Ancient mechanisms for the evolution of the bicoid homeodomain's function in fly development
Qinwen Liu,
Pinar Onal,
Rhea R Datta,
Julia M Rogers,
Urs Schmidt-Ott,
Martha L Bulyk,
Stephen Small,
Joseph W Thornton
Affiliations
Qinwen Liu
Department of Ecology and Evolution, University of Chicago, Chicago, United States
Pinar Onal
Department of Biology, New York University, New York, United States
Rhea R Datta
Department of Biology, New York University, New York, United States
Julia M Rogers
Committee on Higher Degrees in Biophysics, Harvard University, Cambridge, United States; Division of Genetics, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, United States
Department of Organismal Biology and Anatomy, University of Chicago, Chicago, United States
Martha L Bulyk
Committee on Higher Degrees in Biophysics, Harvard University, Cambridge, United States; Division of Genetics, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, United States; Department of Pathology, Brigham and Women’s Hospital and Harvard Medical School, Boston, United States
Stephen Small
Department of Biology, New York University, New York, United States
Department of Ecology and Evolution, University of Chicago, Chicago, United States; Department of Human Genetics, University of Chicago, Chicago, United States
The ancient mechanisms that caused developmental gene regulatory networks to diversify among distantly related taxa are not well understood. Here we use ancestral protein reconstruction, biochemical experiments, and developmental assays of transgenic animals carrying reconstructed ancestral genes to investigate how the transcription factor Bicoid (Bcd) evolved its central role in anterior-posterior patterning in flies. We show that most of Bcd’s derived functions are attributable to evolutionary changes within its homeodomain (HD) during a phylogenetic interval >140 million years ago. A single substitution from this period (Q50K) accounts almost entirely for the evolution of Bcd’s derived DNA specificity in vitro. In transgenic embryos expressing the reconstructed ancestral HD, however, Q50K confers activation of only a few of Bcd’s transcriptional targets and yields a very partial rescue of anterior development. Adding a second historical substitution (M54R) confers regulation of additional Bcd targets and further rescues anterior development. These results indicate that two epistatically interacting mutations played a major role in the evolution of Bcd’s controlling regulatory role in early development. They also show how ancestral sequence reconstruction can be combined with in vivo characterization of transgenic animals to illuminate the historical mechanisms of developmental evolution.
