Questions: The Drosophila Segmentation Gene Hierarchy

Question 1

A fly mutant larva is missing every other segment-width region along its body. Which class of gene is most likely mutated?

• A. A maternal effect gene such as bicoid
• B. A gap gene
• C. A pair-rule gene
• D. A Hox gene

Answer: (C)

Pair-rule genes are expressed in seven stripes, one in every second segment-width region. Losing a pair-rule gene deletes pattern elements in alternating regions, giving a larva with about half the normal number of segments. Fushi tarazu, Japanese for 'too few segments,' was named for this phenotype.

Question 2

The seven stripes of even-skipped expression are all driven by a single enhancer that responds identically in every stripe.

• T. True
• F. False

Answer: False

even-skipped has several modular enhancers. The stripe 2 enhancer is activated by Bicoid and Hunchback and repressed by Giant on its anterior side and Krüppel on its posterior side, which confines expression to a narrow band. Other enhancers drive other stripes, and some drive two stripes each.

Question 3

Why can a transcription factor such as Bicoid form a gradient across the early fly embryo without any cell-to-cell signaling?

Model answer: The early Drosophila embryo is a syncytium: its nuclei divide without forming cell membranes, so they all share one cytoplasm. Bicoid protein, translated from mRNA at the anterior pole, diffuses through this shared cytoplasm and enters nuclei directly, forming a concentration gradient from front to back.

Once cell membranes form, at the cellular blastoderm stage, later tiers such as the segment polarity genes must use secreted signals like Wingless and Hedgehog to maintain segment boundaries.

Question 4

What is the main job of the gap genes in the segmentation hierarchy?

• A. To specify the identity of each segment
• B. To divide the embryo into broad regions by reading maternal gradients and to regulate pair-rule genes
• C. To form the boundaries within each segment
• D. To control the dorsal-ventral axis

Answer: (B)

Gap genes such as hunchback, Krüppel, knirps and giant are expressed in broad, overlapping domains. Their mutants lack a contiguous block of segments, a 'gap' in the pattern. Their proteins then act together on the enhancers of pair-rule genes.