The modification of transcriptional regulation has become increasingly appreciated as a major contributor to morphological evolution. However, the role of negative-acting control elements (e.g. silencers) in generating morphological diversity has been generally overlooked relative to positive-acting “enhancer” elements. The highly variable body coloration patterns among Drosophilid insects represents a powerful model system in which the molecular alterations that underlie phenotypic diversity can be defined. In a survey of pigment phenotypes among geographically disparate Japanese populations of Drosophila auraria, we discovered a remarkable degree of variation in male-specific abdominal coloration. In testing the expression patterns of the major pigment-producing enzymes, we found that phenotypes uniquely correlated with differences in the expression of ebony, a gene required for yellow-colored cuticle. Assays of ebony’s transcriptional control region indicated that a lightly pigmented strain harbored cis-regulatory mutations that caused correlated changes in its expression. Through a series of chimeric reporter constructs between light and dark strain alleles, we localized function-altering mutations to a conserved silencer that mediates a male-specific pattern of ebony repression. This suggests that the light allele was derived through the loss of this silencer’s activity. Furthermore, examination of the ebony gene of D. serrata, a close relative of D. auraria which secondarily lost male-specific pigmentation revealed the parallel loss of this silencer element. These results demonstrate how loss-of-function mutations in a silencer element resulted in increased gene expression. We propose that the mutational inactivation of silencer elements may represent a favored path to evolve gene expression, impacting morphological traits.
Copyright © 2015, Johnson et al.
National Science Foundation, Alfred P. Sloan Foundation, University of Dayton Research Institute, American Heart Association, Howard Hughes Medical Institute Precollege and Undergraduate Science Education Program
Johnson, Winslow C.; Ordway, Alison J.; Watada, Masayoshi; Pruitt, Jonathan N.; Williams, Thomas M.; and Rebeiz, Mark, "Genetic Changes to a Transcriptional Silencer Element Confers Phenotypic Diversity within and between Drosophila Species" (2015). Biology Faculty Publications. 143.
Intraspecific variation in D. auraria pigmentation does not follow a traditional geographical cline
S1_Table.docx (39 kB)
Additional strains used in this study
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Yellow is expressed similarly in lines that contain the light or dark phenotype
S2_Table.docx (47 kB)
Primers for in situ hybridization probes
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Tan is expressed similarly in lines that exhibit the light or dark phenotype
S3_Table.docx (56 kB)
Primers for constructing reporter constructs, including chimeric reporters
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Variable ebony expression in lines that contain a mixture of light and dark pigmentation phenotypes
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Isolation of the ebony upstream region from montium subgroup species
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Chimeric constructs localize causative changes to the conserved promoter-proximal silencer region
S7_Fig.jpg (178 kB)
The expansion of ebony expression is fixed among males in D. serrata strains