9. Three embryos, X (wild type), Y (mutant for bicoid) and Z (mutant for nanos) were injected with bicoid mRNA in their posterior pole at early cleavage stage. What would be the phenotypes of the resulting embryos?
(1) Embryo X will develop head on both anterior and posterior side, while embryos Y and Z will develop head on posterior side only.
(2) Embryos X and Z will develop head on both anterior and posterior side, while embryo Y will
develop head on posterior end only.
(3) Embryos X, Y and Z will develop head on both anterior as well as posterior side.
(4) Embryo X will develop head on anterior side, embryo Y will develop no head, while embryo Z will develop head on anterior as well as posterior side.

In Drosophila embryogenesis, maternal genes such as bicoid and nanos play crucial roles in establishing the anterior-posterior axis. Bicoid mRNA is localized at the anterior pole in wild type embryos, creating a protein gradient necessary for head and thorax formation. Nanos acts primarily at the posterior, repressing hunchback translation.

Injecting bicoid mRNA at the posterior pole of embryos at early cleavage stages creates an ectopic source of Bicoid protein at the posterior end. This experimental manipulation has distinct phenotypic consequences depending on the embryo genotype:

• In wild type embryos (X), endogenous anterior bicoid mRNA is present, so the additional bicoid mRNA at the posterior generates heads at both anterior and posterior ends.

• In bicoid mutant embryos (Y), no endogenous bicoid is available at the anterior, so only the posterior injection site produces Bicoid protein, resulting in head formation at the posterior end only.

• In nanos mutant embryos (Z), normal anterior bicoid expression occurs, and the posterior injection produces an ectopic source, causing heads at both poles.

Therefore, the experimental phenotypes resulting from posterior bicoid mRNA injection are:

(2) Embryos X and Z will develop head on both anterior and posterior side, while embryo Y will develop head on posterior end only.

Answer: (2) Embryos X and Z will develop head on both anterior and posterior side, while embryo Y will develop head on posterior end only.