DNA sequence, and the homeodomain protein sequence) were conserved not only in different homeotic genes in flies but also across the whole animal kingdom, including humans. This conservation of genetic structure and function has become the cornerstone of modern developmental biology, forming the basis for the usefulness of model organisms in understanding human developmental mechanisms.

D. melanogaster has a genome size of approximately 180 Mb, a third of which is centric heterochromatin (regions rich in simple sequence repeats that remain condensed during interphase). The 120 Mb of euchromatin (unique sequence, decondensed during interphase) are located on two large autosomes, one dot chromosome, and paired XY sex chromosomes. The 120 Mb of euchromatic DNA have now been sequenced (Adams et al. 2000), and are estimated to encode approximately 13,600 genes, somewhat fewer than the C. elegans genome but with comparable functional diversity. The polytene chromosomes of Drosophila provide a cytogenetic map of the euchromatic portion of the genome, and by means of in situ hybridization to those large chromosomes, molecular markers have been identified within most subdivisions of the map.

Transgenic Technologies

One of the principal tools for Drosophila research is the availability of a transposon, called the P-element, which can be used as a vehicle for introducing nearly any genetic construct into the Drosophila genome at high efficiency via a relatively easy process of transformation. In addition, flies possessing single P-elements, containing dominant markers, such as the bacterial β-galactosidase gene, can be used as mutagens to disrupt coding sequences and as markers of the disrupted gene for cloning. Similarly, P-elements lacking a strong promoter for the expression of β-galactosidase can insert adjacent to enhancer elements that activate the enzyme in an enhancer-specific manner and thus identify potential new genes for study. By a combination of saturation screens and newer insertional mutagenesis experiments, it is possible to accumulate large sets of mutations of known genes and of related gene functions. Among reverse genetic technologies, for determining the function of a gene identified only by its nucleotide sequence, is insertional mutagenesis using transposable elements or RNAi, which has recently been shown to be effective in Drosophila as well as C. elegans.

Signaling Pathways in Development

One of the most useful outcomes of the genetic analysis of D. melanogaster development has been the identification of developmental pathways that are conserved in most organisms. Initially, these sets of genes were identified because of their similar phenotypes. As indicated above, using epistasis relationships, genes could be put into developmental sequence. More recently, the innovative use of a

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