Pleiotropic functions of a conserved insect-specific Hox peptide motif.
The proteins that regulate developmental processes in animals have generally been well conserved during evolution. A few cases are known where protein activities have functionally evolved. These rare examples raise the issue of how highly conserved regulatory proteins with many roles evolve new functions while maintaining old functions. We have investigated this by analyzing the function of the ;QA' peptide motif of the Hox protein Ultrabithorax (Ubx), a motif that has been conserved throughout insect evolution since its establishment early in the lineage. We precisely deleted the QA motif at the endogenous locus via allelic replacement in Drosophila melanogaster. Although the QA motif was originally characterized as involved in the repression of limb formation, we have found that it is highly pleiotropic. Curiously, deleting the QA motif had strong effects in some tissues while barely affecting others, suggesting that QA function is preferentially required for a subset of Ubx target genes. QA deletion homozygotes had a normal complement of limbs, but, at reduced doses of Ubx and the abdominal-A (abd-A) Hox gene, ectopic limb primordia and adult abdominal limbs formed when the QA motif was absent. These results show that redundancy and the additive contributions of activity-regulating peptide motifs play important roles in moderating the phenotypic consequences of Hox protein evolution, and that pleiotropic peptide motifs that contribute quantitatively to several functions are subject to intense purifying selection.
Hittinger
Chris Todd
CT
Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53706, USA.
Stern
David L
DL
Carroll
Sean B
SB
D020816:Amino Acid Motifs; D000818:Animals; D017124:Conserved Sequence / Q000502:physiology*; D029721:Drosophila Proteins / Q000235:genetics / Q000502:physiology*; D004331:Drosophila melanogaster / Q000235:genetics / Q000254:growth & development; D005121:Extremities / Q000254:growth & development; D005801:Genes, Homeobox; D018398:Homeodomain Proteins / Q000235:genetics / Q000502:physiology*; D010641:Phenotype; D017384:Sequence Deletion; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*
D029721:Drosophila Proteins; D018398:Homeodomain Proteins; D014157:Transcription Factors; C066177:Ubx protein, Drosophila