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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Forming Complex Pattern: Establishing Positional Information
Genetics In Process 16-3: Mutational analysis of early Drosophila development

The initial insights into the genetic control of pattern formation have emerged from studies of the fruit fly Drosophila melanogaster. The reason that Drosophila development has proved to be a gold mine to researchers is that developmental problems can be simultaneously approached by the use of genetic and molecular techniques. Let's consider the basic genetic and molecular techniques that are employed.

The Drosophila embryo has been especially important in understanding the formation of the basic animal body plan. One important reason is that the formation of the larval exoskeleton in the Drosophila embryo lends itself to easy identification of body plan mutant phenotype. The exoskeleton of the Drosophila larva is laid down as a mosaic in the embryo. Each structure of the exoskeleton is built by the epidermal cell or cells underlying that structure. With its intricate pattern of hairs, indentations, and other structures, the exoskeleton provides numerous landmarks to serve as indicators of the fates acquired by the many epidermal cells. In particular, there are many anatomical structures that are distinct along the anteriorposterior (AP) and dorsalventral (DV) axes. Furthermore, because all the nutrients necessary to develop to the larval stage are prepackaged in the egg, mutant embryos in which the AP or DV cell fates are drastically altered can nonetheless develop to the end of embryogenesis and produce a mutant larva. The exoskeleton of such a mutant larva mirrors the mutant fates assigned to subsets of the epidermal cells and can thus identify genes worthy of detailed analysis.

Researchers, most notably Christiane Nüsslein-Volhard, Eric Wieschaus, and their colleagues, have performed extensive mutational screens, essentially saturating the genome for mutations that alter the AP or DV patterns of the larval exoskeleton. These mutational screens identified two broad classes of genes affecting the basic body plan: zygotically acting genes and maternal-effect genes (see the adjoining diagram). The zygotically acting genes are those in which the gene products contributing to early development are expressed exclusively in the zygote. They are part of the DNA of the zygote itself and are the "standard" sorts of genes that we are used to thinking about. Recessive mutations in zygotically acting genes elicit mutant phenotypes only in homozygous mutant animals. The alternative categorythe maternal-effect genesaffects early development through contributions of gene products from the ovary of the mother to the developing oocyte. In maternal-effect mutations, the phenotype of the offspring depends on the genotype of the mother, not of the offspring. A recessive maternal-effect mutation will produce mutant animals only when the mother is a mutant homozygote.

Equally important to the mutational identification of genes affecting the body plan is the ease with which these genes can be cloned and characterized at the molecular level. Any Drosophila gene can be cloned if its chromosomal map location has been well established, by using recombinant DNA techniques like those described in Chapters 10 and 11. The analysis of the cloned genes often provides valuable information on the function of the protein productusually by identifying close relatives in amino acid sequence of the encoded polypeptide through comparisons with all the protein sequences stored in public data bases. In addition, one can investigate the spatial and temporal pattern of expression of (1) an mRNA, by using histochemically tagged single-stranded DNA sequences complementary to the mRNA to perform RNA in situ hybridization, or (2) a protein, by using histochemically tagged antibodies that specifically bind to that protein.

Extensive use is also made of in vitro mutagenesis techniques. P elements are used for germ-line transformation in Drosophila (see Chapter 11). A cloned pattern formation gene is mutated in a test tube and put back into the fly. The mutated gene is then analyzed to see how the mutation alters the gene's function.

© 1999 by W. H. Freeman and Company. All rights reserved.