Trap a Gene and Find Out its Function: Toward Functional Genomics in Drosophila

Many declared aims of the genome projects have been achieved. The total genomic sequences of several relatively noncomplex/complex organisms (such as E. coli, yeast, Caenorhabditis, Drosophila) are being determined, and the nucleotide sequencing of the entire human genome will be complete in the near future. However, this achievement is not the end of the road but rather the first step toward the functional understanding of the genome of humans and other organisms. The determined linear nucleotide sequences remain only lists of A, C, G and T, unless they are given functional significance. The coding sequences of genes can be identified in a relatively reliable manner by computational methods, but the exact function of their protein products can rarely be determined without obtaining much additional information, e.g., by biochemical or cell biological methods. Thus, following sequencing, the next step must be to assign functions to the identified genes. The final goal of genome research today may look futuristic, but the knowledge of the function of every single gene and the interactions between them will finally allow us to understand the development and functioning of an organism as a whole. Gene-trapping methodology is a powerful strategy for cloning and identifying functional genes, as it marks a gene with a tag and simultaneously generates a corresponding genetic variation for that particular locus. Therefore, gene trapping is an extremely useful tool for functional genomics, establishing a correlation between the physical and genetic maps of the genome. The relative simplicity of its genome and the availability of huge bodies of genetic and molecular information make Drosophila melanogaster one of the most important model organisms. Its genome will serve as a "reference" for the in-depth analysis of the organization of more complex eukaryotic genomes. Multifaceted approaches to Drosophila functional genomics and the dual-tagging gene trap system newly developed for functional analysis of Drosophila genes are discussed in this review.