Discussion
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?
Answer: D
The expected phenotype is head structure at both anterior and posterior poles of the embryo.
bicoid mRNA is normally localised at the anterior pole. Its protein forms a gradient that specifies head and thorax structures. Injecting excess bicoid mRNA at the posterior pole creates a second high point of Bicoid there, so the posterior end is also instructed to form a head, while the embryo's own anterior Bicoid still produces a head at the front. The result is a double-headed (bicephalic) embryo, with no normal tail structures.
A normal embryo would ignore the injection. A head only at the posterior would require the anterior Bicoid to be lost, and a tail at the anterior would need posterior determinants there, neither of which the injection provides.
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