FIGURE 7-2 Life cycle of Drosophila melanogaster. The larva hatches 1 day after the egg is fertilized. First, second, and third instar are larval stages, each ending with a molt. During pupation most of the larval tissues are destroyed and replaced by adult tissues derived from the imaginal discs that were growing in the larva. Times are given for the life cycle at 25°C. Source: Adapted from Wolpert et al. (1998).

ery of the giant polytene salivary gland chromosomes in the 1930s provided a cytological basis for those genetic theorems and thus made Drosophila a key organism for genetic analysis.

As discussed in Chapter 6, the use of fruit flies for developmental studies awaited the saturation screens for lethal and female sterile mutations. These screens were conducted in the late 1970s and 1980s and led to the discovery of cascades of gene functions responsible for the organization of the egg and early pattern formation in the embryo. The advent of recombinant DNA and cloning quickly led to the isolation and sequencing of key genes, which affect the regional specification of body parts. Such genes were defined by the homeotic mutations studied by E. B. Lewis. These studies led to the startling discovery in 1983 that sequences of amino acids coded for by homeotic genes (the homeobox

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