MBE Advance Access published online on October 5, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm216
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Research Article |
Relaxed Constraint and Evolutionary Rate Variation Between Basic Helix-Loop-Helix (bhlh) Floral Anthocyanin Regulators in ipomoea
a Duke University, Department of Biology, Box 90338, Durham, NC, 27708-0338. Fax: 919-660-7293, Phone: 919-684-2295
Corresponding Author E-mail: mstreisf{at}duke.edu
Received for publication July 16, 2007. Revision received September 24, 2007. Accepted for publication September 27, 2007.
Regulatory genes are believed to play a large role in morphological diversification and are often characterized by elevated rates of evolution. Whether this rapid evolution is primarily due to adaptive differentiation or relaxed selective constraint remains an open question. We attempted to distinguish between these alternative outcomes in two transcription factors known to regulate the expression of anthocyanin pigmentation genes in flowers. We cloned the full-length coding region from two basic Helix-Loop-Helix (bHLH) transcription factors from several species of Ipomoea with diverse flower colors and determined the selective forces operating on them. In both genes, rapidly evolving sites and indel mutations are clustered in non-binding domains, but the extent of rate acceleration in these domains is reduced relative to most previously characterized plant transcription factors. Moreover, codon models of substitution rates and models evaluating the magnitude of change to physical amino acid properties demonstrate little evidence for adaptive evolution and suggest that elevated non-synonymous substitution rates in these domains represent relaxed selective constraint. Although both genes show qualitatively similar patterns, their rates of evolution differ significantly due to an increased rate of non-synonymous substitutions in the non-binding domains in one copy, suggesting substantial differences in functional constraint on each gene. In general, these results provide additional evidence demonstrating that decreased constraint as opposed to positive selection is largely responsible for the frequently observed pattern of rapid evolution in particular domains of plant transcription factors. More specifically, they suggest that most of the amino acid substitutions are neutral and do not implicate a role for natural selection on these regulatory genes in the diversification of flower color in Ipomoea.
Key Words: Ipomoea • positive selection • rate variation • anthocyanin • basic Helix-Loop-Helix • transcription factor