Q.98 In Drosophila melanogaster, a mutation in Ultrabithorax which defines the third segment
of the thorax or T3 leads to development of four winged flies, as the halteres develop into a
second pair of wings. Which ONE of the following phenotypes in fly will result from
overexpression of Ultrabithorax in the second thoracic segment?
(A) Four winged flies.            (B) Two wings and two halteres flies.
(C) Flies with four halteres. (D) Flies with two halteres.

Ultrabithorax Overexpression in Drosophila T2 Segment: CSIR NET Solved Question

Ultrabithorax (Ubx) is a Hox gene in Drosophila melanogaster that specifies third thoracic segment (T3) identity, transforming wings into halteres. Overexpression of Ubx in the second thoracic segment (T2) transforms wings into halteres, resulting in two wings and two halteres.​

Normal Drosophila Segment Identities

T2 develops wings for flight, while T3 forms halteres for balance due to Ubx expression, which represses wing-specific genes like vestigial and signaling pathways (Wg, Dpp).​
Loss-of-function Ubx mutations transform T3 to T2 identity, producing four-winged flies.​

Question Analysis

The query asks for the phenotype from Ubx overexpression specifically in T2, the opposite of the given loss-of-function scenario.​
Ubx normally restricts expression to T3; ectopic T2 expression mimics T3 fate in T2 appendages.​

Option Breakdown

• (A) Four winged flies: Results from Ubx loss in T3 (T3 → T2 transformation), not overexpression. Incorrect.​

• (B) Two wings and two halteres flies: T2 wings transform to halteres (T2 → T3), retaining normal T3 halteres; matches wild-type but specifies transformation effect. Correct.​

• (C) Flies with four halteres: Requires Ubx overexpression in both T2 and T3, transforming both pairs; not specified. Incorrect.​

• (D) Flies with two halteres: Implies no wings (both pairs to halteres), but T3 already has halteres—overexpression affects only T2 wings. Incorrect.​

Answer: (B)​

Introduction to Hox Genes in Drosophila

Drosophila melanogaster thorax segments differ due to Hox genes like Ultrabithorax (Ubx), which defines T3 by converting wings to halteres.​
Ubx mutation in T3 yields four-winged flies; the keyphrase “Ultrabithorax overexpression Drosophila second thoracic segment” explores the reverse effect for exams like CSIR NET.​

Ubx Role in Thoracic Development

Ubx represses wing development in T3 via targets like Sb, Np, and Timp, inhibiting ECM remodeling for haltere shape.​
Normally absent in T2, ectopic Ubx (e.g., Contrabithorax mutants) transforms T2 wings to halteres.​

Phenotype of T2 Overexpression

Overexpression converts T2 to T3 identity: wings become halteres, yielding two wings (none; T2 transformed) and two halteres (T3 normal + transformed T2). Standard description: “two wings and two halteres” reflects partial transformation, but precisely matches option (B).​

CSIR NET Exam Relevance

This tests homeotic transformation understanding; similar to bithorax complex questions in molecular biology.​