Q.93 In an experiment, excess amount of bicod mRNA (more than wild–type expression
level) was injected into the posterior pole of a wild–type Drosophila embryo at
pre–blastodermal stage. Out of the following options, which one represents the
best expected phenotype in the resulted developing embryo?
(A) Normal embryo with head structure at anterior and tail structure at posterior pole
(B) Head structure only at posterior pole of the embryo
(C) Tail structure at anterior and head structure at posterior poles of the embryo
(D) Head structure at both anterior and posterior poles of the embryo

Correct Answer: (D) Head structure at both anterior and posterior poles of the embryo.​

Bicoid mRNA normally localizes at the anterior pole of wild-type Drosophila embryos, forming a protein gradient that specifies head and thorax structures. Injecting excess bicoid mRNA into the posterior pole at the pre-blastodermal stage creates an ectopic anteriorizing signal there, leading to duplicated head structures. This occurs because the endogenous anterior bicoid source remains active alongside the new posterior source.​

Option Analysis

• (A) Normal embryo: Incorrect, as posterior bicoid injection disrupts normal anterior-posterior polarity by generating a second Bicoid gradient, preventing typical tail formation at the posterior.​

• (B) Head structure only at posterior: Incorrect for wild-type embryos, which retain anterior bicoid mRNA; head forms at both poles due to dual sources, unlike bicoid mutants.​

• (C) Tail at anterior, head at posterior: Incorrect, as Bicoid promotes anterior (head) fates and represses posterior ones; no mechanism swaps poles without additional mutations.​

• (D) Head at both poles: Correct, as excess posterior-injected bicoid mRNA produces Bicoid protein gradients from both ends, inducing mirror-image head structures.​

Introduction to Bicoid mRNA Injection Posterior Pole Drosophila Embryo Phenotype

In Drosophila embryogenesis, bicoid mRNA injection posterior pole Drosophila embryo phenotype reveals key insights into anterior-posterior axis formation. This classic experiment, vital for CSIR NET Life Sciences, tests maternal effect genes like bicoid, which specifies head structures via an anterior gradient. Excess bicoid mRNA at the posterior pole before blastoderm stage creates dual gradients, yielding a “double-head” phenotype.​

Bicoid Role in Drosophila Axis Formation

Bicoid mRNA localizes anteriorly during oogenesis, translating into a nuclear protein gradient highest at the anterior pole. High Bicoid activates head/thorax genes (e.g., hunchback) and represses posterior factors like caudal. Nanos at the posterior counters this by inhibiting hunchback translation, ensuring polarity. Pre-blastodermal injection timing allows diffusion before nuclear cycles.​

Expected Phenotype Breakdown

Wild-type embryos have anterior bicoid; posterior injection adds an ectopic source. Result: Bicoid gradients from both poles induce head structures bilaterally, replacing posterior tail with a mirror-image head. This “two-headed” embryo confirms Bicoid’s sufficiency as a morphogen.​

Why Not Other Options?

Normal development (A) fails due to ectopic signaling. Posterior-only head (B) ignores anterior source. Reversed polarity (C) mismatches Bicoid’s anteriorizing role. Only (D) fits wild-type data.​

CSIR NET Exam Relevance

This question assesses maternal gradients, morphogen function, and experimental phenotypes—core for Unit 1 (Developmental Biology). Similar queries test bicoid/nanos mutants. Master via diagrams of gradients for scoring.​