Q. 103 In an experiment involving Drosophila development, a large amount of purified bicoid mRNA was
injected into the posterior end of a wild-type embryo, the resulting developing embryo will have
(A) normal development with one each of head, thorax and abdomen.
(B) head in the middle with two thoraces and two abdomens.
(C) a head with two thoraces and an abdomen.
(D) two heads and two thoraces with an abdomen segment in the middle.

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Drosophila melanogaster, the fruit fly, serves as a powerful model organism in developmental biology, especially for studying anterior-posterior (A-P) axis formation. The bicoid gene plays a pivotal role as a maternal effect gene, with its mRNA deposited at the anterior end of the egg. This mRNA translates into Bicoid protein, forming a concentration gradient—high anteriorly, low posteriorly—that activates head and thorax genes while repressing abdominal ones.

In the classic experiment, researchers inject purified bicoid mRNA into the posterior end of a wild-type early embryo. This disrupts the natural gradient, creating high Bicoid levels at both ends. The result? A mirror-image duplication of anterior structures, yielding two heads and two thoraces with an abdomen segment in the middle.

Correct Answer: (D) two heads and two thoraces with an abdomen segment in the middle

High Bicoid at the posterior mimics anterior conditions, triggering duplicate head (e.g., via hunchback activation) and thorax formation. The central region retains some posterior identity due to lower Bicoid overlap and other gradients like Nanos, resulting in abdominal segments sandwiched between duplicated anterior halves. This phenotype, often called a “double-anterior” embryo, confirms Bicoid’s role as a morphogen.

Explanation of All Options

• (A) normal development with one each of head, thorax and abdomen.
  Incorrect. Extra posterior Bicoid overrides normal polarity, preventing standard single-axis development.

• (B) head in the middle with two thoraces and two abdomens.
  Incorrect. This reverses the effect; high Bicoid specifies anterior, not posterior, structures. Posterior duplication would require abdominalizing factors like nanos mRNA.

• C) a head with two thoraces and an abdomen.
  Incorrect. While it includes anterior elements, it lacks the symmetric double-head from dual high-Bicoid poles and doesn’t match the mirrored phenotype.

• (D) two heads and two thoraces with an abdomen segment in the middle.
  Correct, as detailed above. Experimental evidence from Nüsslein-Volhard’s Nobel-winning work (1980s) shows larvae with acron (head-like) structures at both ends, thoracic segments flanking a reduced abdomen.

This experiment underscores morphogen gradients in pattern formation, relevant to genetics, molecular biology, and evo-devo studies. For visuals, search “bicoid injection Drosophila embryo” on academic databases like PubMed.