The correct answer is (C) A ventralized egg with two dorsal appendages.

Drosophila Oogenesis Basics

In normal Drosophila oogenesis, the oocyte nucleus positions asymmetrically at the anterior-dorsal cortex during stages 7-9. This localization drives gurken (grk) mRNA and protein accumulation nearby, signaling via EGFR to overlying follicle cells to specify dorsal fate and repress ventral genes like pipe. Pipe expression remains restricted to ventral follicle cells, enabling ventral eggshell structures and normal dorsoventral (DV) polarity in the embryo; dorsal follicle cells form two dorsal appendages anteriorly.​

Experiment Setup

Transplanting a second nucleus into the anterior oocyte, opposite the existing one, creates two nuclei at anterior positions ~180° apart around the cortex. Each nucleus independently induces local grk signaling, dorsalizing two opposing anterior regions of follicle cells. This expands dorsal fate across much of the eggshell circumference, leaving little equatorial/ventral region for pipe expression.​

Option Analysis

• (A) A ventralized egg with no dorsal appendages: Incorrect, as two nuclei produce dual grk signals, forming two dorsal appendages rather than eliminating them; ventralization occurs secondarily.​

• (B) A dorsalized egg with two dorsal appendages: Partially correct on dorsalization and appendages, but binuclear experiments show ventralized eggshells due to pipe loss, not fully dorsalized ones.​

• (C) A ventralized egg with two dorsal appendages: Correct. Dual dorsal signals create two appendages (opposing, sometimes fused) and repress pipe ventrally, yielding ventralized eggs that often fail to support normal embryogenesis despite compensatory mechanisms.​

• (D) A dorsalized egg with four dorsal appendages: Incorrect; two nuclei typically induce two (not four) appendages, as each acts equivalently without multiplying signals.​

In Drosophila oocyte nucleus transplantation experiments, transplanting a nucleus to the anterior region opposite the existing one disrupts dorsoventral polarity, resulting in a ventralized egg with two dorsal appendages. This classic setup tests understanding of oocyte signaling for competitive exams like CSIR NET Life Sciences.

Gurken Signaling Mechanism

The oocyte nucleus localizes grk mRNA anterior-dorsally, signaling EGFR to dorsalize follicle cells and restrict pipe ventrally. Dual nuclei induce two dorsal domains, forming two appendages while eliminating ventral cues for a ventralized phenotype.​

Phenotypic Outcomes

• Two opposing grk sources expand dorsal follicle fates.

• Dorsal appendages form at both anterior sites.

• Ventral repression leads to pipe absence, ventralizing the eggshell.​

Exam Relevance

This question assesses polarity establishment; option C matches binuclear oocyte data where embryos often show normal DV despite eggshell defects via compensation.​