Conserved Developmental Processes

Researchers increasingly suspected similarities of development between fruit flies and mice and began to look systematically for homologs of Drosophila developmental genes in mice, frogs, and chicks. In the late 1980s, this was a new research approach. Its success has favored the impression that at a gross level, nematodes, flies, and mice are “all the same organism” and that what is learned about one will have relevance to the others. In a genetically tractable organism, such as Drosophila or C. elegans, a gene is isolated by using a screen for a particular kind of developmental failure, and then the role of its encoded product in development is efficiently deciphered in that organism. Homologs of “developmentally interesting” genes are then sought in vertebrates, such as mice or frogs, in which mutant searches are still daunting due to the comparatively small populations and slow development. The homolog’s function is thereafter studied in the vertebrate, for which the Drosophila or C. elegans information is used as a guide. The mouse is attractive for such studies, because the homologous gene can be knocked out and the phenotype of the null mutant examined to learn about the function of the encoded product.

A surprising array of developmental components and processes is shared between Drosophila and vertebrates (i.e., between arthropods and chordates). In addition to the EMX, OTX, and HOX organization of the body plan, they share the compartments of the dorsoventral dimension (which are thought to be inverted in orientation in one group relative to the other); the presence and mode of organogenesis of limbs (appendages), eyes, heart, visceral mesoderm, and gut; the steps of cytodifferentiation during neurogenesis and myogenesis; and even segmentation. Although the anatomical structures themselves are very different between arthropods and chordates, a number of the underlying steps of development are the same. These are listed in more detail in Table 6-1. The last common ancestor of chordates and arthropods was, it seems, a pre-Cambrian animal of much greater complexity than previously realized. Divergent groups of metazoa (members of the animal kingdom) can be treated as “the same organism” in the experimental analysis of many fundamentals of development. From all of those similarities, the value of model systems for gaining an understanding of difficult basic problems in mammalian development, including that of humans, is undeniable. Humans, flies, and even roundworms are less different than widely thought just 10 years ago.

Signaling Pathways in Development

An important realization to come from the Drosophila research concerns the pervasive use of cell-cell signaling in most aspects of development, starting with the termini and dorsoventral dimension (see Figures 6-1A-D) and extending to organogenesis of many kinds. Inductive signaling was thought to be important in vertebrate development, as mentioned above, but insects and other invertebrates



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