A limitation of the above approach is that only genetically well-defined pathways can be tested for toxicological responses, and only one pathway can be tested at a time. More global tests will soon be possible that take advantage of the complete genomic sequence of the nematode. (As discussed in Chapter 5, functional genomics refers to the profiling of all transcription from the genome.) The targets of most signaling pathways in development are transcription factors; thus, the end result of the signal is a change in the pattern of transcripts synthesized by responding cells. DNA microchips carrying ordered arrays of cDNAs representing all the C. elegans expressed genes will soon be available, allowing rapid comparisons of mRNA populations from different stages of development and from wild-type and mutant animals at the same stage. As data from such tests accumulate, it should be possible to define “fingerprints” of the mRNA changes that result from perturbation of specific signaling pathways. Once these fingerprints are defined, comparison of animals treated versus not treated with a developmental toxicant could indicate whether a particular pathway or pathways are being affected. Such DNA microchips with arrays for detecting 6,000 mRNAs have been successfully used to detect changes of gene expression in yeast cells in mating circumstances or not (Spellman et al. 1998) and in cultured human cells exposed or unexposed to serum (Iyer et al. 1999).
Several genes known to encode proteins affecting toxicant responses in mammals have homologs in C. elegans. Examples are the genes for the aryl hydrocarbon receptor (AHR) and the AHR nuclear translocator ARNT (Powell-Coffman et al. 1998), whose normal functions are not yet fully understood in any animal. As an approach toward understanding the role of such gene products, their normal functions can be conveniently investigated in C. elegans by reverse genetics using the RNAi technique described above.
Both the low cost of rearing flies and the ease of constructing special stocks are special virtues of Drosophila for the testing of potentially dangerous chemicals. One difficulty, however, will be to determine the effective dosage of the chemical being tested and the most effective route of delivery. Development of the eye and wing takes place during pupal development, although most of the growth of the eye and wing primordia (imaginal discs) occurs in the larva. The pupal case is relatively impermeable to anything except very lipophilic compounds. These compounds can be dissolved in acetone and applied directly to the