Developmental biology seeks to understand how a single cell, the fertilized egg, can give rise to all the complexity of form and function observed in a multicellular organism. One way to address this extraordinarily complicated question is to approach one early and fundamental aspect: in the earliest stages of development, how is head distinguished from tail, and back from front? What are the positional cues that specify the initial asymmetry along these body axes, and how are the structures the structures in between elaborated?
In many organisms, the first indications of these body axes are already visible at the single cell stage. In some cases asymmetry is triggered by fertilization, when sperm entry provides a cue that defines one end of a future axis. In other cases, the unfertilized egg itself contains molecular information, deposited by the mother during oogenesis, that will ultimately specify the poles of the head-tail, or “anterior-posterior,” and back-front, or “dorsal-ventral,” axes. Since we can trace the origin of these basic body axes back to asymmetries at the single cell stage, addressing larger questions about embryonic development begins with an understanding of the mechanisms that generate asymmetry in the egg. As described below, in these issues we can recognize fundamental themes in developmental biology, such as how an inductive signal can trigger cell fate changes or how molecules can adopt specific subcellular localization patterns.
Why the fly?
We study the generation of asymmetry during the development of the fruit fly, Drosophila melanogaster. One of the major advantages of this model organism is the ability to isolate mutations that disrupt a particular process of interest; identification of the affected gene thus reveals a component critical to the proper function of that process. This approach is powerful because it allows the identification of developmentally important genes, even with no prior knowledge of their possible function. Furthermore, the Drosophila genome project continues to provide a wealth of tools – the complete genome sequence, gene expression data, libraries of mapped random mutations, etc. – that allow the increasingly rapid identification of the genes affected in mutant strains and facilitate the study of particular genes of interest.