Christiane Nüsslein-Volhard and her colleagues carried out several experiments in an attempt to understand what determines the anterior and posterior ends of a Drosophila larva (reviewed in C. NüssleinVolhard, H. G. Frohnhofer, and R. Lehmann. 1987. Science 238:1675– 1681). They isolated fruit flies with mutations in the bicoid gene (bcd−). These flies produced embryos that lacked a head and thorax. When they transplanted cytoplasm from the anterior end of an egg from a wild-type female into the anterior end of an egg from a mutant bicoid female, normal head and thorax development took place in the embryo. However, transplanting cytoplasm from the posterior end of an egg from a wild-type female into the anterior end of an egg from a bicoid female had no effect. Explain these results in regard to what you know about proteins that control the determination of the anterior–posterior axis.

Bicoid gene is the maternal effect gene whose protein concentration gradient patters the anterior-posterior axis in Drosophila embryogenesis. It was the primary protein that is demonstrated to act as a morphogen. Morphogens are proteins whose concentration gradient will affect the developmental fate of the surrounding region.

The specified bicoid gene is the one which codes for bicoid protein. This protein is being existing in a gradient manner in the egg and thus its concentration is higher at the anterior end and lower in posterior end.

This bicoid protein is responsible for the stimulation of the development of the anterior end.

The posterior region (including the hindgut) expands and extends towards the anterior pole along the dorsal side of the embryo. At this time, segments of the embryo become visible, creating a striped arrangement along the anterior-posterior axis.