Why and how do insects have only six legs, but always six legs? Why and how do flies have only one pair of wings, while butterflies have two? During their development, segmented animals - including humans - produce a series of iterated structures that are subsequently identified by genes called "architects" (the Hox genes). This segmented structure is clearly visible when we consider our spinal column. In insects, the number and quality of dorsal (wings) and ventral (legs) appendages are determined by specific architect genes.
In this third lecture, I examine examples where variations in the regulation of these architect genes lead to drastic changes in the arrangement of these appendages. I show that, in all cases, the resulting morphologies always remind us of an existing animal. For example, the addition of a pair of wings in the fly reminds us of the structure of a dragonfly, suggesting the evolutionary mechanisms that are likely to have been associated with these radical transformations. This lecture also clarifies the notion of gene architects and introduces the concepts of "homeosis" and "tagmosis", two important concepts in this evo-devo discipline.
