LIMB DEVELOPMENT IN A PRIMITIVE CRUSTACEAN, TRIOPS LONGICAUDATUS - SUBDIVISION OF THE EARLY LIMB BUD GIVES RISE TO MULTIBRANCHED LIMBS

Recent advances in developmental genetics of Drosophila have uncovered some of the key molecules involved in the positioning and outgrowth o f the leg primordia. Although expression patterns of these molecules h ave been analyzed in several arthropod species, broad comparisons of m echanisms of limb development among arthropods remain somewhat specula tive since no detailed studies of limb development exist for crustacea ns, the postulated sister group of insects. As a basis for such compar isons, we analysed limb development in a primitive branchiopod crustac ean, Triops longicaudatus. Adults have a series of similar limbs with eight branches or lobes that project from the main shaft. Phalloidin s taining of developing limbs buds shows the distal epithelial ridge of the early limb bud exhibits eight folds that extend in a dorsal ventra l (D/V) are across the body. These initial folds subsequently form the eight lobes of the adult limb. This study demonstrates that, in a pri mitive crustacean, branched limbs do not arise via sequential splittin g. Current models of limb development based on Drosophila do not provi de a mechanism for establishing eight branches along the D/V axis of a segment. Although the events that position limbs on a body segment ap pear to be conserved between insects and crustaceans, mechanisms of li mb branching may not.