RNAi. A powerful tool for reverse genetic analysis has been provided by the discovery that introduction of double-stranded mRNA for a particular gene into C. elegans will specifically inactivate that gene, resulting in loss-of-function phenotypes that generally mimic the gene’s null phenotype for at least a generation or two (Fire et al. 1998). Although the mechanism of this inactivation, referred to as RNAi, is not yet understood and gene expression in some tissues is more susceptible to inactivation than expression in other tissues (Montgomery et al. 1999), it is clear that RNAi will be extremely useful for rapid functional tests of genes identified by genome sequencing as potentially important, for example, in development or in responses to environmental toxicants. Moreover, recent results indicate that the technique is applicable to Drosophila (Kennerdell and Carthew 1998) and perhaps to other organisms as well.

Signaling Pathways in Development

Most of the progress in understanding C. elegans development has come from application of forward genetics as described above, combined with laser ablation experiments to identify required cell interactions. A variety of inductive events, which in C. elegans can be analyzed at the single-cell level, are mediated by signaling pathways that are still under investigation. However, it is already clear that nematode development uses most of the pathways described in Chapter 6, often in developmental contexts similar to those found in more complex metazoans. Two exceptions are the Hedgehog and cytokine signaling pathways, which C. elegans appears to lack (Ruvkun and Hobert 1998).

The Fruit Fly Drosophila

History, Biology, and Genetics

Drosophila melanogaster is the common fruit fly found worldwide in orchards, where adult flies lay eggs on rotting fruit. Since the beginning of this century, fruit flies have been cultured in the laboratory in half-pint milk bottles and more recently in shell vials and plastic tubes by using a solid food, typically composed of agar, cornmeal, dried yeast, and molasses. At 25°C the life cycle takes approximately 2 weeks. Embryogenesis and the first two larval stages require 1 day each; the third larval stage, 2 days; and the pupal stage, 4-5 days (Figure 7-2). Two-day-old adults begin to lay eggs. Because of the short life cycle, ease of rearing in large populations, and the many diverse phenotypes readily visible under a simple dissecting microscope, many mutations have been accumulated in the organism since its initial use by T. H. Morgan and his associates at Columbia University in the 1920s. The study of fly genetics has been instrumental in many classic discoveries in eukaryotic genetics, such as linkage, gene mapping, recombination frequency, and chromosomal aberrations. Discov-

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