THE ROLE OF THE DISTAL-LESS GENE IN THE DEVELOPMENT AND EVOLUTION OF INSECT LIMBS

Background: Arthropod diversity is apparent in the variations in limb number, type, and position along the body axis. Among the insects, for example, butterflies and moths (Lepidoptera) develop larval abdominal and caudal appendages ('prolegs'), whereas flies (Diptera) do not. Comparative studies of the expression and regulation during development of limb-patterning genes, such as Distal-less (Dll), may provide insights into arthropod evolution. Results: We report the cloning of a Dll homolog from the butterfly Precis coenia, and present data showing that it is expressed in all developing limbs (except the mandible), including the prolegs; the relationship between Dll and wingless expression observed in Drosophila is conserved in Precis among all limbs. However, Dll is deployed in distinct spatial and temporal patterns within each limb type. Conclusions: These data suggest that Dll function, suppressed in the abdomen early in insect evolution, has been derepressed in Lepidoptera, and also suggest that there is a common mechanism underlying the formation of all insect appendages. The limb-type-specific patterns of Dll expression (and its exclusion from the mandible) indicate that regulation of Dll expression may be critical to limb morphology, and are inconsistent with Dll functioning in a simple distal-to-proximal concentration gradient.