Question

How does the lack of anterior structures in the exu and swa mutants support the idea of bcd as a morphogen? (Hint: what role do exu and swa normally play in the graded localization of bcd, and how is this affected in those mutants?)

Answer 1

Drosophila oocyte is connected by cytoplasmic bridges to 15 nurse cells. Bicoid mRNA(bcd),oskar(osk) are the major mRNAs which will determines the anterior and posterior poles of the oocytes respectively. Anterior patterning of the Drosophila embryo chiefly depends on localization of Bicoid mRNA to the anterior pole of the developing oocyte.This Bicoid mRNA localization requires the exuperantia (exu) gene, swallow , staufen protiens and an intact microtubule cytoskeleton.

Swa protein alone is not enough to concentrate BCD mRNA anteriorly, while in exu mutants BCD mRNA reach the oocyte but that will not be localized perfectly. Thus, Exu might have involved in preparing or modifying Bicoid mRNA, thereby allowing the later anterior localization of this RNA while Exu itself is not concentrated interiorly but is dispersed throughout the oocyte.

In Swa mutants bcd mRNA enters the oocyte but fails to localize properly in the anterior portion of the oocyte.Where as in case of Exu muatnats the bcd m RNA transport abnormally in the nurse cells and fails to localize anterior cortex. Hence both the genes are essential for the development of drosophila embryo.