Question

In an experiment, excess amount of bicoid 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

Hint:

In Drosophila axis formation, {bicoid} specifies anterior (head), while {nanos} specifies posterior (tail). Mislocalization of these mRNAs causes axis duplication or transformation.

Answer 1

The Correct Option is D

Step 1: Role of \textbf{bicoid.} The bicoid gene is a maternal effect gene in Drosophila that establishes the anterior (head) structures. Its mRNA is normally localized at the anterior pole. The protein forms a gradient that specifies head and thorax development.
Step 2: Experimental manipulation. When excess bicoid mRNA is artificially injected into the posterior pole, the posterior region also experiences high anterior-determining signals.
Step 3: Phenotypic outcome. As a result, the embryo develops head structures at both ends (anterior and posterior), leading to a double-head phenotype. The normal posterior (tail) is lost.
Therefore, the expected phenotype is: head structures at both anterior and posterior poles $⇒$ (D).