The fruit fly Drosophila melanogaster, like other arthropods, is composed of numerous body segments. The fly has several fused head segments, three thoracic segments, eight abdominal segments, and a terminal segment at the end of the abdomen. In the adult fly, these segments are clearly unique, in that a head segment has antennae, but a thoracic segment has legs instead, and an abdominal segment has neither.
About 24 hours after fertilization, a larva appears with distinct segments. The segments all look similar, but their fates to become different adult segments is already determined. Several types of genes are expressed sequentially in the embryo to define these segments. The genes involved in each step code for transcription factors, which in turn control the synthesis of other transcription factors acting on the next set of genes. The genes expressed at the end of this cascade code for proteins that carry out the functions of the cell.
A female fly provides her eggs with the first signals that direct the process of embryo development. These signals are mRNA molecules from the maternal effect genes bicoid and nanos, as well as from other genes (not shown in the animation).
The maternal products trigger a cascade of reactions in which the embyro's own genes become activated. Classes of genes are turned on sequentially, beginning with the gap genes, which define large regions of the embryo; then with the pair rule genes, which identify smaller regions; and finally with the segment polarity genes, which define single body segments.
The Hox genes become activated at the end of this cascade and provide the individual body segments with their ultimate identity. Hox genes instruct the segments to develop according to their positions along the embryo, such that a head segment develops antennae and a thoracic segment develops legs. Mutations in Hox genes cause fascinating body transformations. For example, the Hox gene Antennapedia can mutate so that legs, rather than antennae sprout from the fly's head.
We can learn a lot about development by studying the fruit fly. Although the details may vary, the types of genes and developmental processes in fruit flies have much in common with the genes and developmental processes in other organisms, including humans.
Textbook Reference: Concept 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis