in flower color. Because the anthocyanin pathway that controls pigmentation differences has been well-studied at the molecular level, we are able to use molecular and biochemical approaches to understand gene function. As a consequence, we also can assess empirically whether there are predictable molecular evolutionary patterns that control adaptive evolution. From this, we can determine what role, if any, deleterious pleiotropy plays in determining the mutations that become fixed during evolutionary transitions in flower color.
To test these issues, we are using stable and transient transgenics, quantitative gene expression, and in vitro enzyme expression to identify functional elements that control flower color differences in several species, including M. aurantiacus. In addition, we are using field, lab, and greenhouse experiments to test whether individual mutations in these genes have predictable consequences on plant fitness based on their pathway position or function.
Molecular evolution of transcriptional regulation
The combinatorial interaction of transcription factor proteins with non-coding cis-regulatory DNA plays a major role in determining the spatial and temporal patterning of eukaryotic gene expression. Because sequence changes in either type of regulatory element can have dramatic consequences during development, it is important to understand whether and how these features are involved in adaptive evolution. In this area of my research, I investigate the molecular evolutionary consequences of sequence variation on both cis- and trans-acting regulatory elements of the anthocyanin pathway.
Transcriptional regulation of the anthocyanin pathway genes is in large part controlled by transcription factors from three gene families. These proteins interact to form a multi-protein complex that binds to the cis-regulatory DNA of target anthocyanin genes. While some members of this complex also are involved in regulating developmental pathways besides anthocyanin synthesis, others are more tissue- and function-specific. This leads to the prediction that those transcription factors with the fewest pleiotropic effects should evolve faster than those that are more constrained. To test these ideas, we use sequence data from these transcription factors to test whether evolutionary rates vary predictably based on their expected magnitudes of pleiotropy and functional constraint.
An additional project in this area tests whether sequence differences in cis-regulatory promoter regions are functionally important. Previous studies in Drosophila demonstrated that even though critical regulatory elements have been lost in different species, co-evolved, compensatory mutations in other regions of an enhancer sequence ensure that the overall patterns of gene expression are maintained. I am testing to see whether similar, co-evolved differences can account for sequence variation in a well-studied, anthocyanin gene promoter in the relatives of Arabidopsis thaliana using transient transfection experiments in leaf mesophyl protoplasts.