The precise maintenance of engrailed and wingless gene expression in Drosophila melanogaster segments is vital for sustaining anterior-posterior compartment boundaries during development. This maintenance depends on complex molecular interactions and reciprocal signaling between cells expressing these genes.

Gene Expression Initiation by Pair-Rule Genes

Engrailed is expressed in cells where pair-rule genes even-skipped and fushi tarazu are not active. Conversely, wingless gene expression occurs predominantly in cells influenced by these pair-rule genes, establishing complementary patterns that define segment polarity.

Wingless as a Secreted Protein Activating Engrailed

Wingless protein is secreted and diffuses to nearby cells. It binds to receptors Frizzled and Lrp6 expressed on neighboring cells and activates the intracellular Armadillo pathway—Drosophila’s version of β-catenin signaling. This activation leads to transcriptional upregulation of engrailed, sustaining its expression.

Role of Hedgehog in Maintaining Wingless Expression

Hedgehog protein, produced by engrailed-expressing cells, diffuses to adjacent wingless-expressing cells. It binds the Patched receptor on these cells, relieving repression and enabling continued wingless gene transcription. Hedgehog does not initiate engrailed transcription nor activates its own transcription.

Assessing the Statements

• A: Correct — engrailed expression is in cells lacking even-skipped and fushi tarazu activity.

• B: Incorrect — wingless is not solely expressed in cells with a high concentration of either even-skipped or fushi tarazu, but in cells patterned by pair-rule genes.

• C: Correct — wingless is secreted, binds Frizzled and Lrp6, and activates engrailed via the Armadillo pathway.

• D: Incorrect — Hedgehog does not activate engrailed transcription or its own transcription.

• E: Correct — Hedgehog diffuses and binds to Patched receptor on wingless-expressing cells to maintain wingless expression.

Correct Combination for Maintenance

The correct combination of statements that accurately describes the maintenance of engrailed and wingless expression in Drosophila segments is:

(4) C and E

Conclusion

This dynamic interplay—where secreted Wingless activates engrailed through Frizzled and Armadillo, and Hedgehog signaling sustains wingless expression via Patched receptor binding—forms a feedback loop essential to the maintenance of segment polarity in Drosophila. Understanding these molecular mechanisms exemplifies how intercellular communication preserves developmental boundaries crucial for proper body patterning.