Developmental regulatory mechanisms in the evolution of insect diversity

The major architectural differences between most Arthropod classes and orders involve variations in the number, type and pattern of body appendages. We have utilized the emerging knowledge of appendage formation in fruit flies to begin to address the developmental and genetic basis of morphological diversity among insects. Butterflies, for example, differ from fruit flies in possessing larval abdominal limbs, two pairs of adult wings, and a sophisticated system of wing color pattern formation. We have found that the genetic bases for these three major morphological features involve differences between flies and butterflies at three levels of genetic regulation during development. First, we show that the presence of abdominal limbs in butterflies is associated with striking changes in the regulation of specific homeotic genes in the abdominal segments of the butterfly embryo. Second, we suggest that the twowinged state of the fruit fly and the distinct pattern of the butterfly hindwing are the consequence of many accumulated changes in the target genes regulated by the Ultra-bithorax homeotic gene. And finally, we demonstrate that a new genetic program, involving many of the same genes that specify the conserved global patterning coordinates of fruit fly and butterfly wings, has been superimposed onto the butterfly wing to create their unique color patterning system. These findings demonstrate how morphological diversity arises from the different ways in which conserved sets of regulatory genes are deployed during development.