Consider the even-skipped regulatory sequences in Figure 18.9.
Explain what you expect to see happen to even-skipped stripe 2 if it is expressed in a Krüppel mutant background. What about a hunchback mutant background? A giant mutant background? A bicoid mutant background?

Verified step by step guidance

1. Step 1: Understand the role of the even-skipped (eve) gene and its stripe 2 expression pattern. The eve gene is expressed in seven stripes during Drosophila embryogenesis, and stripe 2 expression is regulated by a combination of activators and repressors binding to its regulatory sequences.
2. Step 2: Identify the regulatory proteins involved in controlling eve stripe 2. Key regulators include Bicoid and Hunchback as activators, and Giant and Krüppel as repressors. Each protein binds to specific sites in the stripe 2 enhancer to shape the stripe's position and intensity.
3. Step 3: Predict the effect of a Krüppel mutant background. Since Krüppel normally represses eve expression outside stripe 2 boundaries, its absence would likely cause the stripe 2 expression domain to expand into regions where Krüppel repression is lost.
4. Step 4: Predict the effect of a Hunchback mutant background. Hunchback acts as an activator for stripe 2, so in its absence, the activation of eve stripe 2 would be reduced or lost, leading to diminished or absent stripe 2 expression.
5. Step 5: Predict the effects of Giant and Bicoid mutant backgrounds. Giant represses eve expression flanking stripe 2, so a giant mutant would cause expansion of stripe 2 expression. Bicoid is an activator required for stripe 2 initiation, so a bicoid mutant would result in loss or severe reduction of stripe 2 expression.

Key Concepts

Even-skipped (eve) Stripe 2 Regulation

The even-skipped gene is expressed in seven stripes during Drosophila embryogenesis, with stripe 2 regulated by a specific enhancer integrating inputs from multiple transcription factors. This stripe's precise spatial expression depends on activators and repressors binding to its regulatory sequences, making it a model for understanding gene regulation in development.

Role of Maternal and Gap Genes in Pattern Formation

Maternal effect genes like bicoid establish initial gradients that activate gap genes such as hunchback, Krüppel, and giant. These gap genes encode transcription factors that define broad embryonic regions and regulate pair-rule genes like even-skipped, shaping the segmented body plan by activating or repressing specific stripes.

Effects of Mutations in Gap Genes on eve Stripe 2 Expression

Mutations in gap genes alter the balance of activators and repressors controlling eve stripe 2. For example, a Krüppel mutant may reduce repression, expanding stripe 2; a hunchback mutant may reduce activation, diminishing stripe 2; a giant mutant may affect repression boundaries; and a bicoid mutant disrupts initial activation, often abolishing stripe 2 expression.