When a particular evolutionary change, such as a color, has evolved independently in related organisms, the change could be due to a mutation in the same stage of a developmental pathway in all the groups, or it could be due to different mutations. The PIs will examine this problem by using the wildflower genus Penstemon, which is made up of nearly 300 species, most of which have blue or purple flowers. However, bright red flowers are found in about 15-20 subgroups, and these changes evolved independently at least 10 times. The red flowers are probably an adaptation to attract hummingbirds as pollinators. The biochemical pathway that produces the types of flower pigments found in Penstemon is well-understood. The PIs intend to test whether these repeated or parallel episodes of adaptive evolution are due to similar genetic changes across subgroups of plants or different genetic changes in different subgroups. This study is one of a very few that addresses the genetic basis of parallel evolution, and whether certain types of genetic changes are more often observed to underlie adaptive evolution than others. Furthermore, it will address the broader question of whether the genetic basis of adaptive evolution is predictable or unpredictable.
The PIs are integrating mentorship of local high school and Duke University undergraduate students into this project. This study system provides a clear and easily described example of parallel adaptive evolution; the PIs will work towards developing it as a potential textbook example for understanding the genetic basis of parallel evolution.
The wildflower genus Penstemon has experienced a number of independent evolutionary shifts from blue to red flowers. The funded research investigated whether these independent evolutionary transitions involve similar genetic mutations. We chose one red-flowered species for intensive study. We crossed this species to a closely related blue-flowered species and performed molecular experiments to identify the pattern of flower color inheritance, determine the number of genetic changes responsible for the flower color divergence, the nature of the genetic changes (i.e. changes to enzyme function or changes to gene expression), and the identity of the genetic loci. We determined that two genetic loci underlie this difference. We have determined the identity of one of these genetic loci (its protein product is F3’5’h, an enzyme of the anthocyanin biosynthetic pathway, which is responsible for producing anthocyanin pigments), and confirmed the nature of this mutation. Although we have not characterized the second genetic mutation, we have evidence that it functions as a transcription factor activating the gene F3’5’h. Additional experiments investigated whether mutations to the function and/or expression of anthocyanin pathway genes are likely responsible for independent origins of red flowers in this plant genus. We chose eight additional red-flowered species along with their blue-flowered close relatives, resulting in eight species pairs for comparative analysis. Our original goals were to compare the function and expression levels for each red-flowered species to its blue-flowered relative. We have made substantial progress on this goal, but it is not complete. Specifically, we have compared the relative gene expression in two additional species, with similar experiments for further species pairs currently underway. We have compared relative enzyme activity for one of three candidate enzymes in six additional species pairs. Although we have not yet performed experiments to explicitly compare relative enzyme activity for the remaining two candidate enzymes, visual inspection of full-length coding sequence for one of these enzymes suggests that it has been inactivated in nearly every red-flowered species we have sampled. Furthermore, the pattern of anthocyanin production in vegetative tissues suggests that the final candidate enzyme is fully functional in each red-flowered species we have sampled. Together, our data suggest that many of the independent evolutionary origins of red flowers within Penstemon have occurred through similar types of mutations to the anthocyanin biosynthesis pathway.
