This chapter discussed several case studies of the genetic mechanisms underlying the divergence of arthropod and vertebrate body plans. In each case, morphological change is related to a change in the expression pattern of developmental genes. Shifts in Hox expression domains between arthropod, vertebrate, and annelid classes correlate with large differences in body organization—particularly transitions in appendage morphology and vertebral identity. Modifications in Hox expression domains within the limb fields of more closely related organisms underlie such morphological features as butterfly prolegs and fruit fly trichome patterning. When Hox or other selector gene domains are conserved, evolutionary changes in the regulation of downstream target genes facilitate the diversification of homologous parts a
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