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← Theoretical B

Q50 — Bicoid, Exuperantia, and Body Axis Determination in Drosophila

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Evolutionary Developmental Biology (Evo-Devo)

During Drosophila development, oogenesis establishes the body plan before fertilization.

The concentration gradient of bicoid mRNA in the oocyte determines the anterior-posterior axis (Figure 1A), a process controlled by the Exuperantia protein. Exuperantia anchors bicoid mRNA at the anterior end of the oocyte, preventing its free diffusion and thereby shaping the distribution of bicoid protein in the developing embryo (Figure 1B).

The concentration gradient of the gurken protein (Figure 1C) and its positive interaction with the torpedo protein is critical for determining the dorsoventral axis of the embryo. Interestingly, a wild-type oocyte inside a torpedo-deficient follicle develops into an entirely ventral embryo, while a torpedo-deficient oocyte inside a wild-type follicle develops a normal body plan.

Three-panel figure: panel A plots bicoid mRNA concentration against anterior-posterior position in the oocyte; panel B plots bicoid protein concentration against position in the embryo, with the embryo divided into acron, head, thorax, abdomen, and telson regions; panel C is a schematic oval oocyte surrounded by follicle cells (FC), with a gradient bar showing gurken concentration from low (dorsal) to high (ventral) and an arrow indicating the gradient direction. Figure 1. A Y-1: bicoid mRNA concentration; Ant: anterior, Pos: posterior. B Y-2: bicoid protein concentration; Ac: acron, H: head, Th: thorax, Ab: abdomen, Te: telson. C O: oocyte, FC: follicle cells, G: concentration gradient of gurken.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. You can deplete bicoid in male testes to get mutant embryos lacking head and thorax regions.
B. An Exuperantia loss-of-function mutant embryo will have an extended acron.
C. Torpedo gene expression is not required in oocytes for establishing the normal dorsal-ventral axis.
D. The arrow in Figure 1C points from the dorsal to the ventral part of the future embryo.
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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF