Genetic regulation of engrailed and wingless in Tribolium segmentation and the evolution of pair-rule segmentation

Abstract

In Drosophila, primary pair-rule genes establish the parasegmental boundaries and indirectly control the periodic expression of the segment polarity genes engrailed ( en) and wingless ( wg) via regulation of secondary pair-rule genes. Although orthologs of some Drosophila pair-rule genes are not required for proper segmentation in Tribolium, segmental expression of Tc-en and Tc-wg is conserved. To understand how these segment polarity genes are regulated, we examined the results of expressing one or two pair-rule genes in the absence of the other known pair-rule genes. Expression of one or both of the secondary pair-rule genes, Tc-sloppy-paired ( Tc-slp) and Tc-paired ( Tc-prd), activated Tc-wg in the absence of the primary pair-rule genes, Tc-even-skipped ( Tc-eve), Tc-runt ( Tc-run) and Tc-odd-skipped ( Tc-odd). Tc-eve alone failed to activate Tc-wg or Tc-en, but in combination with Tc-run or Tc-prd activated Tc-en. These results, interpreted within the pair-rule gene expression patterns, suggest separate models for the genetic regulation of the juxtaposed expression of Tc-wg and Tc-en at odd- and even-numbered parasegmental boundaries, respectively. Conserved interactions between eve and prd at the anterior boundary of odd-numbered parasegments may reflect an ancestral segmentation mechanism that functioned in every segment prior to the evolution of pair-rule segmentation.