Lewis' Discoveries

Homeotic Genes and Segment Specialization

Homeotic genes control the further specialization of larval segments. Bithorax and Antennapedia are two complexes of homeotic genes in the fly's DNA (see fig. below). Edward B. Lewis studied the bithorax and found that the genes in the DNA are arranged in the same general order as their expression pattern along the head-to-tail axis.

Flies and Mammals, Similar Genes

In his co-linearity principle, Edward B. Lewis also found that the genetic expression domains overlap and that the first gene in the complex becomes active a little earlier than the second and so on. This is indicated in the figure above by the black bars below the fly embryo. Later research showed that the homeotic genes of the fly are homologous to homeotic genes in other types of animals, including man.

A normal adult fruit fly, enlarged 40 times. To the right the fly's ill-fated cousin, a mutant with 4 wings but no balance organs. This now famous little "monster" was a starting point for Lewis in his research on homeotic transformations.

Lewis found that the extra pair of wings was due to a duplication of an entire body segment, the 2nd thoracic segment. Inactivity of the first gene of the bithorax-complex in the appropriate larval segment caused other homeotic genes to respecify the 3rd thoracic segment into one that forms wings instead of halteres.

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